Chapter III: Morphology (2)

In the genus _Gyrophora_, the rhizinae are simple strands of hyphae (_G. polyrhiza_) or they are corticate structures (_G. murina_, _G. spodochroa_ and _G. vellea_). They are also present in species of _Solorina_, _Ricasolia_, _Sticta_ and _Physcia_ and very sparingly in _Cetraria_ (_Platysma_).

_c._ HAPTERA. Sernander[369] has grouped all the more distinctively aerial organs of attachment, apart from rhizinae, under the term “hapteron” and he has described a number of instances in which cilia and even the growing points of the thallus may become transformed to haptera or sucker-like sheaths.

The long cilia of _Physcia ciliaris_ occasionally form haptera at their tips where the hyphae are loose and in active growing condition. Contact with some substance induces branching by which a spreading sheath arises; a plug-like process may also be developed which pierces the substance encountered—not unfrequently another lobe of its own thallus. The long flaccid fronds of _Evernia furfuracea_ are frequently connected together by bridge-like haptera which rise at any angle of the thallus or from any part of the surface.

The spinous hairs that border the thalline margins in _Cetraria_ may also, in contact with some body—often another frond of the lichen—form a hapteron, either while the spermogonium, which occupies the tip of the spine, is still in a rudimentary stage, or after it has discharged its spermatia. The small sucker sheath may in that case arise either from the apex of the cilium, from the wall of the spermogonium or from its base. By means of these haptera, not only different individuals become united together, but instances are given by Sernander in which _Cetraria islandica_, normally a ground lichen, had become epiphytic by attaching itself in this way to the trunk of a tree (_Pinus sylvestris_).

In _Alectoria_, haptera are formed at the tip of the thallus filament as an apical cone-like growth from which hyphae may branch out and penetrate any convenient object. A species of this genus was thus found clinging to stems of _Betula nana_. Apical haptera are very frequent in _Cladonia rangiferina_ and _Cl. sylvatica_, induced here also by contact. These two plants, as well as several species of _Cetraria_, tend, indeed, to become entirely epiphytic on the heaths of the _Calluna_ formations. Haptera similar to those of _Alectoria_ occur in _Usnea_, _Evernia_, _Ramalina_ and _Cornicularia_ (_Cetraria_). In _Evernia prunastri_ var. _stictoceros_, a heath form, the fronds become attached to the stems and branches of _Erica tetralix_ by hapteroid strands of slender glutinous hyphae which persist on the frond of the lichen after it is detached as small very dark tubercles surmounted, as Parfitt[370] pointed out, by a dark-brown grumous mass of cells. Plug-like haptera may be formed at the base of _Cladoniae_ which attach them to each other and to the substratum. The brightly coloured fronds of _Letharia vulpina_ are attached to each other in somewhat tangled fashion by lateral bridges or by fascicles of hyphae dark-brown at the base but colourless at the apices, exactly like aerial adventitious rhizinae. They grow out from the fronds generally at or near the tips and lay hold of a neighbouring frond by means of mucilage. These haptera are evidently formed in response to friction. Haptera along with other lichen attachments have received considerable attention from Galløe[371]. He finds them arising on various positions of the lichen fronds and has classified them accordingly.

After the haptera have become attached, they increase in size and strength and supply a strong anchorage for the plant; the point of contact frequently forms a basis for renewed growth while the part beneath the hapteron may gradually die off. Haptera are more especially characteristic of fruticose lichens, but Sernander considers that the rhizinae of foliose species may function as haptera. They are important organs of tundra and heath formations as they enable the lichens to get a foothold in well-lighted positions, and by their aid the fronds are more able to resist the extreme tearing strains to which they are subjected in high and unsheltered moorlands.

F. STRENGTHENING TISSUES OF STRATOSE LICHENS

Squamulose and foliose lichens grow mostly in close relation with the support, and the flat expanding thallus, as in the _Parmeliae_, is attached at many points to the substance—tree, rock, etc.—over which the plants spread. Special provision for support is therefore not required, and the lobes remain thin and flaccid. Yet, in a number of widely different genera the attachment to the substratum is very slight, and in these we find an adaptation of existing tissues fitted to resist tearing strains, resistance being almost invariably secured by the strengthening of the cortical layers.

_a._ BY DEVELOPMENT OF THE CORTEX. Such a transformation of tissue is well illustrated in _Heppia Guepini_. The thallus consists of rigid squamules which are attached at one point only; the cortex of both surfaces is plectenchymatous and very thick and even the medulla is largely cellular.

The much larger but equally rigid coriaceous thallus of _Dermatocarpon miniatum_ (Fig. 56) has also a single central attachment or umbilicus, and both cortices consist of a compact many-layered plectenchyma. The same structure occurs in _Umbilicaria pustulata_ and in some species of _Gyrophora_, which, having only a single central hold-fast, gain the necessary stiffening through the increase of the cortical layers.

In the Stictaceae there are a large number of widely-expanded forms, and as the attachment depends mostly on a somewhat short tomentum, strength is obtained here also by the thick plectenchymatous cortex of both surfaces. When areas denuded of tomentum and cortex occur, as in _Lobaria pulmonaria_, the under surface is not sensibly weakened, since the cortical tissue remains connected in a stout and firm reticulation.

_b._ BY DEVELOPMENT OF VEINS OR NERVES. Certain ground lichens belonging to the Peltigeraceae have a wide spreading thallus often with very large lobes. The upper cortex is a many-layered plectenchyma, but the under surface is covered only by a loose felt of hyphae which branch out into a more or less dense tomentum. As the firm upper cortex continues to increase by intercalary growth from the branching upwards of hyphae from the meristematic gonidial zone, there occurs an extension of the upper thallus with which the lower cannot keep pace[372]. A little way back from the edge, the result of the stretching is seen in the splitting asunder of the felted hyphae of the under surface, and in the consequent formation of a reticulate series of ridges known as the veins or nerves; they represent the original tomentose covering, and are white, black or brown, according to the colour of the tomentum itself. The naked ellipsoid interstices show the white medulla, and, if the veins are wide, the colourless areas are correspondingly small. Rhizinae are formed on the nerves in several of the species, and anchor the thallus to the support. In _Peltigera canina_, the under surface is almost wholly colourless, the veins are very prominent (Fig. 55), and are further strengthened by the growth and branching of the parallel hyphae of which they are composed. They serve to strengthen the large and flabby thallus and form a rigid base for the long rhizinae by which the lichen clings to the grass or moss over which it grows.

The most perfect development of strengthening nerves is to be found in _Hydrothyria venosa_[373], a rather rare water lichen that occurs in the streams of North America. It consists of fan-like lobes of thin structure, the cortex being only about one cell thick. The fronds are about 3 cm. wide and they are contracted below into a stalk which serves to attach the plant to the substratum. Several fronds may grow together in a dense tuft, the expanded upper portion floating freely in the water. Frequently the plants form a dense growth over the rocky beds of the stream.

At the point where the stalk expands into the free erect frond, there arise a series of stout veins which spread upwards and outwards. They are definitely formed structures and not adaptations of pre-existing tissues: certain hyphae arise from the medulla at the contracted base of the frond, take a radial direction and, by increase, become developed into firm strands. The individual hyphae also increase in size, and the swelling of the nerve gives rise to a ridge prominent on both surfaces. They seldom anastomose at first but towards the tips they become smaller and spread out in delicate ramifications which unite at various points. There is no doubt, as Bitter[372] points out, that the nerves function as strengthening tissues and preserve the frond from the strain of the water currents which would, otherwise, tear apart the delicate texture.

III. RADIATE THALLUS

1. CHARACTERS OF RADIATE THALLUS

In the stratose dorsiventral thallus, there is a widely extended growing area situated round the free margins of the thallus. In the radiate thallus of the fruticose or filamentous lichens, growth is confined to an apical region. Attachment to the substratum is at one point only—the base of the plant—thus securing the exposure of all sides equally to light. The cortex surrounds the fronds, and the gonidia (mostly Protococcaceae) lie in a zone or in groups between the cortex and the medulla. It is the highest type of vegetative development in the lichen kingdom, since it secures the widest room for the gonidial layer, and the largest opportunity for photosynthesis.

Shrubby upright lichens consist mostly of strap-shaped fronds, either simple or branched, which may be broadened to thin bands (Fig. 57) or may be narrowed and thickened till they are almost cylindrical. The fronds vary in length according to the species from a few millimetres upwards: those of _Roccella_ have been found measuring 30 cm. in length; those of _Ramalina reticulata_, the largest of all the American lichens, extend to considerably more.

Lichens of filamentous growth are more or less cylindrical (Fig. 58). They are in some species upright and of moderate length, but in a few pendulous forms they grow to a great length: specimens of _Usnea longissima_ have been recorded that measured 6 to 8 metres from base to tip.

The radiate type of thallus occurs in most of the lichen groups but most frequently in the Gymnocarpeae. In gelatinous Discolichens it is represented in the Lichinaceae. It is rare among Pyrenocarpeae: there is one very minute British lichen in that series, _Pyrenidium actinellum_, and one from N. America, _Pyrenothamnia_, that are of fruticose habit.

2. INTERMEDIATE TYPES OF THALLUS

Between the foliose and the fruticose types, there are intermediate forms that might be, and often are, classified now in one group and now in the other. These are chiefly: _Physcia_ (_Anaptychia_) _ciliaris_, _Ph. leucomelas_ and the species of _Evernia_.

In the two former the habit is more or less fruticose as the plants are affixed to the substratum at a basal point, but the fronds are decumbent and the internal structure is of the dorsiventral type: there is an upper “fibrous” cortex of closely compacted parallel hyphae, a gonidial zone—the gonidia lying partly in the cortex and partly among the loose hyphae of the medulla—and a lower cortex formed of a weft of hyphae which also run somewhat parallel to the surface. Both species are distinguished by the numerous marginal cilia, either pale or dark in colour. These two lichens are greyish-coloured on the upper surface and greyish or whitish below.

_Evernia furfuracea_ with a basal attachment[374], and with a partly horizontal and partly upright growth, has a dorsiventral thallus, dark greyish-green above and black beneath, with occasional rhizinae towards the base. The cortex of both surfaces belongs to the “decomposed” type; the gonidial zone lies below the upper surface, and the medullary tissue is of loose hyphae. In certain forms of the species isidia are abundant on the upper surface, a character of foliose rather than of fruticose lichens. _E. furfuracea_ grows on trees and very frequently on palings.

_E. prunastri_, the second species of the genus, is more distinctly upright in habit, with a penetrating basal hold-fast and upright strap-shaped branching fronds, light-greyish green on the “upper” surface and white on the other (Fig. 59). The internal structure is sub-radiate; both cortices are “decomposed”; the gonidial zone consists of somewhat loose groups of algae, very constant below the “upper” surface, with an occasional group in the pith near to the lower cortex in positions that are more exposed to light. There is also a tendency for the gonidial zone to pass round the margin and spread some way along the under side. The medulla is of loose arachnoid texture and the whole plant is very limp when moist. It grows on trees, often in dense clusters.

3. FRUTICOSE AND FILAMENTOUS

A. GENERAL STRUCTURE OF THALLUS

The conditions of strain and tension in the upright plant are entirely different from those in the decumbent thallus, and to meet the new requirements, new adaptations of structure are provided either in the cortex or in the medulla.

CORTICAL STRUCTURES. With the exception of the distinctly plectenchymatous cortex, all the other types already described recur in fruticose lichens; in various ways they have been modified to provide not only covering but support to the fronds.

_a._ =The fastigiate cortex.= This reaches its highest development in _Roccella_ in which the branched hyphal tips, slightly clavate and thick-walled, lie closely packed in palisade formation at right angles to the main axis (Fig. 45). They afford not only bending power, but give great consistency to the fronds. The cortex is further strengthened in _R. fuciformis_[375] by the compact arrangement of the medullary hyphae that run parallel with the surface, and among which occur single thick-walled filaments. The plant grows on maritime rocks in very exposed situations; and the narrow strap-shaped fronds, as stated above, may attain a length of 30 cm., though usually they are from 10 to 18 cm. in height. The same type of cortex, but less highly differentiated, affords a certain amount of stiffness to the cylindrical much weaker fronds of _Thamnolia_.

_b._ =The fibrous cortex.= This type is found in a number of lichens with long filamentous hanging fronds. It consists of parallel hyphae, rarely septate and rarely branched, but frequently anastomosing and with strongly thickened “sclerotic” walls. Such a cortex is the only strengthening element in _Alectoria_, and it affords great toughness and flexibility to the thong-like thallus. It is also present in _Ramalina_ (_Alectoria_) _thrausta_, a species with slender fronds (Fig. 60).

In _Usnea longissima_ the cortex both of the fibrillose branchlets and of the main axis is fibrous, and is composed of narrow thick-walled hyphae which grow in a long spiral round the central strand. The hyphae become more frequently septate further back from the apex (Fig. 61). Such a type of cortex provides an exceedingly elastic and efficient protection for the long slender thallus.

The same type of cortex forms the strengthening element in the fruticose or partly fruticose members of the family Physciaceae. One of these, _Teloschistes flavicans_, is a bright yellow filamentous lichen with a somewhat straggling habit. The fronds are very slender and are either cylindrical or slightly flattened. The hyphae of the outer cortex are compactly fibrous; added toughness is given by the presence of some longitudinal strands of hyphae in the central pith.

Another still more familiar grey lichen, _Physcia ciliaris_, has long flat branching fronds which, though dorsiventral in structure, are partly upright in habit. Strength is secured as in _Teloschistes_ by the fibrous upper cortex. Other species of _Physciae_ are somewhat similar in habit and in structure.

In _Dendrographa leucophaea_, a slender strap-shaped rock lichen, Darbishire[376] has described the outer cortex as composed of closely compacted parallel hyphae resembling the strengthening cortex of _Alectoria_ and very different from the fastigiate cortex of the _Roccellae_ with which it is usually classified.

B. SPECIAL STRENGTHENING STRUCTURES

_a._ SCLEROTIC STRANDS. This form of strengthening tissue is characteristic of _Ramalina_. With the exception of _R. thrausta_ (more truly an _Alectoria_) all the species have a rather weak cortical layer of branching intricate thick-walled hyphae, regarded by Brandt[377] as plectenchymatous, but more correctly by Hue[378] as “decomposed” on account of the gelatinous walls and diminishing lumen of the irregularly arranged cells.

In _R. evernioides_, a plant with very wide flat almost decumbent fronds of soft texture, in _R. ceruchis_ and in _R. homalea_ there is a somewhat compact medulla which gives a slight stiffness to the thallus. The other species of the genus are provided with strengthening mechanical tissue within the cortex formed of closely united sclerotic hyphae that run parallel to the surface (Fig. 62). In a transverse section of the thallus, this tissue appears sometimes as a continuous ring which may project irregularly into the pith (_R. calicaris_); more frequently it is in the form of strands or bundles which alternate with the groups of gonidia (_R. siliquosa_, _R. Curnowii_, etc.). In _R. fraxinea_ these strands may be scarcely discernible in young fronds, though sometimes already well developed near the tips. Occasionally isolated strands of fibres appear in the pith (_R. Curnowii_), or the sclerotic projections may even stretch across the pith to the other side (_R. strepsilis_) (Fig. 75 B).

In the _Cladoniae_ support along with flexibility is secured to the upright podetium by the parallel closely packed hyphae that form round the hollow cylinder a band called the “chondroid” layer from its cartilage-like consistency.

_b._ CHONDROID AXIS. The central medullary tissue in _Ramalina_ is, with few exceptions, a loose arachnoid structure; often the fronds are almost hollow. In one species of _Usnea_, _U. Taylori_, found in polar regions, there is a similar loose though very circumscribed medullary and gonidial tissue in the centre of the somewhat cylindrical thallus, and a wide band of sclerotic fibres towards the cortex.

In all other species of _Usnea_ the medulla itself is transformed into a strong central strand of long-celled thick-walled hyphae closely knit together by frequent anastomoses (Fig. 63 A). This central strand of the Usneas is known as the “chondroid axis.” A narrow band of loose air-containing hyphae and a gonidial zone lie round the central axis between it and the outer cortex (Fig. 63 A, _b_). At the extreme apex, the external cortical hyphae grow in a direction parallel with the long axis of the plant, but further back, they branch out at right angles and become swollen and mostly “decomposed” as in the cortex of _Ramalina_.

In _Letharia_ (_L. vulpina_, etc.) the structure is midway between _Ramalina_ and _Usnea_: the central axis is either a solid strand of chondroid hyphae or several separate strands.

In three other genera with upright fruticose thalli, _Sphaerophorus_, _Argopsis_ and _Stereocaulon_, rigidity is maintained by a medulla approaching the chondroid type. In _Sphaerophorus_ the species may have either flattened or cylindrical branching stalks, but in all of them, the centre is occupied by longitudinal strands of hyphae. _Argopsis_, a monotypic genus from Kerguelen, has a cylindrical branching thallus with a strong solid axis; it is closely allied to _Stereocaulon_, a genus of familiar moorland lichens. The central tissue of the stalks in _Stereocaulon_ is also composed of elongate, thick-walled conglutinate hyphae, formed into a strand which is, however, not entirely solid.

C. SURVEY OF MECHANICAL TISSUES

Mechanical tissues scarcely appear among fungi, except perhaps as stoutish cartilaginous hyphae in the stalks of some Agarics (_Collybiae_, etc.), or as a ring of more compact consistency round the central hyphae of rhizomorphic strands. It is practically a new adaptation of hyphal structure confined to lichens of the fruticose group, where there is the same requirement as in the higher plants for rigidity, flexure and tenacity.

Rigidity is attained as in other plants by groups or strands of mechanical tissue situated close to the periphery, as they are so arranged in _Ramalina_ and _Cladonia_; or the same end is achieved by a strongly developed fastigiate cortex as in _Roccella_. Bending strains to which the same lichens are subjected, are equally well met by the peripheral disposition of the mechanical elements.

Tenacity and elasticity are provided for in the pendulous forms either by a fibrous cortex as in _Alectoria_, or by the chondroid axis in _Usnea_. Haberlandt[379] has recorded some interesting results of tests made by him as to the stretching capacity of a freshly gathered pendulous species in which the central strand was from ·5 to 1 mm. thick. He found he could draw it out 100 to 110 per cent. of its normal length before it gave way. In an upright species the frond broke when stretched 60 to 70 per cent. In both of the plants tested, the central strand retained its elasticity up to 20 per cent. of stretching. The outer cortical tissue was cracked and broken in the experiments. Schulte[380] calculated somewhat roughly the tenacity of _Usnea longissima_ and found that a piece of the main axis 8 cm. long carried up to 300 grms. without breaking.

D. RETICULATE FRONDS

In the upright radiate thallus, more especially among the _Ramalinae_, though also among _Cladoniae_[381], there has appeared a reticulate thallus resulting from the elongate splitting of the tissues, and due to unequal growth tension and straining of the gelatinous cortex when swollen with moisture. In several species of _Ramalina_, the strap-shaped frond is hollow in the centre; and strands of strengthening fibres give rise to a series of cortical ridges. The thinner tissue between is frequently torn apart and ellipsoid openings appear which do not however pierce beyond the central hollow. Such breaks are irregular and accidental though occurring constantly in _Ramalina fraxinea_, _R. dilacerata_, etc.

A more complete type of reticulation is always present in a Californian lichen, _Ramalina reticulata_, in which the large flat frond is a delicate open network from tip to base (Fig. 64). It grows on the branches of deciduous trees and hangs in crowded tufts up to 30 cm. or more in length. Usually it is so torn, that the real size attainable can only be guessed at. It is attached at the base by a spreading discoid hold-fast, and, in mature plants, consists of a stoutish main axis from which side branches are irregularly given off. These latter are firm at the base like the parent stalk, but soon they broaden out into very wide fronds. Splitting begins at the tips of the branches while still young; they are then spathulate in form with a slightly narrower recurved tip, below which the first perforations are visible, small at first, but gradually enlarging with the growth of the frond.

_Ramalina reticulata_ is an extremely gelatinous lichen and the formation of the network was supposed by Lutz[382] to be entirely due to the swelling of the tissues, or the imbibition of water, causing tension and splitting. A more exact explanation of the phenomenon is given by Peirce[383]: he found that it was due to the thickened incurved tip, which, on the addition of moisture, swells in length, breadth and thickness, causing it to bend slightly upwards and then curve backwards over the thallus, thus straining the part immediately behind. These various movements result in the splitting of the frond while it is young and the cortices are thin and weak.

Peirce made a series of experiments to test the capacity of the tissues to support tensile strains. In a dry state, a piece of the lichen held a weight up to 150 grms.; when wet it broke with a weight of 30 grms. It was also observed that the thickness of the frond doubled on wetting.

E. ROOTING BASE IN FRUTICOSE LICHENS

Fruticose and filamentous lichens are distinguished by their mode of attachment to the substratum: instead of a system of rhizinae or of hairs spread over a large area, there is usually one definite rooting base by which the plant maintains its hold on the support.

Intermediate between the foliose and fruticose types of thallus are several species which are decumbent in habit, but which are attached at one (or sometimes more) definite points, with but little penetration of the underlying substance. One such lichen, _Evernia furfuracea_, has been classified now as foliose, and again as fruticose. The earliest stage of the thallus is in the form of a rosette-like sheath which bears rhizinae on the under surface, very numerous at the centre of the sheath, but entirely wanting towards the periphery. A secondary thallus of strap-shaped rather narrow fronds rises from the sheath and increases by irregular dichotomous branching. These branches, which are considered by Zopf[384] as adventitious, may also come into contact with the substratum and produce a few rhizinae at that point; or if the frond is more closely applied, the irritation thus produced causes a still greater outgrowth of rhizinae and the formation of a new base from which other fronds originate. These renewed centres of growth are not of very frequent occurrence; they were first observed and described by Lindau[385] in another species, _Evernia prunastri_, and were aptly compared by him to the creeping stolons of flowering plants.

_Evernia furfuracea_ grows frequently on dead wood, palings, etc., as well as on trees. _E. prunastri_ grows invariably on trees, and has a more constantly upright fruticose habit; in this species also, a basal sheath is present, and the attachment is secured by means of rhizoidal hyphae which penetrate deeply into the periderm of the tree, taking advantage of the openings afforded by the lenticels. The sheath hyphae are continuous with the medullary hyphae of the frond, and gonidia are frequently enclosed in the tissues; the sheath spreads to some extent over the surface of the bark, and round the base of the fronds, thus rendering the attachment of the lichen to the tree doubly secure.

Among _Ramalinae_, the development of the base was followed by Brandt[386] in one species, _R. Landroensis_, an arboreal lichen from S. Tyrol. A rosette-like sheath was formed consisting solely of strands of thick-walled hyphae which spread over the bark. There were no gonidia included in the tissue.

A different type of attachment was found by Lilian Porter[387] in corticolous _Ramalinae_—_R. fraxinea_, _R. fastigiata_, and _R. pollinaria_. The lichens were anchored to the tree by strands of closely compacted hyphae longitudinally arranged and continuous with the cortical hyphae. These enter the periderm of the tree by cracks or lenticels, and by wedge action cause extensive splitting. The strands may also spread horizontally and give rise to new plants. The living tissues of the tree were thus penetrated and injured, and there was evidence that hypertrophied tissue was formed and caused erosion of the wood.

Several _Ramalinae_—_R. siliquosa_, _R. Curnowii_, etc.—grow on rocks, often in extremely exposed situations, in isolated tufts or in crowded swards (Fig. 65). The separate tufts are not unfrequently connected at the base by a crustaceous thallus. It is possible also to see on the rock, here and there, small areas of compact thalline granules that have scarcely begun to put out the upright fronds. These granules are corticate on the upper surface and contain gonidia; from the lower surface, slender branching hyphae in rhizoid-like strands penetrate down between the inequalities and separable particles of the rock, if the formation is granitic. They frequently have groups of gonidia associated with them, and they continue to ramify and spread, the pure white filaments often enough enclosing morsels of the rock. The upright fronds are continuous with the base and are thus securely anchored to the substratum.

On a smooth rock surface such as quartzite a continuous sward of _Ramalina_ growth is impossible. The basal hyphae being unable to penetrate the even surface of the rock, the attachment is slight and the plants are easily dislodged. They do however succeed, sometimes, in taking hold, and small groups of fronds arise from a crustaceous base which varies in depth from ·5 to 1 mm. The tissues of this base are very irregularly arranged: towards the upper surface loose hyphae with scattered groups of algae are traversed by strands of gelatinized sclerotic hyphae similar to the strengthening tissues of the upright fronds, while down below there are to be found not only slender hyphae, but a layer of gonidia visible as a white and green film on the rock when the overlying particles are scaled off.

Darbishire[388] found that attachment to the substratum by means of a basal sheath was characteristic of all the genera of Roccellaceae. He looks on this sheath, which is the first stage in the development of the plant, as a primary or proto-thallus, analogous to the primary squamules of the _Cladoniae_, and he carries the analogy still further by treating the upright fronds as podetia. The sheath of the Roccellaceae varies in size but it is always of very limited extent; it is mainly composed of medullary hyphae, and gonidia may or may not be present. The whole structure is permanent and important, and is generally protected by a well-developed upper cortex similar in structure to that of the upright thallus, _i.e._ of a fastigiate type. There is no lower cortex.

The two British species of _Roccella_—_R. fuciformis_ and _R. phycopsis_—grow on maritime rocks, the latter also occasionally on trees. In _R. fuciformis_, the attaching sheath is a flat structure which slopes up a little round the base of the upright frond. It is about 2 mm. thick, the cortex occupying about 40 µ of that space; a few scattered gonidia are present immediately below. The remaining tissue of the sheath is composed of firmly wefted slender filaments. Towards the lower surface, there is a more closely compacted dark brown layer from which pass out the hyphae that penetrate the rock.

The sheath of _R. phycopsis_ is a small structure about 3 to 4 mm. in width and 1·5 mm. thick. A few gonidia may be found below the dense cortical layer, but they tend to disappear as the upright fronds become larger and the shade, in consequence, more dense. Lower down the hyphae take an intensely yellow hue; mixed with them are also some brown filaments. A somewhat larger sheath 7 to 8 mm. wide forms the base of _R. tinctoria_. In structure it corresponds—as do those of the other species—with the ones already described.

In purely filamentous species such as _Usnea_ there is also primary sheath formation: the medullary hyphae spread out in radiating strands which force their way wherever possible into the underlying substance; on trees they enter into any chink or crevice of the outer bark like wedges; or they ramify between the cork cells which are split up by the mere growth pressure. By the vertical increase of the base, the fronds may be hoisted up and an intercalary basal portion may arise lacking both gonidia and cortical layer. Very frequently several bases are united and the lichen appears to be of tufted habit.

A basal sheath provides a similar firm attachment for _Alectoria jubata_ and allied species: these are slender mostly dark brown lichens which hang in tangled filaments from the branches of trees, rocks, etc.

These attaching sheaths differ in function as well as in structure from the horizontal thallus of the Cladoniaceae. They may be more truly compared with the primary thallus of the red algae _Dumontia_ and _Phyllophora_ which are similarly affixed to the substratum, while upright fronds of subsequent formation bear the fructifications.

IV. STRATOSE-RADIATE THALLUS

1. STRATOSE OR PRIMARY THALLUS

A. GENERAL CHARACTERISTICS

This series includes the lichens of one family only, the Cladoniaceae, the genera of which are characterized by the twofold thallus, one portion being primary, horizontal and stratose, the other secondary and radiate, the latter an upright simple or branching structure termed a “podetium” which narrows above, or widens to form a trumpet-shaped cup or “scyphus” (Fig. 66). The apothecia are terminal on the podetium or on the margins of the scyphi; in a few species they are developed on the primary thallus. Some degree of primary thallus-formation has been demonstrated in all the genera, if not in all the species of the family. The genus _Cladina_ was established to include those species of _Cladonia_ in which, it was believed, only a secondary podetial thallus was present, but Wainio[389] found in _Cladonia sylvatica_ a granular basal crust and, in _Cladonia uncialis_, minute round scales with crenate margins measuring from ·5 to 1 mm. in width. In some species (subgenus _Cladina_) the primary thallus is quickly evanescent, in others it is granular or squamulose and persistent. Where the basal thallus is so much reduced as to be practically non-existent, apothecia are rarely developed and soredia are absent. Renewal of growth in these lichens is secured by the dispersal of fragments of the podetial thallus; they are torn off and scattered by the wind or by animals, and, if suitable conditions are met, a new plant arises.

_Cladonia_ squamules vary in size from very small scales as in _Cl. uncialis_ to the fairly large foliose fronds of _Cl. foliacea_ which extend to 5 cm. in length and about 1 cm. or more in width. It is interesting to note that when the primary thallus is well developed, the podetia are relatively unimportant and frequently are not formed. As a rule the squamules are rounded or somewhat elongate in form with entire or variously cut and crenate margins. They may be very insignificant and sparsely scattered over the substratum, or massed in crowded swards of leaflets which are frequently almost upright. In colour they are bluish-grey, yellowish or brownish above, and white beneath (red in _Cl. miniata_), frequently becoming very dark-coloured towards the rooting base. These several characteristics are specific and are often of considerable value in diagnosis. In certain conditions of shade or moisture, squamules are formed on the podetium; they repeat the characters of the basal squamules of the species.

B. TISSUES OF THE PRIMARY THALLUS

The stratose layers of tissue in the squamules of _Cladonia_ are arranged as in other horizontal thalli.

_a._ CORTICAL TISSUE. In nearly all these squamules the cortex is of the “decomposed” type. In a few species there is a plectenchymatous formation—in _Cl. nana_, a Brazilian ground species, and in two New Zealand species, _Cl. enantia_ f. _dilatata_ and _Cl. Neo-Zelandica_. The principal growing area is situated all round the margins though generally there is more activity at the apex. Frequently there is a gradual perishing of the squamule at the base which counterbalances the forward increase.

The upper surface in some species is cracked into minute areolae; the cracks, seen in section, penetrate almost to the base of the decomposed gelatinous cortex. They are largely due to alternate swelling and contraction of the gelatinous surface, or to extension caused, though rarely, by intercalary growth from the hyphae below. The surface is subject to weathering and peeling as in other lichens; but the loss is constantly repaired by the upward growth of the meristematic hyphae from the gonidial zone; they push up between the older cortical filaments and so provide for the expansion as well as for the renewal of the cortical tissue.

_b._ GONIDIAL TISSUE. The gonidia consisting of Protococcaceous algae form a layer immediately below the cortex. Isolated green cells are not unfrequently carried up by the growing hyphae into the cortical region, but they do not long survive in this compact non-aerated tissue. Their empty membranes can however be picked out by the blue stain they take with iodine and sulphuric acid.

Krabbe[390] has described the phases of development in the growing region: he finds that differentiation into pith, gonidial zone and cortex takes place some little way back from the edge. At the extreme apex the hyphae lie fairly parallel to each other; further back, they branch upwards to form the cortex, and to separate the masses of multiplying gonidia, by pushing between them and so spreading them through the whole apical tissue. The gonidia immediately below the upper cortex, where they are well-lighted, continue to increase and gradually form into the gonidial zone; those that lie deeper among the medullary hyphae remain quiescent, and before long disappear altogether.

Where the squamules assume the upright position (as in _Cladonia cervicornis_), there is a tendency for the gonidia to pass round to the lower surface, and soredia are occasionally formed.

_c._ MEDULLARY TISSUE. The hyphae of the medulla are described by Wainio as having long cells with narrow lumen, and as being encrusted with granulations that may coalesce into more or less detachable granules; in colour they are mostly white, but pale-yellow in _Cl. foliacea_ and blood-red in _Cl. miniata_, a subtropical species. They are connected at the base of the squamules with a filamentous hypothallus which penetrates the substratum and attaches the plant. In a few species rhizinae are formed, while in others the hyphae of the podetium grow downwards, towards and into the substratum as a short stout rhizoid.

_d._ SOREDIA. Though frequent on the podetia, soredia are rare on the squamules, and, according to Wainio[391], always originate at the growing region, from which they spread over the under surface—rather sparsely in _Cl. cariosa_, _Cl. squamosa_, etc., but abundantly in _Cl. digitata_ and a few others. In some instances, they develop further into small corticate areolae on the under surface (_Cl. coccifera_, _Cl. pyxidata_ and _Cl. squamosa_).

2. RADIATE OR SECONDARY THALLUS

A. ORIGIN OF THE PODETIUM

The upright podetium, as described by Wainio[392] and by Krabbe[393], is a secondary product of the basal granule or squamule. It is developed from the hyphae of the gonidial zone, generally where a crack has occurred in the cortex and rather close to the base or more rarely on or near the edge of the squamule (_Cl. verticillata_, etc.). At these areas, certain meristematic gonidial hyphae increase and unite to form a strand of filaments below the upper cortex but above the gonidial layer, the latter remaining for a time undisturbed as to the arrangement of the algal cells.

This initial tissue—the primordium of the podetium—continues to grow not only in width but in length: the basal portion grows downwards and at length displaces the gonidial zone, while the upper part as a compact cylinder forces its way through the cortex above, the cortical tissue, however, taking no part in its formation; as it advances, the edges of the gonidial and cortical zones bend upwards and form a sheath distinguishable for some time round the base of the emerging podetium.

Even when the primary horizontal thallus is merely crustaceous, the podetia take origin similarly from a subcortical weft of hyphae in an areola or granule.

B. STRUCTURE OF THE PODETIUM

_a._ GENERAL STRUCTURE. In the early stages of development the podetium is solid throughout, two layers of tissue being discernible—the hyphae forming the centre of the cylinder being thick-walled and closely compacted, and the hyphae on the exterior loosely branching with numerous air-spaces between the filaments.

In all species, with the exception of _Cl. solida_, which remains solid during the life of the plant, a central cavity arises while the podetium is still quite short (about 1 to 1·5 mm. in _Cl. pyxidata_ and _Cl. degenerans_). The first indication of the opening is a narrow split in the internal cylinder, due to the difference in growth tension between the more free and rapid increase of the external medullary layer and the slower elongation of the chondroid tissue at the centre. The cavity gradually widens and becomes more completely tubular with the upward growth of the podetium; it is lined by the chondroid sclerotic band which supports the whole structure (Fig. 67).

_b._ GONIDIAL TISSUE. In most species of Cladoniaceae, a layer of gonidial tissue forms a more or less continuous outer covering of the podetium, thus distinguishing it from the purely hyphal stalks of the apothecia in Caliciaceae. Even in the genus _Baeomyces_, while the podetia of some of the species are without gonidia, neighbouring species are provided with green cells on the upright stalks clearly showing their true affinity with the _Cladoniae_. In one British species of _Cladonia_ (_Cl. caespiticia_) the short podetium consists only of the fibrous chondroid cylinder, and thus resembles the apothecial stalk of _Baeomyces rufus_, but in that species also there are occasional surface gonidia that may give rise to squamules.

Krabbe[394] concluded from his observations that the podetial gonidia of _Cladonia_ arrived from the open, conveyed by wind, water or insects from the loose soredia that are generally so plentiful in any _Cladonia_ colony. They alighted, he held, on the growing stalks and, being secured by the free-growing ends of the exterior hyphae, they increased and became an integral part of the podetium. In more recent times Baur[395] has recalled and supported Krabbe’s view, but Wainio[396], on the contrary, claims to have proved that in the earliest stages of the podetium the gonidia were already present, having been carried up from the gonidial zone of the primary thallus by the primordial hyphae. Increase of these green cells follows normally by cell-division or sporulation.

Algal cells have been found to be common to different lichens, but in _Cladoniae_ Chodat[397] claims to have proved by cultures that each species tested has a special gonidium, determined by him as a species of _Cystococcus_, which would render colonization by algae from the open much less probable. In addition, the fungal hyphae are specific, and any soredia (with their combined symbionts) that alighted on the podetium could only be utilized if they originated from the same species; or, if they were incorporated, the hyphae belonging to any other species would of necessity die off and be replaced by those of the podetium.

_c._ CORTICAL TISSUE. In some species a cortex of the decomposed type of thick-walled conglutinate hyphae is present, either continuous over the whole surface of the podetium, as in _Cl. gracilis_ (Fig. 68), or in interrupted areas or granules as in _Cl. pyxidata_ (Fig. 69) and others. In _Cl. degenerans_, the spaces between the corticated areolae are filled in by loose filaments without any green cells. _Cl. rangiferina_, _Cl. sylvatica_, etc. are non-corticate, being covered all over with a loose felt of intricate hyphae.

In the section _Clathrinae_ (_Cl. retepora_, etc.) the cortex is formed of longitudinal hyphae with thick gelatinous walls.

_d._ SOREDIA. Frequently the podetium is coated in whole or in part by granules of a sorediate character—coarsely granular in _Cl. pyxidata_, finely pulverulent in _Cl. fimbriata_. Though fairly constant to type in the different species, they are subject to climatic influences, and, when there is abundant moisture, both soredia and areolae develop into squamules on the podetium. A considerable number of species have thus a more or less densely squamulose “form” or “variety.”

C. DEVELOPMENT OF THE SCYPHUS

Two types of podetia occur in _Cladonia_: those that end abruptly and are crowned when fertile by the apothecia or spermogonia (pycnidia), or if sterile grow indefinitely tapering gradually to a point (Fig. 70); and those that widen out into the trumpet-shaped or cup-like expansion called the scyphus (Fig. 69). Species may be constantly scyphiferous or as constantly ascyphous; in a few species, and even in individual tufts, both types of podetium may be present.

Wainio[398], who has studied every stage of development in the _Cladoniae_, has described the scyphus as originating in several different ways:

_a._ FROM ABORTIVE APOTHECIA. In certain species the apothecium appears at a very early stage in the development of the podetium of which it occupies the apical region. Owing to the subsequent formation of the tubular cavity in the centre of the stalk, the base of the apothecium may eventually lie directly over the hollow space and, therefore, out of touch with the growing assimilating tissues; or even before the appearance of the tube, the wide separation between the primordium of the apothecium and the gonidia, entailing deficient nutrition, may have produced a similar effect. In either case central degeneration of the apothecium sets in, and the hypothecial filaments, having begun to grow radially, continue to travel in the same direction both outwards and upwards so that gradually a cup-shaped structure is evolved—the amphithecium of the fruit without the thecium.

The whole or only a part of the apothecium may be abortive, and the scyphus may therefore be entirely sterile or the fruits may survive at the edges. The apothecia may even be entirely abortive after a fertile commencement, but in that case also the primordial hyphae retain the primitive impulse not only to radial direction, but also to the more copious branching, and a scyphus is formed as in the previous case. It must also be borne in mind that the tendency in many _Cladonia_ species to scyphiform has become hereditary.

Baur[399], in his study of _Cl. pyxidata_, has taken the view that the origin of the scyphus was due to a stronger apical growth of the hyphae at the circumference than over the central tubular portion of the podetium, and that considerable intercalary growth added to the expanding sides of the cup.

Scyphi originating from an abortive apothecium are characteristic of species in which the base is closed (Wainio’s Section _Clausae_), the tissue in that case being continuous over the inside of the cup as in _Cl. pyxidata_, _Cl. coccifera_ and many others.

_b._ FROM POLYTOMOUS BRANCHING. Another method of scyphus formation occurs in _Cl. amaurocrea_ and a few other species in which the branching is polytomous (several members rising from about the same level). Concrescence of the tissues at the base of these branches produces a scyphus; it is normally closed by a diaphragm that has spread out from the different bases, but frequently there is a perforation due to stretching. These species belong to the Section _Perviae_.

_c._ FROM ARRESTED GROWTH. In most cases however where the scyphus is open as in _Cl. furcata_, _Cl. squamosa_, etc., development of the cup follows on cessation of growth, or on perforation at the summit of the podetium. Round this quiescent portion there rises a circle of minute prominences which carry on the apical development. As they increase in size, the spaces between them are bridged over by lateral growth, and the scyphus thus formed is large or small according to the number of these outgrowths. Apothecia or spermogonia may be produced at their tips, or the vegetative development may continue. Scyphi formed in this manner are also open or “pervious.”

_d._ GONIDIA OF THE SCYPHUS. Gonidia are absent in the early stages of scyphus formation when it arises from degeneration of the apical tissues, either fertile or vegetative; but gradually they migrate from the podetium, from the base of young outgrowths, or by furrows at the edge, and so spread over the surface of the cup. Soredia may possibly alight, as Krabbe insists that they do, and may aid in colonizing the naked area. Their presence, however, would only be accidental; they are not essential, and scyphi are formed in many non-sorediate species such as _Cl. verticillata_. The cortex of the scyphus becomes in the end continuous with that of the podetium and is always similar in type.

_e._ SPECIES WITHOUT SCYPHI. In species where the whole summit of the podetium is occupied by an apothecium, as in _Cl. bellidiflora_, no scyphus is formed. There is also an absence of scyphi in podetia that taper to a point. In those podetia the hyphae are parallel to the long axis and remain in connection with the external gonidial layer so that they are unaffected by the central cavity. Instances of tapering growth are also to be found in species that are normally scyphiferous such as _Cl. fimbriata_ subsp. _fibula_, and _Cl. cornuta_, as well as in species like _Cl. rangiferina_ that are constantly ascyphous.

The scyphus is considered by Wainio[400] to represent an advanced stage of development in the species or in the individual, and any conditions that act unfavourably on growth, such as excessive dryness, would also hinder the formation of this peculiar lichen structure.

D. BRANCHING OF THE PODETIUM

Though branching is a constant feature in many species, regular dichotomy is rare: more often there is an irregular form of polytomy in which one of the members grows more vigorously than the others and branches again, so that a kind of sympodium arises, as in _Cl. rangiferina_, _Cl. sylvatica_, etc.

Adventitious branches may also arise from the podetium, owing to some disturbance of the normal growth, some undue exposure to wind or to too great light, or owing to some external injury. They originate from the gonidial tissue in the same way as does the podetium from the primary thallus; the parallel hyphae of the main axis take no part in their development.

In a number of species secondary podetia arise from the centre of the scyphus—constantly in _Cl. verticillata_ and _Cl. cervicornis_, etc., accidentally or rarely in _Cl. foliacea_, _Cl. pyxidata_, _Cl. fimbriata_, etc. Wainio[401] has stated that they arise when the scyphus is already at an advanced stage of growth and that they are to be regarded as adventitious branches.

The proliferations from the borders of the scyphus are in a different category. They represent the continuity of apical growth, as the edges of the scyphus are but an enlarged apex. These marginal proliferations thus correspond to polytomous branching. In many instances their advance is soon stopped by the formation of an apothecium and they figure more as fruit stalks than as podetial branches.

E. PERFORATIONS AND RETICULATION OF THE PODETIUM

Perforations in the podetial wall at the axils of the branches are constant in certain species such as _Cl. rangiferina_, _Cl. uncialis_, etc. They are caused by the tension of the branches as they emerge from the main stalk. A tearing of the tissue may also arise in the base of the scyphus, due to its increase in size, which causes the splitting of the diaphragm at the bottom of the cup.

Among the _Cladoniae_ the reticulate condition recurs now and again. In our native _Cladonia cariosa_ the splitting of the podetial wall is a constant character of the species, the carious condition being caused by unequal growth which tears apart the longitudinal fibres that surround the central hollow.

A more advanced type of reticulation arises in the group of the _Clathrinae_ in which there is no inner chondroid cylinder. In _Cladonia aggregata_, in which the perforations are somewhat irregular, two types of podetia have been described by Lindsay[402] from Falkland Island specimens: those bearing apothecia are short and broad, fastigiately branched upwards and with reticulate perforations, while podetia bearing spermogonia are slender, elongate and branched, with fewer reticulations. An imperfect network is also characteristic of _Cl. Sullivani_, a Brazilian species. But the most marvellous and regular form of reticulation occurs in _Cl. retepora_, an Australian lichen (Fig. 71): towards the tips of the podetia the ellipsoid meshes are small, but they gradually become larger towards the base. In this species the outer tissue, though of parallel hyphae, is closely interwoven and forms a continuous growth at the edges of the perforations, giving an unbroken smooth surface and checking any irregular tearing. The enlargement of the walls is solely due to intercalary growth. The origin of the reticulate structure in the _Clathrinae_ is unknown, though it is doubtless associated with wide podetia and rendered possible by the absence of an internal chondroid layer. The reticulate structure is marvellously adapted for the absorption of water: _Cl. retepora_, more especially, imbibes and holds moisture like a sponge.

F. ROOTING STRUCTURES OF CLADONIAE

The squamules of the primary thallus are attached, as are most squamules, to the supporting substance by strands of hyphae which may be combined into simple or branching rhizinae and penetrate the soil or the wood on which the lichen grows. There is frequently but one of these rooting structures and it branches repeatedly until the ultimate branchlets end in delicate mycelium. Generally they are grey or brown and are not easily traced, but when they are orange-coloured, as according to Wainio[403] they frequently are in _Cladonia miniata_ and _Cl. digitata_, they are more readily observed, especially if the habitat be a mossy one.

In _Cl. alpicola_ it has been found that the rooting structure is frequently as thick as the podetium itself. If the podetium originates from the basal portion of the squamule, the hyphae from the chondroid layer, surrounding the hollow centre, take a downward direction and become continuous with the rhizoid. Should the point of insertion be near the apex of the squamule, these hyphae form a nerve within the squamule or along the under surface, and finally also unite with the rhizoid at the base, a form of rooting characteristic of _Cl. cartilaginea_, _Cl. digitata_ and several other species.

Mycelium may spread from the rhizinae along the surface of the substratum and give rise to new squamules and new tufts of podetia, a method of reproduction that is of considerable importance in species that are generally sterile and that form no soredia.

Many species, especially those of the section _Cladina_, soon lose all connection with the substratum, there being a continual decay of the lower part of the podetia. As apical growth may continue for centuries, the perishing of the base is not to be wondered at.

G. HAPTERA