Chapter III: Morphology (4)

Soredia have been compared to the gemmae of the Bryophytes and also to the slips and cuttings of the higher plants. There is a certain analogy between all forms of vegetative reproduction, but soredia are peculiar in that they include two dissimilar organisms. In the lichen kingdom there has been evolved this new form of propagation in order to secure the continuance of the composite life, and, in a number of species, it has almost entirely superseded the somewhat uncertain method of spore germination inherited from the fungal ancestor, but which leaves more or less to chance the encounter with the algal symbiont.

From a phylogenetic point of view we should regard the sorediate lichens as the more highly evolved, and those which have no soredia as phylogenetically young, though, as Lindau[502] has pointed out, soredia are all comparatively recent. They probably did not appear until lichens had reached a more or less advanced stage of development, and, considering the polyphyletic origin of lichens, they must have arisen at more than one point, and probably at first in circumstances where the formation of apothecia was hindered by prolonged conditions of shade and moisture.

That soredia are ontogenetic in character, and not, as Nilson[503] has asserted, accidental products of excessively moist conditions is further proved by the non-sorediate character of those species of crustaceous lichens belonging to _Lecanora_, _Verrucaria_, etc. that are frequently immersed in water. Bitter[504] found that the soredia occurring on _Peltigera spuria_ were not formed on the lobes which were more constantly moist, nor at the edges where the cortex was thinnest: they always emerged on young parts of the thallus a short way back from the edge.

Bitter[504] points out that in extremely unfavourable circumstances—in the polluted atmosphere near towns, or in persistent shade—lichens, that would otherwise form a normal thallus, remain in a backward sorediose state. He considers, however, that many of these formless crusts are autonomous growths with specific morphological and chemical peculiarities. They hold these outposts of lichen vegetation and are not found growing in any other localities. The proof would be to transport them to more favourable conditions, and watch development.

4. ISIDIA

A. FORM AND STRUCTURE OF ISIDIA

Many lichens are rough and scabrous on the surface, with minute simple or divided coral-like outgrowths of the same texture as the underlying thallus, though sometimes they are darker in colour as in _Evernia furfuracea_. They always contain gonidia and are covered by a cortex continuous with that of the thallus.

This very marked condition was considered by Acharius[505] as of generic importance and the genus, _Isidium_, was established by him, with the diagnostic characters: “branchlets produced on the surface, or coralloid, simple and branched.” In the genus were included the more densely isidioid states of various crustaceous species such as _Isidium corallinum_ and _I. Westringii_, both of which are varieties of _Pertusariae_. Fries[506], with his accustomed insight, recognized them as only growth forms. The genus was however still accepted in English Floras[507] as late as 1833, though we find it dropped by Taylor[508] in the _Flora Hibernica_ a few years later.

The development of the isidial outgrowth has been described by Rosendahl[509] in several species of _Parmelia_. In one of them, _P. papulosa_, which has a cortical layer one cell thick, the isidium begins as a small swelling or wart on the upper surface of the thallus. At that stage the cells of the cortex have already lost their normal arrangement and show irregular division. They divide still further, as gonidia and hyphae push their way up. The full-grown isidia in this species are cylindrical or clavate, simple or branched. They are peculiar in that they bear laterally here and there minute rhizoids, a development not recorded in any other isidia. The inner tissue accords with that of the normal thallus and there is a clearly marked cortex, gonidial zone and pith. A somewhat analogous development takes place in the isidia of _Parmelia proboscidea_; in that lichen they are mostly prolonged into a dark-coloured cilium.

In _Parmelia scortea_ the cortex is several cells thick, and the outermost rows are compressed and dead in the older parts of the thallus; but here also the first appearance of the isidium is in the form of a minute wart. The lower layers (4 to 6) of living cortical cells divide actively; the gonidia also share in the new growth, and the protuberance thus formed pushes off the outer dead cortex and emerges as an isidium (Fig. 85). They are always rather stouter in form than those of _P. papulosa_ and may be simple or branched. The gonidia in this case do not form a definite zone, but are scattered through the pith of the isidium.

Here also should be included the coralloid branching isidia that adorn the upper surface and margins of the thallus of _Umbilicaria pustulata_. They begin as small tufts of somewhat cylindrical bodies, but they sometimes broaden out to almost leafy expansions with crisp edges. Most frequently they are situated on the bulging pustules where intercalary growth is active. Owing to their continued development on these areas, the tissue becomes slack, and the centre of the isidial tuft may fall out, leaving a hole in the thallus which becomes still more open by the tension of thalline expansion. New isidia sprout from the edges of the wound and the process may again be repeated. It has been asserted that these structures are only formed on injured parts of the thallus—something like gall-formations—but Bitter[510] has proved that the wound is first occasioned by the isidial growth weakening the thallus.

B. ORIGIN AND FUNCTION OF ISIDIA

Nilson[511] (later Kajanus[512]) insists that isidia and soredia are both products of excessive moisture. He argues that lichen species, in the course of their development, have become adapted to a certain degree of humidity, and, if the optimum is passed, the new conditions entail a change in the growth of the plant. The gonidia are stimulated to increased growth, and the mechanical pressure exerted by the multiplying cells either results in the emergence of isidial structures where the cortex is unbroken, or, if the cortex is weaker and easily bursts, in the formation of soralia.

This view can hardly be accepted; isidia as well as soredia are typical of certain species and are produced regularly and normally in ordinary conditions; both of them are often present on the same thallus. It is not denied, however, that their development in certain instances is furthered by increased shade or moisture. In _Evernia furfuracea_ isidia are more freely produced on the older more shaded parts of the thallus. Zopf[513] has described such an instance in _Evernia olivetorina_ (_E. furfuracea_), which grew in the high Alps on pine trees, and which was much more isidiose when it grew on the outer ends of the branches, where dew, rain or snow had more direct influence. He[514] quotes other examples occurring in forms of _E. furfuracea_ which grew on the branches of pines, larches, etc. in a damp locality in S. Tyrol. The thalli hung in great abundance on each side of the branches, and were invariably more isidiose near the tips, because evidently the water or snow trickled down and was retained longer there than at the base.

Bitter[515] has given a striking instance of shade influence in _Umbilicaria_. He found that some boulders on which the lichen grew freely had become covered over with fallen pine needles. The result was at first an enormous increase of the coralline isidia, though finally the lichen was killed by the want of light.

Isidia are primarily of service to the plant in increasing the assimilating surface. Occasionally they grow out into new thallus lobes. The more slender are easily rubbed off, and, when scattered, become efficient organs of propagation. This view of their function is emphasized by Bitter who points out that both in _Evernia furfuracea_ and in _Umbilicaria pustulata_ other organs of reproduction are rare or absent. Zopf[513] found new plants of _Evernia furfuracea_ beginning to grow on the trunk of a tree lower down than an old isidiose specimen. They had developed from isidia which had been detached and washed down by rain.

VI. HYMENOLICHENS

A. SUPPOSED AFFINITY WITH OTHER PLANTS

Lichens in which the fungal elements belong to the Hymenomycetes are confined to three tropical genera. They are associated with blue-green algae and are most nearly related to the Thelephoraceae among fungi. The spores are borne, as in that family, on basidia.

The best known Hymenolichen, _Cora Pavonia_ (Fig. 86), was discovered by Swartz[516] during his travels in the W. Indies (1785-87) growing on trees in the mountains of Jamaica, and the new plant was recorded by him as _Ulva montana_. Gmelin[517] also included it in _Ulva_ in close association with _Ulva_ (_Padina_) _Pavonia_, but that classification was shortly after disputed by Woodward[518] who thought its affinity was more nearly with the fungi and suggested that it should be made the type of a new genus near to _Boletus_ (_Polystictus_) _versicolor_. Fries[519] in due time made the new genus _Cora_, though he included it among algae; finally Nylander[520] established the lichenoid character of the thallus and transferred it to the Lecanorei.

It was made the subject of more exact investigation by Mattirolo[521] who recognized its affinity with _Thelephora_, a genus of Hymenomycetes. Later Johow[522] went to the West Indies and studied the Hymenolichens in their native home. The genera and species described by Johow have been reduced to _Cora_ and _Dictyonema_; a new genus _Corella_ has since been added by Wainio[523].

Johow found that _Cora_ grew on the mountains usually from 1000 to 2000 ft. above sea-level. As it requires for its development a cool damp climate with strong though indirect illumination, it is found neither in sunny situations nor in the depths of dark woods. It grows most freely in diffuse light, on the lower trunks and branches of trees in open situations, but high up on the stem where the vegetation is more dense. It stands out from the tree like a small thin bracket fungus, one specimen placed above another, with a dimidiate growth similar to that of _Polystictus versicolor_. Both surfaces are marked by concentric zones which give it an appearance somewhat like _Padina Pavonia_. These zones indicate unequal intercalary growth both above and below. The whole plant is blue-green when wet, greyish-white when dry, and of a thin membranaceous consistency.

B. STRUCTURE OF THALLUS

There is no proper cortex in any of the genera, but in _Cora_ there is a fastigiate branching of the hyphae in parallel lines towards the upper surface; just at the outside they turn and lie in a horizontal direction, and, as the branching becomes more profuse, a rather compact cover is formed. The gonidia, which consist of blue-green _Chroococcus_ cells, lie at the base of the upward branches and they are surrounded with thin-walled short-celled hyphae closely interwoven into a kind of cellular tissue. The medulla of loose hyphae passes over to the lower cortex, also of more or less loose filaments. The outermost cells of the latter very frequently grow out into short jagged or crenate processes (Fig. 87).

In _Corella_, the mature lichen is squamulose or consists of small lobes; in _Dictyonema_ there is a rather flat dimidiate expansion; in both the alga is _Scytonema_, the trichomes of which largely retain their form and are surrounded by parallel growths of branching hyphae. The whole tissue is loose and spongy.

_Corella_ spreads over soil on a white hypothallus without rhizinae. In the other two genera which live on soil, or more frequently on trees, there is a rather extensive formation of hold-fast tissue. When the dimidiate thallus grows on a rough bark, rhizoidal strands of hyphae travel over it and penetrate between the cracks; if the bark is smooth, there is a more continuous weft of hyphae. In both cases a spongy cushion of filamentous tissue develops at the base of the lichen between the tree and the bracket thallus. There is also in both genera an encrusting form which Johow regarded as representing a distinct genus _Laudatea_, but which Möller found to be merely a growth stage. Möller[524] judged from that and from other characteristics that the same fungus enters into the composition of both _Cora_ and _Dictyonema_ and that only the algal constituents are different.

C. SPORIFEROUS TISSUES

As in Hymenomycetes, the spores of Hymenolichens are exogenous, and are borne at the tips of basidia which in these lichens are produced on the under surface of the thallus. In _Cora_ the fertile filaments may form a continuous series of basidia over the surface, but generally they grow out in separate though crowded tufts. As these tufts broaden outwards, they tend to unite at the free edges, and may finally present a continuous hymenial layer. Each basidium bears four sterigmata and spores (Fig. 87 _e_); paraphyses exactly similar to the basidia are abundant in the hymenium. In _Dictyonema_ the hymenium is less regular, but otherwise it resembles that of _Cora_. No hymenium has as yet been observed in _Corella_; it includes, so far as known, one species, _C. brasiliensis_, which spreads over soil or rocks.