Chapter XI: Part 11

Thus, if anthocyanin is present, together with a small amount of acid, the leaves are turned violet, as in the case of the autumn leaves of the Dogwood and the Spindle Tree; or purple, like those of the Service Tree. A larger proportion of acid produces, with the anthocyanin, the brownish green tint of the Alder leaves; or the brownish yellow of the Oak; while still larger proportions will turn the anthocyanin yellow, orange, red, or scarlet, according to the quantity in which the latter is present. Thus we can account for the rich yellow of the Maple in autumn, the orange of the Aspen leaves, the beautiful scarlet tints of the Mountain Ash and the Barberry, and the grand display of varied colours exhibited by the autumn Beeches.

Again, before the leaves are shed, the buds that are destined to produce the new branches of the following spring are already formed. These may be seen on all deciduous trees and shrubs, some of them in the axils of the leaves, and others at the tips of the present twigs. Each bud is the embryo branch of the following year. Some of them are destined to produce leafy branches only; some are to develop into branches bearing both floral leaves and flowers; while others are to produce flowers without floral leaves; and it is interesting to note that, even at this stage, sections of the buds, examined with the aid of a microscope, will reveal the future leaves and flowers compactly concealed within their scaly, protective coverings.

In October we may see the well-formed catkins of the Birch that are to bloom in the following April, in company with the ripe fruiting catkins of the present year. The Alder also bears its catkins that are to flower five months later, together with the woody remains of the female catkins of the previous spring; and the Hazel may be seen with its ripe nuts and its future flowers both on the same twig.

The leaves, having manufactured the materials necessary for the formation of the buds that are to produce the leaves and flowers of the following year, and then transferred their remaining store of nutrient matter to a suitable storehouse for the winter, are now practically empty and lifeless. Had they remained alive and active, they would have endangered the life of the tree by giving off more moisture than could be replaced by the inactive roots. In their present, lifeless condition they are useless to the tree; but by falling to the ground, and decomposing where they lie, they improve the soil by the addition of organic matter as well as of the mineral salts they contained.

In countries where a moderate temperature is maintained throughout the year, the growth of plants and trees goes on without interruption, and the fall of the leaf is hardly noticeable; for the older leaves die and fall one by one, as they become incapable of performing their functions for want of light, and new ones are being continuously formed close to the tips of the twigs. But where the growth is interrupted, either in hot countries during periods of drought, or in temperate countries by the approach of a cold season, the whole of the foliage is shed within a short period, and new leaves as suddenly appear when favourable conditions return.

In our own latitudes, as we all know, the defoliation of the trees is caused by the approach of cold weather, which decreases the activity of the roots, so that the leaves become dry and lifeless. It is very commonly supposed that the fall of the leaf is caused by frost; but this is not the case. The leaves are shed during the cool days of autumn, even though the temperature does not fall to freezing point; but it is equally certain that the leaf-fall is accelerated by the frost when it comes, for the little moisture remaining in the leaves is then frozen, rendering the structures so brittle that they are easily snapped by the wind.

The real cause of the rupture of the leaf is the formation of what is called the '_separation layer_.' This consists of soft, succulent cells, really in several layers, which are formed across the leaf-stalk, usually at the base, where the bundles of vessels passing from the twig to the leaf are narrower. The walls of these cells are thin, and are easily separated; and as they extend inwards from the surface all round, they break through the old cells, thus weakening the junction. When the growth of the separation layer is complete, it requires very little force to break off the leaf, and the process is aided by the formation of certain organic acids which act on the cell-walls, causing them to dissolve; and when the leaf has finally separated from the twig, it will be found that the scar left is a clean-cut surface, such as would be produced by the incision of a sharp knife.

The recognition of the above facts introduces to us a difficulty for which we can find no explanation:--If the leaf-fall is not caused by frost, but by certain structural alterations that take place in the tree itself, how are we to account for the fact that the tree produces the changes which are necessary for its own preservation every year, just at the proper season? Plants and trees do not foresee the coming period of cold weather that necessitates the performance of the functions which they execute, and yet they instinctively prepare for the winter in the manner described above.

Our autumn observations teach us that there are interesting differences in the times and progress of leaf-fall of different species of trees, and also of trees of the same species when exposed to different external conditions. On open ground, where the trees are fully exposed both to the sun's rays and to the cool autumn breezes, the leaves lose their moisture and fall earlier than would the same species in more sheltered situations; and they retain their moisture and position latest in damp, shady woods. On high hills, where the exposure is extreme, the leaves, which, by the way, do not appear till late in the spring, fall early on account of the low temperature, and consequent decrease of root activity, in the autumn.

Further, we note that while in some trees, such as the Ash, Hornbeam, Beech and Hazel, the leaves fall first at the tips of the branches, and the defoliation extends fairly regularly towards the trunk, in other species, including Willows, Poplars, and the Lime, the branches become bare first at their bases, and finally at their tips.

Even during the depths of winter we may see a number of dead leaves still attached to the twigs of certain trees, notably the Oak and the Beech; but where we find practically all the foliage remaining on the tree or on special branches of a tree, we may generally assume that the tree, or the branches in question, are dead--that they died during the summer, before the separation layers of the leaves had been formed. We can also understand, from what has been said, why the dead leaves remain attached to a cut branch, and yet fall from the living tree from which it was severed.

In our own country some plants and trees retain their leaves throughout the year, so that we speak of them as evergreens. Many of these include herbaceous plants of a hardy nature, some of which remain fresh and green even in exposed situations, while others grow in more sheltered places. In either case they are plants whose roots remain more or less active in the cold season; and some of them, especially the evergreen shrubs, have rather thick leaves which contain a considerable quantity of sap, and which are surrounded by an outer covering or epidermis that does not allow the water within to pass out so readily as in the case of the deciduous leaves.

In addition to the observations previously mentioned, we should do well, at this season of the year, to study the autumn fruits of our trees and shrubs, most of which still remain attached to the twigs.

Some of these fruits lose most of their moisture as they ripen, thus becoming very light, and are provided with wings that cause them to be dispersed more or less by the wind.

The so-called 'keys' of the Ash are one-seeded fruits, extended at the end into a long, narrow wing with a slight twist. As a result of this peculiarity they usually fall less rapidly to the ground, spinning as they descend, and are thus carried farther than they otherwise would be by the wind. The fruits of the Sycamore and the Maple are somewhat similarly winged, and each of these consist of two carpels which separate sooner or later--generally after they have reached the ground.

On the Birch trees we may now see the ripe female catkins, consisting of hundreds of minute fruits, closely packed together, each provided with a wing on either side. They are very light, and easily blown a considerable distance by the wind; and late in the autumn we may observe the stalks of the catkins, from which some of the fruits have been blown, still on the trees.

The wings that thus aid in the dispersion of fruits are not always part of the fruit itself. In the Hornbeam it is a three-lobed, persistent bract that performs this function; and the fruits of the Lime are also blown away by the aid of a large bract from the middle of which the fruit-stalk projects.

Some of our trees present a glorious aspect during the autumn months, displaying conspicuous and more or less brightly-coloured fruits in combination with the varied autumn tints of their leaves. The red foliage of the Mountain Ash or Rowan is accompanied by the still brighter clusters of scarlet fruits--little apple-like pomes, about the size of holly 'berries'; and the Wayfaring Tree bears pretty clusters of flattened, oval, one-seeded berries which are first red, and then nearly black. The Guelder Rose, while still in full leaf, is often very heavily laden with its bright red, semi-transparent berries; and the violet foliage of the Dogwood is intermingled with clusters of little berry-like drupes which, at first green, have now changed to a rich purple-black. Then there is the Spindle Tree, with its pretty red lobed capsules which split, when ripe, at its angles, disclosing as many cells as there are lobes (usually four), each with a single seed enclosed in an orange jacket. Occasionally we meet with the Strawberry Tree, during early autumn, bearing both flower and fruit at the same time. This tree flowers in September and October, but the fruits which accompany the flowers are those of the previous year, for they require more than twelve months to come to maturity. The fruit is a large berry, of an orange-red colour, with a granulated surface that gives it somewhat the appearance of the strawberry. It should be mentioned that the Strawberry Tree is not indigenous to England, and is seldom seen outside parks and gardens; but it grows wild in Ireland, and is very abundant round Killarney and in other parts.

In conclusion, we must note one autumn flower of the woods which is exceedingly common--that of the Ivy (_Hedera Helix_), belonging to the order _Araliaceæ_. The Ivy is an evergreen climber, fixing itself by means of little rootlike suckers attached to the main stem and its branches, while the lower branches trail along the ground. The leaves are thick and glossy, usually of a deep green colour, but often beautifully variegated. Those attached to the trailing and climbing stems have three or five lobes, are always turned with one surface towards the light, and are so arranged as to obtain the maximum of light, the less exposed leaves below catching the rays which pass between the lobes of those which are more favourably situated.

The branches of the tree do not, as a rule, produce flowers as long as they are able to climb; but as soon as they reach the summit of the tree or wall to which they cling, or reach a situation where there is a sufficient abundance of light and air, they change their character in a remarkable way. They now become bushy, cease to produce suckers, and give rise to undivided leaves that turn in all directions for light and air. At the tip of each twig is formed a cluster of yellowish-green flowers, arranged in a short raceme or in an umbel. These flowers have an inconspicuous calyx which forms a border round the middle of the ovary, and five short petals. There are also five stamens, and united styles. The fruit is a smooth, black berry, containing from two to five seeds.

XXIII

PARASITIC PLANTS

A number of plants extract more or less of the organic material they require from other plants, and thus save themselves the labour of building up this material themselves. These are termed parasites; but we must be careful to distinguish between them and certain other plants which, though apparently parasitic, are not really so. One plant may climb on another, perhaps even producing "rootlets" by which it clings to its living support, and yet it may not be a parasite in the proper sense of the term, for it may not absorb the slightest amount of nutritious matter except from the soil and the air. It is not at all uncommon for the Honeysuckle to twine its stems round the trunk and branches of a young tree, with the result that the tree becomes stunted, and assumes a starved appearance, especially in its lower parts; and yet the Honeysuckle is not a parasite. It has withdrawn nothing from the tree which supports it, but has coiled itself so tightly round it as to interfere with the circulation of its sap. The lower part of the tree is especially affected because the strangulating coils of the climber prevent the downward flow of the sap contained in the vessels of the bast or inner bark, and this is the sap which holds the constructive materials that have been built up in the leaves, under the influence of light.

Many of the parasitic plants are of microscopic dimensions, and others are larger species belonging to the Fungi or Mushroom group. Some, however, are flowering plants, and these only fall within the scope of our work.

We shall first deal with parasites which have no green leaves or chlorophyll, and are therefore entirely dependent on outside sources for their supply of organic material, starting with the interesting Dodders (_Cuscuta_), which coil themselves round herbs, shrubs, or even trees, and produce sucking organs on their stems that come in contact with their host.

These are all smooth plants, with globular clusters of yellowish-pink flowers, the calyx being of the same colour as the corolla. The former is deeply divided into four or five parts, and the corolla has four or five spreading lobes with as many scales inside its broad tube. The ovary has two distinct styles, and the fruit is a globular capsule. The following summary of distinguishing features will enable the reader to identify the British species of the genus:--

1. The Greater Dodder (_Cuscuta europæa_).--A plant of a greenish yellow colour, generally more or less tinged with red, with flowers in sessile, globular clusters nearly half an inch in diameter, each individual flower being about a tenth of an inch. This species is not abundant. It may be met with in hop-fields, and is also parasitic on nettles, various shrubs, and trees, including the elder and the ash.

2. The Flax Dodder (_C. Epilinum_).--Very much like _C. europæa_, but the flowers are fewer in number, larger, and more fleshy. The calyx is nearly as long as the corolla, with sharply-pointed segments; and the corolla tube is always globular. This species is not indigenous, but is sometimes met with in flax-fields.

3. The Lesser Dodder (_C. Epithymum_).--A more slender plant, with thread-like stems, and flowers in small, compact, globular heads, with red calyx and cylindrical corolla. This species occurs principally on sunny heaths, where it is parasitic on shrubby plants, such as thyme and ling. It is much more common than the foregoing.

4. The Clover Dodder (_C. Trifolii_).--Very much like the Lesser Dodder, of which it is sometimes regarded as a variety. Its calyx is of a very pale colour, and is almost as long as the tube of the corolla, which is cylindrical in form. It is rare, but sometimes appears in undesirable numbers in clover fields.

All the species produce their flowers in August and September, but _C. europæa_ may often be seen in bloom very early in July.

The seeds of the Dodder fall from the opened capsules during late summer and early autumn, alighting on the soil, or on the decomposing foliage that covers the ground, or on the rough barks of the tree that served as a host for the parasitic plant. The seeds of many other plants fall about the same time, but those of the Dodder do not begin to germinate until about a month later than the majority of these, in the following season, and consequently the young Dodder plants do not appear before their future hosts have had time to grow sufficiently large to support and nourish them. Perennial plants, too, which are attacked by the Dodder, have also produced strong shoots and leaves from their roots or underground stems by the time that the parasite begins its search for ready-made organic food; and it is clear that if the Dodder seeds germinated earlier in the season, the young plants would starve for want of suitable herbs to give them support and nourishment.

When the seed germinates it sends out a filament which penetrates into the soil and fixes the seedling firmly. The other end grows upward, carrying up with it a little swollen mass of food-reserve, sufficient to support the growing seedling until it has had some chance of reaching a suitable host. The upper end of the seedling now sends out a filament which rapidly elongates, and, growing upward, searches for some stem on which to climb.

All this time the little mass of food-reserve is being rapidly exhausted, and if the young seedling fails to reach a suitable plant on which to climb it soon dies, for its lower extremity is unable to absorb sufficient food material from the soil; and the plant itself, having no chlorophyll, cannot decompose carbonic acid gas and build up organic material to add to its substance.

Again, should the young plant fail to reach a favourable support, so that it is of necessity compelled to trail along the ground, the filaments which would soon produce suckers when attached to a living plant have no power to form any structures capable of extracting food material from a damp soil.

Circumstances being more favourable, however, the upper filament eventually finds a stem, and immediately begins to twine itself round it, making a few close coils in a clockwise direction. Should the support prove to be a dead stem, little wartlike swellings are produced at points where the two touch, and these serve as a means of attachment for the climbing filament, but no suckers are formed. If, however, the filament surrounds a living stem, each of the swellings gives rise to suckers that penetrate into the tissues of the latter, and withdraw the organic food necessary for the continued existence of the plant.

The Dodder now grows rapidly, giving off branches which search in all directions for additional supports, sometimes climbing from one plant to another, and producing new suckers whenever a favourable situation has been reached. The plant has now all it requires both in the way of mechanical support and nourishment, and its lower part, thus rendered useless, soon withers, breaking all connexion with the soil on which the seed originally germinated. New branches continue to form, each one producing additional suckers for the extraction of food from the host or hosts, until a tangled mass of clinging stems is the result. Then the globular clusters of little flowers appear, followed by balls of small capsules which throw off their lids when ripe, allowing the seeds to be shaken out by the wind. The Dodder plant now withers, leaving, in the autumn, its dead tangles of climbing filaments still attached to the withered herbs on which it fed, or to the branches of the tree which served as its host.

Other parasitic plants possessing no chlorophyll, and therefore incapable of building up organic compounds for themselves, derive their food from the roots of trees and shrubs.

Among these is the Toothwort (_Lathræa_), which is carnivorous as well as parasitic, and is described in our chapter (XXIV) dealing with carnivorous plants, so that we need only refer here to its habit as a parasite.

The seed of this plant germinates on the damp ground to which it falls in early summer. The young root penetrates into the soil, deriving its nourishment entirely from the food reserve that was stored up in the seed, and soon sends out lateral branches in search of the roots of a suitable host. If it fails to attain this end by the time that the reserve is exhausted, it dies; but if it succeeds in reaching the root of an Elm, Hazel, Hornbeam, Ash, Poplar, or other tree, it fastens itself to it, and develops suckers which penetrate into the substance of the root to extract its sap. The parasite now grows very rapidly, producing its underground stems, with their fleshy, overlapping scales, as described on p. 352.

The Broomrapes of the same order (_Orobanchaceæ_) are very similar in their parasitic habits to the Toothwort, and, like the latter, they possess no chlorophyll. The seeds germinate on the damp soil, producing a long, narrow embryo that grows downward into the ground until it reaches the root of some herb or shrub. It then gives off suckers which penetrate into the root, and, with the aid of the organic food thus obtained, forms a tuberous swelling on its surface. Flowering stems are afterwards produced, and these, rising above the soil, bear terminal spikes of lipped flowers, followed by capsules containing many seeds.

There are several British species of this genus (_Orobanche_), and their flowering stems, which are usually unbranched, produce scale-like leaves of the same colour as themselves. Each flower of the spike is in the axil of a bract resembling the scales of the lower part of the stem; and in some species there is a pair of smaller bracts close to the base of the calyx. The corolla is either tubular or bell-shaped, and more or less distinctly lipped. Each flower has four stamens, arranged in pairs, and a two-lobed stigma. The following outline of leading features will serve for the identification of the common Broomrapes:--

1. The Great Broomrape (_O. Rapum_).--A plant from twelve to eighteen inches high, of a pale yellow colour at first, but afterwards turning to a dull purple brown. Stem thick, especially below, and unbranched. Scales lanceolate. Flowers sessile, whitish, with only one bract, forming a spike from six to nine inches long. This species is moderately common, and is parasitic on the roots of Furze and Broom. Time of flowering--May to July.

2. The Clove Broomrape (_O. caryophyllacea_).--Very similar to the Great Broomrape in colour, but usually smaller, and easily distinguished by the sweet clove-like scent of its flowers. Spike not so dense as in the last species, and the corolla tube not so broad. The plant is not uncommon in the southern counties of England. It is parasitic on the roots of the Great Hedge Bedstraw, and flowers from May to July.

3. The Tall Broomrape (_O. elatior_).--Also much like the Great Broomrape, of which it is perhaps a variety. It retains its original yellowish colour for a longer period, and is parasitic on the Great Knapweed, flowering from June to August.

4. The Least Broomrape (_O. minor_).--A yellow or pale brown plant, from six inches to over a foot in height, more slender than the preceding species, with smaller flowers. The flowers are whitish, but more or less tinged with purple, and bloom from June to October. It is parasitic on a number of different plants, including the Ivy, Clovers, Hawkweed, Wild Carrot, &c., and is found in many districts in South and Central England.

* * * * *

We have now to consider those parasites which bear leaves possessing chlorophyll granules, and are therefore able to build up a portion of the organic compounds necessary for their development. Most of these, at least as far as the British flowering species are concerned, have also true roots which grow into the soil and absorb mineral food, like those of the non-parasitic plants allied to them, so that it is difficult to understand why they should require the additional nourishment stolen from the roots of neighbouring plants. One, however, the well-known Mistletoe, grows on trees at a distance from the ground, and therefore obtains the whole of its food, with the exception of carbonic acid gas, direct from its host.

This plant--the Mistletoe (_Viscum album_), of the order _Loranthaceæ_--is attached to the tree on which it grows by a thick stem that becomes woody when old. Its branches are of a yellowish-green colour, and are repeatedly forked in such a manner as to form a dense tuft that often reaches a diameter of two feet or more. The leaves are of the same colour as the branches, and are rather thick and fleshy. The flowers grow in the forks of the branches, on very short stalks, and are imperfect, the males and females being on separate plants. The former are in clusters of about three or four, in a cuplike, fleshy bract, each flower having four thick, triangular petals with an anther on the middle. The females are either solitary or in clusters of two or three, with a similar bract, and very small petals. The fruit is a white, glutinous berry, almost transparent, with only one seed.

The Mistletoe grows on a variety of trees, including the Apple, Pear, Black Poplar, and Oak; and thrives most luxuriantly on those which have a soft tissue beneath the bark. It is found principally in the southern and western counties of England, and flowers from March to May.

There is no doubt but that the seeds of the Mistletoe are distributed from tree to tree by the agency of birds, especially the thrushes, which devour the berries in large numbers. The seed of the berry is protected by a covering which remains quite untouched by the digestive fluids of the bird, and consequently it is expelled intact with the excrement, and frequently drops to a branch of the tree, where it lodges in a crevice of the bark, and is securely fixed in its place by the slimy excrement in which it is embedded.

Here the seed germinates, sending out a little rootlet that always turns towards the bark on which it rests, and subsists for a time on the food-reserve that it contains. When the young root reaches the bark it becomes flattened against the surface, and spreads out, forming a disc that holds the seedling firmly to the tree.

A projection (the _sinker_) is then sent inwards from the disc, and this penetrates the bark, reaching the wood beneath, but does not enter the latter. This terminates the growth of the seedling for the first year, but as soon as the warm weather of the following spring commences, the sinker begins to spread over the surface of the outer ring of wood, while at the same time a new annual ring of wood begins to form outside, thus surrounding and banking in the sinker. It would appear, on making a section of the tree, as if the sinker had actually pushed its growth through the outer ring of wood, whereas it does not penetrate the wood at all, but is only banked up by the new wood that grows round it. This is repeated year by year, until the sinker is at last quite deeply set in the branch, being surrounded by the wood of several annual rings.

During the second year's growth the sinker sends out little roots which run up and down the stem, beneath the bark, and these give rise to new sinkers that grow down to the surface of the wood, and become, in turn, embedded in the new layers of wood that form round them. And while the young Mistletoe plant is thus securing a firm hold on its host, and withdrawing ready-made organic compounds from its sap, the outer green stem develops, and soon gives rise to the first pair of leaves.

If food is obtained in abundance, as is the case when the host is a tree of a soft and sappy nature, the growth is rather rapid; but otherwise the development is comparatively slow. In any case the age of the parasite may be ascertained by counting the number of annual rings of wood that lie outside the deepest sinker; and by this means it has been found that the Mistletoe may attain an age of over thirty years.

We have now to consider a group of plants, the parasitic habits of which would scarcely be suspected by an ordinary observer. They are green plants, with well-developed foliage leaves, and true roots which absorb mineral food from the soil. Their seedlings grow in the same way as those of non-parasitic species, deriving no nourishment from neighbouring plants, but obtaining all their food from the air and the soil, and building up all the organic compounds required for their growth by the agency of their own chlorophyll.

It is difficult to understand why these plants should afterwards produce suckers on their roots in order to obtain nourishment from other species, but they do this, and experiments have proved that the food thus obtained is more or less essential to their development. Some of them die while still young if grown apart from other species, and the others, under similar conditions, though they reach what we may term the adult stage, remain somewhat weak and stunted, and produce but few flowers and fruits.

Most of the plants referred to belong to the order _Scrophulariaceæ_, and among them we may mention the Eyebright (_Euphrasia_), the Yellow Rattle (_Rhinanthus_), the Cow-wheat (_Melampyrum_), and the Lousewort (_Pedicularis_). They generally appear in large numbers close together, often in such abundance as to determine the general colour of the ground on which they grow, and yet they do not apparently cause much damage to the grass and other plants which they rob.

These green parasites are described in various chapters, according to their habitats and their flowering seasons; so we shall do no more here than to briefly refer to their parasitic habits.

The Eyebright (p. 274) grows on heaths and downs, where it derives organic food from the roots of the neighbouring grasses. The Lousewort, too (p. 118), which grows in marshes and moist meadows, is parasitic principally on the roots of grasses, apparently without affecting the latter. The last-named species is a perennial, the roots of which have to find hosts that are capable of supporting it year by year. If the host of the present year should happen to die in the autumn, the suckers that were attached to its roots soon die, and the parasite has to seek a new source of supply. This it does by extending its roots until it reaches a new host, and then producing new suckers. Thus we are able to understand the origin of the long roots so often seen on the Lousewort, and also the reason why these roots never grow downwards into the soil, but always horizontally, just beneath the surface. Further, since the roots extend themselves in search of food at times when the supply is temporarily diminished or stopped, it is clear that some reserve is necessary for the elongation referred to. Such a reserve exists in the older, thick portion of the perennial root, near the base of the stem.

In the case of the Cow-wheat (p. 146) no suckers are produced until the lateral branches of the root of the seedling reach a moderate length; but in order to increase the chances of finding a suitable host these branches are developed in large numbers, and extend themselves in all directions. The suckers produced on them cling very firmly to the root-fibres of the host, which they almost completely embrace.

The suckers of the Yellow Rattle (p. 118) are globular, often nearly one-eighth of an inch in diameter, and partly surround the root-fibres of the plants to which they are attached.

XXIV

CARNIVOROUS PLANTS

Quite a number of plants, belonging to different orders, are provided with the means of capturing small animals, and of digesting their prey and absorbing the nutrient matter thus obtained into their own systems. In this way they are enabled to obtain nitrogenous material which, in the ordinary way, is absorbed in the form of mineral solutions, from the soil, by the agency of the roots. The greater number of these carnivorous plants are to be found in tropical lands; but a few are British, and are of such an interesting nature that we propose to devote a short chapter to a description of their peculiar structure and habits.

The plants to which we refer are often spoken of as insectivorous species; but although in nearly all cases the animal food consists almost entirely of insects, it is not entirely derived from this one group of animal life, and therefore the term carnivorous is rather more appropriate.

In pools we sometimes meet with floating plants that have no true roots, at least at the time of flowering, but consist of a tuft of long, rootlike, submerged branches, bearing much-divided leaves, and sending leafless stalks of yellow flowers above the surface of the water. These plants are the Bladderworts (_Utricularia_), of the order _Lentibulaceæ_, and are so called because they have little air-bladders either attached to the leaves or supported on leafless branches.

The leaves are divided into numerous very narrow segments, thus presenting a proportionately large amount of surface to the water for the absorption of dissolved gases required by the plant; and the flowers consist of a deeply two-lobed calyx; a spurred corolla, with its mouth closed or nearly closed by means of a convex 'palate'; two stamens; and a one-celled ovary that ripens into a globular fruit.

As to the little air-bladders mentioned above, they form, perhaps, the most interesting feature of the plant, for they are the traps by means of which small aquatic creatures are caught, and also the organs concerned in the absorption of nutritive products derived from the prey. Each bladder has an opening, guarded by a kind of valve which allows easy ingress, but no exit. It does not seem to produce any secretion which would hasten the death of the creatures entrapped, nor does it appear to produce any kind of digestive fluid, as is the case with other carnivorous plants; but small aquatic creatures, such as water-fleas, cyclops, very small larvæ, &c., entering the bladders for shelter or some other purpose, are securely imprisoned until they die of starvation or suffocation; and their bodies then decay, giving rise to soluble gases and other products which are absorbed into the plant by special cells within the bladder.

There are three British species of these plants--the Greater, the Lesser, and the Intermediate Bladder-worts. The first of these--_Utricularia vulgaris_--is rather local in its distribution, and is easily distinguished from the other two by its superior size, having floating branches from a few inches to a foot in length. The second (_U. minor_) is much more common. Its floating branches are only two or three inches long at the time of flowering, but they grow longer after; and the flowers are pale yellow, with a short, broad spur. The third (_U. intermedia_), which is very local, has also pale yellow flowers, but with a much longer spur; and the bladders are at the ends of leafless branches.

In the preceding chapter we gave an account of certain plants which are parasitic on other plants and trees, deriving more or less of their nutriment from their vegetable hosts. One of these--the Tooth-wort (_Lathræa squamaria_), of the order _Orobanchaceæ_--is not only a parasite, deriving nourishment from the roots of trees, but is also a carnivorous species, feeding on minute animals which are captured and digested by its peculiar leaves; and therefore it may be conveniently considered here.

The whole plant is of a fleshy character, and lives entirely underground, attached to the roots of the Hazel, Elm, or other tree, except during April and May, when it sends up thick flowering stems, from four to ten inches high, bearing a few broad, fleshy scales which gradually pass into bracts, and a one-sided spike or raceme of flowers. The stem and scales above ground are of a pale rose colour, and the flowers are either brown, flesh-colour or slightly bluish. The latter are numerous, closely placed, and either sessile or shortly stalked. The calyx is bell-shaped, nearly half an inch long, with four broad lobes; and the corolla, which is about half as long again as the calyx, is distinctly lipped.

The whole plant is devoid of chlorophyll, and consequently has not the power of building up organic compounds after the manner of green plants; and, being parasitic on the roots of trees, it derives but little organic material from its host. To compensate for this the underground portion is so constructed that it can capture minute animals which exist in the soil, and has the power of digesting them and of absorbing the products of digestion.

The underground stems are quite white, and are thickly covered with broad, cordate, fleshy leaves that closely overlap one another. There appears to be nothing very remarkable in these underground leaves until one has been removed from the stem and closely examined; and then we find that what appears to be the apex of the leaf is really its middle; and that what seems to be, at first sight, the under surface, is really an extension of the upper side; for the leaf is bent backwards in such a manner as to bring its apex close to the stem, immediately below its base. This peculiar folding of the leaf results in the formation of an irregular cavity, and the tip of the leaf, brought close to its base, is curled upward, close to the stem, in such a way as to form a little canal, with several small openings by which the cavity may be reached. It will not be easy to make out this strange folding of the leaf by an examination of the exterior only, but a longitudinal section, made with a sharp knife or razor, will show it clearly.

When minute animals enter the cavity of the leaf through the little openings above mentioned, they are seized by means of small filaments that protrude from the lining cells; and although no special digestive secretion has been discovered in the leaves, it appears certain that the creatures entrapped are really dissolved, for nothing remains of them after a time except the harder, indigestible portions. Also, there is every reason to believe that the products of digestion are absorbed, probably by the same filaments that are concerned in the capture of the microscopic prey.

Perhaps the most interesting of the carnivorous plants are those which exhibit distinct movements in connexion with the capture of their prey, and among these are the British Butterworts and Sundews, which grow in bogs and other wet places.

There are three British species of Butterwort (_Pinguicula_), similar in structure and habit, all growing in bogs and on wet rocks. They have each a rosette of entire, radical leaves, the lowest of which lie close against the soil or rock on which the plant grows; and violet or yellow flowers on leafless peduncles. The calyx has four or five teeth, arranged in two lips; and the corolla, which is also lipped, has a broad, open throat, and a spur.

The commonest species is the Common Butterwort (_P. vulgaris_), which is found in bogs and wet places, principally in the hilly, humid districts of the West of Britain and Ireland, flowering from May to July. Its leaves are succulent and clammy, of a pale green colour, and covered all over with little glistening spots. The flower stems are three or four inches high, each bearing a single violet flower. In this species the throat of the corolla is bell-shaped, and the spur is as long as the rest of the corolla.

A second species--the Alpine Butterwort (_P. alpina_)--with smaller, pale yellow flowers appearing in June and July, is found only in Scotland; while a third, known as the Pale Butterwort (_P. lusitanica_), also with pale yellow flowers, and a curved spur, occurs in South-West England as well as in the boggy districts of Ireland and the West of Scotland, flowering from June to October.

The carnivorous habits of all species are the same. The horizontal leaves lie flat on the wet soil, with their margins turned upward forming a kind of shallow trough; and the upper surface of each is dotted with many hundreds of minute glands which secrete a colourless, sticky fluid, thus giving to the leaf its glistening and clammy appearance.

If any mineral or other non-nutritious substance be placed on a leaf, the contact stimulates the little glands, causing them to discharge a larger quantity of fluid, but no change seems to take place in the character or composition of the secretion. But if any nitrogenous organic substance, such as an insect or a small piece of meat, be brought in contact with the glands, not only will the secretion increase in quantity, but it will also assume an acid character, and contain a ferment which is capable of digesting the nitrogenous material. In fact, the secretion produced under these circumstances possesses the same properties as the gastric fluid of the stomachs of animals.

The animal food of the Butterworts consists of small insects and other little creatures. If an insect alights on the leaf, it is caught by the sticky secretion of the glands, and every effort to escape causes it to become more and more besmeared with the mucilage, till, at last, it is no longer able to move; and its death is probably hastened by the stoppage of its spiracles or breathing-holes.

If the insect is a small one, and it settles near the edge of the leaf, the curved margin slowly bends over it until it is more or less enclosed, and the larger number of glands thus brought in contact with its body pour out their digestive secretion, which slowly dissolves the nourishing portions, leaving nothing but the legs, wings, and other indigestible parts. A larger insect, alighting similarly near the edge of the leaf, could hardly be enclosed by the bending of the margin near it; but it is pushed towards the middle as the edge curls over, and then the opposite side also bends over it, till the insect is more or less enclosed, when it is digested as mentioned above.

The digestion of an insect and the absorption of nutrient matter by the cells of the leaf occupy from twenty to thirty hours, and when the whole is accomplished the leaf slowly expands, assuming its normal position, and exposing the indigestible residue of its prey to be blown away or washed off by the rain.

It has been observed that the Butterworts are not exclusively animal feeders, for their leaves readily digest any pollen cells or the spores of the lower plants that are carried to them by the wind.

Equally interesting are the habits of the Sundew (_Drosera_), of which there are three species, all readily distinguished from every other British plant by the glandular hairs that cover the long-stalked, radical leaves. They have leafless flower-stalks, each bearing a one-sided spike or raceme of white flowers. The sepals, petals, and stamens each number five; and the ovary, which ripens into a one-celled capsule of three or four valves, has three or four forked styles.

The commonest species--the Round-leaved Sundew (_Drosera rotundifolia_)--is abundant and widely distributed, and may be seen among the bog-mosses, sometimes almost completely covering rather large patches of marshland. Its leaves are round, from a quarter of an inch to near half an inch in diameter, spreading in such a manner that they lie close to or near the ground. The flower-stems are slender, erect, from three to six inches long; and the white flowers, which are in a one-sided raceme, bloom during July and August.

The Long-leaved Sundew (_D. longifolia_ or _D. intermedia_) has oval leaves, tapering gradually into the stalk. They are more erect than the leaves of the last species, and are not half so broad as they are long. The plant flowers at the same time as the latter, but is not nearly so common.

The third species--the Great English Sundew (_D. anglica_)--is still rarer. Its leaves are still longer and narrower, being sometimes an inch or more in length, and more erect; and the flower-stalk sometimes attains a length of eight inches.

The carnivorous habits of these plants are very similar to those of the Butterworts, but the movements connected with the capture of the prey are more marked in the red filaments which cover the upper surface of the leaves than in the leaves themselves. Those filaments which are situated on the margin of the leaf are longest, and spread outwards, while the others are erect and decrease in length from the edge towards the middle.

Each filament is swollen at its extremity, and supports an enticing globule of glistening fluid which it secretes, for the enlarged extremity is really a minute gland. The fluid, though quite clear, is so viscid that it can be drawn out into threads, and it serves a purpose similar to that of the sticky globules on the spiral thread of a spider's web.

If some grains of sand or other inorganic material be sprinkled on the leaf, the sticky secretion of the glands is appreciably increased, and at the same time assumes an acid character; but it contains no digestive ferment, nor do the filaments change their position to any considerable extent. When, however, a small insect alights on the leaf, attracted by the glistening drops which are probably mistaken for nectar, the secretion not only increases and becomes acid, but a digestive ferment is produced, and the little creature is soon besmeared with the fluid, its condition becoming more and more hopeless through its struggles, till at last further movements are impossible and it dies of suffocation.

A few minutes later the filaments of the leaf immediately around the insect begin to bend towards it, and others a little farther off soon partake in the movement, which may finally extend more or less to all the filaments of the leaf, and thus a large number of glands are brought in contact with the prey. The process of digestion now goes on, and, in a day or two, all the digestible portions of the insect are dissolved and absorbed, and the filaments that were concerned in the work have resumed their original position, leaving the indigestible portions to dry and to be eventually blown away.

The principal food of the Sundews consists of small insects such as ants, midges, flies, small butterflies and moths, caddis-flies, and even small species of dragon-flies. Some of these, more particularly the long-bodied dragon-flies, the smallest of which are over an inch in length, are much too large to be caught and devoured by a single leaf; and in this case it is not at all uncommon for two or more leaves to be concerned in the capture and digestion of a single insect, each one converging its filaments towards the part of the body within its reach, and each one digesting and absorbing the portion against which it can apply its glands.

Insects, however, do not constitute the sole food of these plants, for small worms, spiders, centipedes, &c., are caught and digested in the manner described; and the plants may also be fed artificially on small pieces of meat or other nitrogenous substances, which give rise to the same processes and movements as we have observed in connection with the natural mode of feeding.

LIST OF FLOWERS

CLASSIFIED ACCORDING TO THEIR HABITATS AND HABITS

The following list of wild flowers, classified according to their habits and principal habitats, will assist the student in his attempts to identify unknown species. A general acquaintance with the chief distinguishing features of the orders, or, failing this, a frequent reference to these features as given in Chapter I, will be a valuable help; and, the order once determined, the few particulars added to each name will generally narrow the search down to one or two species, leaving the final decision to the more detailed description given in the text.

The first number given after each name is the height, or, in the case of climbing and trailing species, the length of the plant; and this is followed by the colour and diameter, of the flower, or, in the case of the _Dipsaceæ_, _Compositæ_, and some other plants in which the flowers are densely clustered, the diameter of the cluster or head.

Abbreviations are used as follows:--

W. = white
Y. = yellow
G. = green
R. = red
P. = pink
C. = crimson
V. = violet
Bl. = blue
Br. = brown
Pu. = purple
Cr. = cream
Li. = lilac
Ro. = rose
O. = orange
Sc. = scarlet
p. = pale
d. = dark or deep.

A combination of two of the above denotes an intermediate colour. Thus--G.Y. denotes a greenish yellow; Pu. Br., a purple-brown, &c.

1. WOODS AND THICKETS--SPRING (HERBACEOUS PLANTS).

PAGE

Wood Anemone. 4-8 ins. W. 1 in., 48

Green Hellebore. 12-20 ins. G. 1 in., 49

Stinking Hellebore. 1-2 ft. G. 1/2 in., 49

Goldilocks. 6-10 ins. Y. 5/8 in., 50

Columbine. 1-2 ft. W., Bl. or Pu. 1 in., 50

Dog Violet. 3-6 ins. Bl. or Pu. 5/8 in., 50

Wood Sorrel. 4-6 ins. W. 5/8 in., 52

Wood Strawberry. 2-6 ins. W. 1/2 in., 53

Sweet Woodruff. 8 ins. W. 1/4 in., 54

Lesser Periwinkle. 1-2 ft. Bl. 7/8 in., 54

Toothwort. 5-10 ins. Pu.Br. 3/8 in., 54

Bugle. 3-12 ins. Bl. or Pu. 3/8 in., 55

Yellow Dead Nettle. 10-18 ins. Y. 5/8 in., 55

Primrose. 4-7 ins. p.Y. 1 to 1-1/4 in., 56

Lady's Slipper. 1 ft. Br. and Y. 2 ins., 58

Broad-leaved Garlic. 6-12 ins. W. 3/4 in., 59

Sand Garlic. 2-3 ft. R.Pu. 1/4 in., 59

Star of Bethlehem. 6-12 ins. W. 1 in. or more, 59

Blue-bell. 6-18 ins. Bl. 1/2 in., 60

Daffodil. 12-18 ins. Y. 2 ins., 48

Hairy Sedge., 60

Wood Melic Grass, 60

2. SPRING FLOWERING TREES AND SHRUBS.

Barberry. 4-7 ft. p.Y. 3/8 in., 61

Sycamore. 40-50 ft. Y.G. 1/4 in., 62

Maple. 15-20 ft. Y.G. 1/4 in., 63

Spindle Tree. 4-10 ft. Y.G. 3/8 in., 64

Wild Cherry. 4-8 ft. W. 5/8 in., 64

Bird Cherry. 12-15 ft. W. 1/2 in., 64

Gean. 12-18 ft. W. 5/8 in., 65

Sloe or Blackthorn. 4-8 ft. W. 1/2 in., 65

Bullace. 5-8 ft. W. 1/2 in., 66

Hawthorn. 12-20 ft. W. 1/2 in., 66

Wild Pear. 20-30 ft. W. 1 in., 66

Crab Apple. 10-20 ft. W. and P. 1-1/2 in., 66

Service Tree. 12-20 ft. W. 5/8 in., 67

White Beam. 10-30 ft. W. 1/2 in., 68

Mountain Ash. 10-30 ft. Cr.W. 7/16 in., 68

Black Currant. 3-5 ft. Y.G. 5/16 in., 69

Red Currant. 3-5 ft. Y.G. 1/4 in., 69

Wayfaring Tree. 10-20 ft. W. 3/16 in., 69

Ash. 40-50 ft. Br. 1/8 in., 69

Spurge Laurel. 2-4 ft. Y.G. 1/4 in., 70

Mezereon. 2-4 ft. p.R. 3/16 in., 70

Common Elm. 50-120 ft. Br. Clusters 1/2 in., 71

Wych Elm. 40-100 ft. Br. Clusters 1/2 in., 71

Oak. 40-100 ft. G., 72

Beech. 40-100 ft. G., 73

Hornbeam. 20-60 ft. G., 74

Hazel. 8-16 ft. Y.G., 74

Common Birch. 20-50 ft. G., 75

Dwarf Birch. 1-3 ft. G., 75

Alder. 20-50 ft. R.Br., 75

White Poplar. 60-100 ft. Pu.Br., 76

Grey Poplar. 60-100 ft. Pu.Br., 76

Aspen. 30-80 ft. d.Br., 76

Black Poplar. 50-60 ft. Pu.Br., 77

Scots Pine. 50-100 ft. G.Y., 77

Yew. 20-50 ft. G. 1/8 in., 78

NOTE.--The colours given above, in the case of trees bearing catkins,
are generally those of the more conspicuous male flowers.

3. WAYSIDES AND WASTE GROUND--SPRING.

Celandine. 1-2 ft. Y. 3/4 in., 81

Shepherd's Purse. 6-18 ins. W. 1/10 in., 81

Scurvy Grass. 4-8 ins. W. 1/4 in., 82

Whitlow Grass. 1-4 ins. W. 1/8 in., 82

Yellow Rocket. 1-2 ft. Y. 5/16 in., 83

Early Winter Cress. 1-2 ft. Y. 3/16 in., 84

Garlic Mustard. 1-2 ft. W. 1/4 in., 84

Thale Cress. 6-10 ins. W. 1/8 in., 84

Rape. 1-2 ft. Y. 3/8 in., 85

Wild Turnip. 1-2 ft. Y. 3/8 in., 85

Sweet Violet. 3-6 ins. V., Li. or W. 3/4 in., 85

Ciliated Pearlwort. 2-4 ins. W. 1/8 in., 85

Procumbent Pearlwort. 2-3 ins. W. 1/8 in., 85

Greater Stitchwort. 1-2 ft. W. 3/4 in., 86

Lesser Stitchwort. 1-3 ft. W. 3/16 in., 87