Fig. 37. A, a piece of a twig of an
Imbauba-tree (Cecropia adenopus), with
the leaves cut off. At the leaf-bases are
the hair-cushions (P). E, the opening
for the associated ant (Azteca instabilis).
B, a piece of the hair-cushion with the
egg-shaped nutritive corpuscles (nk).
After Schimper.
This alone would not suffice to protect the tree against the leaf-cutting ants, for how should the Aztec ants living inside notice the presence of the lightly climbing leaf-cutters? But that is provided for, for the Aztecs also frequent the outside of the trunk, and just where attack would be most disastrous, namely, at the stalks of the young leaves. At these places there is a peculiar velvet-like cushion of hair (P), from which grow little stalked white papillæ (Fig. 37, B), which are rich in nourishment, and are not only eaten by the ants, but are harvested by them, being carried off into the ants' dwellings, presumably to feed their larvæ. In this case, then, a particular organ, offering special attraction to ants, has been developed by the plant at the places more especially threatened; while, as regards the ants, it is probable that only the instincts of feeding and habitat require to be modified, since courage and thirst for battle are present in all ants, almost any species being ready at any time to throw itself on any other which intrudes into its domain.
It should be noted that not all the candelabra-trees live in symbiosis with ants, and so secure a means of defence against the leaf-cutters. Schimper found in the primitive forests of South America several species of Cecropia which never had ants in the chambers of their hollow stem. But these species did not exhibit the nutritive cushions at the base of the leaf-stalk; these contrivances for attracting and retaining the presence of partner ants were altogether absent. Indeed, only one species, Cecropia peltata, has produced these peculiar structures, and, as they are of no direct use to the tree, we must say that it has produced them only for the ants. Here, again, natural selection must have gradually brought about the development of these nutritive cushions, though as yet we do not know what the beginnings of the process may have been. In no case can the origin of these cushions be referred to any direct influence of the environmental conditions.
We may now pass to the association of two species of plants, of which the lichens furnish the best-known and probably most complete illustration. Till about twenty years ago the lichens, which in so many diverse forms clothe the bark of trees, the stones, and the rocks, were regarded as simple plants like the flowering plants, the ferns, or the mosses; and many lichenologists occupied themselves with the exact systematic distinction of about a thousand species, each of which could be as well and exactly classified, according to form, colour, habitat, and minute structure, as any other kind of plant. Then De Bary and Schwendener discovered that the lichens were made up of two kinds of plants, fungi and algæ, so intimately associated with and adapted to one another, that on coming together they always assume the same specific form.
Fig. 38. A fragment of a Lichen (Ephebe kerneri), magnified 450 times.
a, the green alga-cells. P, the fungoid filaments. After Kerner.
The framework, and therefore the largest part, and the one which determines the form of a lichen, is due to the fungus (Fig. 38). Colourless threads of fungus ramify in a definite manner according to the species of fungus, and in the network of spaces left by this ramification green alga-cells (a) lie singly, or in rows, or groups. The fungus is propagated by multitudes of minute spores, which it produces periodically, and these are disseminated in the air by the bursting of the sporangia and are carried away by the wind in the form of fine dust; the alga multiplies simply by continual division into two, but it also, like the whole lichen, can survive desiccation, and, after falling to pieces, is likewise carried through the air as microscopic dust.
The partnership of the two plants rests on a basis of mutual benefit; the fungus, like all fungi, is without chlorophyll, and cannot therefore decompose carbonic acid gas or elaborate its own organic food-stuffs; it receives these from the alga. The alga has in the network of the fungus a safe shelter and basis of attachment, for the fungus is able to bore into the bark of trees and even into stones; besides which it absorbs water and salts, and supplies these to the partner alga. We here see the mutual advantage derived from the partnership, which is really an extremely intimate one. Fungus spores, sown by themselves, spring up and develop some branchings of fungoid hyphæ, a so-called mycelium, but without the requisite partner alga these remain weak and soon die away. The alga, on the other hand, can, in some cases, though not in all, survive without the fungus if the necessary conditions of its life be supplied to it, but it grows differently and more luxuriantly in association with the fungus.
The same species of alga may be found associated with different species of fungi, and then each partnership forms a distinct species of lichen of definite and characteristic appearance; Stahl even succeeded in making new species of lichen artificially by bringing the spores of a lichen-forming fungus into contact with alga-cells, with which they had never been associated in free nature.
The most remarkable feature of this remarkable association seems to me to be the formation of common reproductive bodies—an adaptation in face of which all doubt as to the theory of selection must disappear. Periodically there are developed in the substance of the lichen small corpuscles, the so-called soredia, each of which consists of one or more alga-cells surrounded and kept together by threads of the fungus. When they are developed in large numbers they form a floury dust over the maternal lichen, which 'breaks up' and leaves them, like the spores of the fungus, to be carried away by the wind. If these alight on favourable soil nothing more is needed than the external conditions of development, light, warmth, and water, to enable the lichen to spring up anew. The great advantage to the preservation of 'species' is obvious, for, when multiplication by the ordinary method occurs among lichens, the spores of the fungus, even if they have fallen on good ground, can only develop into a new lichen if chance bring to them the proper partner alga.