nitrogen.” As this is not the incorporation of either into the living
substance, but is only its manufacture into the complex substances
which we find in the plant, it seems preferable to limit the term
“assimilation” to the processes by which foods are actually taken
into the protoplasm.
Symbiosis.—Though green plants thus possess a very complete
mechanism for the manufacture of their different foodstuffs, it is
not always exercised to the fullest extent. Many of them are known
to supplement it, and some almost entirely to replace it, by absorbing
the food they need in a fully prepared condition from their
environment. It may be that they procure it from decomposing
organic matter in the soil, or they may get it by absorption from other
plants to which they attach themselves, or they may in rare cases
obtain it by preying upon insect life. The power of green plants,
not even specialized in any of these directions, to absorb certain
carbohydrates, particularly sugars, from the soil was demonstrated
by Acton in 1889. Similar observat1ons have been made in the
case of various compounds of nitrogen, though these have not been
so complex as the proteids. It was formerly the custom to regard
as parasites all these plants which inserted roots or root-like organs
into the tissues of other plants and absorbed the contents of the
latter. The most conspicuous case, perhaps, of all these is the
mistletoe, which flourishes luxuriantly upon the apple, the poplar
and other trees. Bonnier has drawn attention to the fact that the
mistletoe in its turn, remaining green in the winter, contributes
food material to its host when the latter has lost its leaves. The
relationship thus existing he showed to be mutually beneficial, each
at one time or another supplying the necessities of the other. Such a
relationship is known as symbiosis, and the large majority of the
cases of so called parasitism among green plants can be referred to
it. Bonnier showed that the same relationship could be proved in
the cases of such plants as the rattle (Rhinanthus), the eye-bright
(Euphrasia), and other members of the Natural Orders, Scrophulariaceae
and Santalaceae, which effect a union between their roots and
the roots of other plants growing near them. The union taking
place underground, while the bulk of both partners in the symbiosis
rises into the air, renders the association a little difficult to see,
but there is no doubt that the plants in question do afford each other
assistance, forming, as it were, a kind of partnership. The most
pronounced case of parasitism, that of Cuscuta, the dodder, which
infests particularly clover fields, appears to differ only in degree
from those mentioned, for the plant, bare of leaves as it is, yet contains
a little chlorophyll. The advantages it can offer to its host
are, however, infinitesimal when compared with the injury it does it. Many other cases of symbiosis have been investigated with
some completeness, especially those in which lower plants than the
Phanerogams are concerned. The relations of the Alga and the
Fungus, which have formed a close associationship in the structure
known as the Lichen, were established many years ago. Since about
1880 our knowledge of the species which can enter into such
relationships has been materially extended, and the fungal constituents
of the Lichens are known to include Basidiomycetes as
well as Ascomycetes.
Mycorhizas.—The most interesting cases, however, in which Fungi form symbiotic relationships with green plants have been discovered in connexion with forest trees. The roots of many of the latter, while growing freely in the soil are found to be surrounded with a dense feltwork of fungal mycelium, which sometimes forms a mass of considerable size. The plants showing it are not all forest trees, but include also some Pteridophytes and some of the prothallia of the Ferns, Club-mosses, Liverworts and Horsetails. The true nature of the relationship was first recognized by Pfeffer in 1877, but few cases were known till recent years. Very complete examination, however, has now been made of many instances, and the name mycorhiza has been given to the symbiotic union. Two classes are recognized. In the first, which are called ectotropic, the fungal filaments form a thick felt or sheath round the root, either completely enclosing it or leaving the apex free. They seldom penetrate the living cells, though they do so in a few cases. The root-hairs penetrate between masses of the hyphae of the Fungus. This type of mycorhiza is found among the Poplars, Oaks and Fir trees. The other type is called endotropic. The fungal filaments either penetrate the epidermis of the root, or enter it from the stem and ramify in the interior. Some make their way through the cells of the outer part of the cortex towards the root-tip, and form a mycelium or feltwork of hyphae, which generally occupies two or three layers of cells. From this branches pass into the middle region of the cortex and ramify through the interior half of its cells. They often cause a considerable hypertrophy of the tissue. From the outer cortical mycelium, again, branches pass through the epidermis and grow out in the soil. In such cases the roots of the plants are usually found spreading in soils which contain a large amount of humus, or decaying vegetable matter. The organic compounds of the latter are absorbed by the protruding fungal filaments, which take the place of root-hairs, the tree ceasing to develop the latter. The food so absorbed passes to the outer cortical mycelium, and from this to the inner hyphae, which appear to be the organs of the interchange of substance, for they are attracted to the neighbourhood of the nuclei of the cells, which they enter, and in which they form agglomerations of interwoven filaments. The prothalli of the Pteridophytes, which form similar symbioses, show a somewhat different mode of arrangement, the Fungi occupying the external or the lower layers of the thalloid body.
The discovery of the widespread occurrence of this mycorhizal symbiosis must be held to be one of the most important results of research upon the nutritive processes of plants during the closing decade of the 19th century. Among green plants the symbionts include Conifers, Orchids, Heaths, Oaks, Poplars and Beeches, though all do not derive equal advantages from the association. Monotropas afford an extreme case of it, having lost their chlorophyll almost entirely, and come to depend upon the Fungi for their nutriment. The fungal constituents vary considerably. Each species of green plant may form a mycorhiza with two or three different Fungi, and a single species of Fungus may enter into symbiosis with several green plants. The Fungi that have been discovered taking part in the union include Eurotium, Pythium, Boletus, Agaricus, Lactarius, Penicillium and many others of less frequent occurrence. All the known species belong to the Oomycetes, the Pyrenomycetes, the Hymenomycetes or the Gasteromycetes. The habit of forming mycorhizas is found more frequently in warm climates than cold; indeed, the percentage of the flora exhibiting this peculiarity seems to increase with a certain regularity from the Arctic Circle to the equator.
Fixation of Nitrogen.—Another, and perhaps an even more important, instance of symbiotic association has come to the front during the same period. It is an alliance between the plants of the Natural Order Leguminosae and certain bacterium-like forms which find a home within the tissues of their roots. The importance of the symbiosis can only be understood by considering the relationship in which plants stand with regard to the free nitrogen of the air. Long ago the view that this gas might be the source of the combined nitrogen found in different forms within the plant, was critically examined, particularly by Boussingault, and later by Lawes and Gilbert and by Pugh, and it was ascertained to be erroneous, the plants only taking nitrogen into their substance when it is presented to their roots in the form of nitrates of various metals, or compounds of ammonia. Many writers in recent years, among whom may be named especially Hellriegel and Wilfarth, Lawes and Gilbert, and Schlœsing and Laurent, have shown that the Leguminosae as a group form conspicuous exceptions to this rule. While they are quite capable of taking up nitrates from the soil where and so long as these are present, they can grow and thrive in soil which contains no combined nitrogen at all, deriving their supplies of this element in these cases from the air. The phenomena have been the subject of very careful and critical examination for many years, and may be regarded as satisfactorily established. The power of fixing atmospheric nitrogen by the higher plants seems to be confined to this solitary group, though it has been stated by various observers with more or less emphasis that it is shared by others. Frank has claimed to have found oats, buckbeans, spurry, turnips, mustard, potatoes and Norway maples exercising it; Nobbe and others have imputed its possession to Elaeagnus. There is little direct evidence pointing to this extension of the power, and many experimenters directly contradict the statements of Frank.
The power exercised by the Leguminosae is associated with the presence of curious tubercular swellings upon their roots, which are developed at a very early age, as they are cultivated in ordinary soil. If experimental plants are grown in sterilized soil, these swellings do not appear, and the plant can then use no atmospheric nitrogen. The swellings have been found to be due to a curious hypertrophy of the tissue of the part, the cells being filled with an immense number of minute bacterium-like organisms of V, X or Y shape. The development of these structures has been studied by many observers, both in England and on the continent of Europe. They appear to be present in large numbers in the soil, and to infect the Leguminous pant by attacking its root-hairs. One of these hairs can be seen to be penetrated at a particular spot, and the entering body is then found to grow along the length of the hair till it reaches the cortex of the root. It has the appearance of a delicate tube which has granular contents, and is provided with an apex that appears to be open. The wall of the tube is very thin and delicate, and does not seem to be composed of cellulose or any modification of it. Careful staining shows that the granular substance of the interior really consists of a large number of delicate rod-like bodies. As the tube grows down the hair it maintains its own independence, and does not fuse with the contents of the root-hair, whose protoplasm remains quite distinct and separate. After making its way into the interior, the intruder sets up a considerable hypertrophy of the tissue, causing the formation of a tubercle, which soon shows a certain differentiation, branches of the vascular bundles of the root being supplied to it. The rod-like bodies from the interior of the tube, which has considerable resemblance to the zoogloea of many Bacteria, are liberated into the interior of the cells of the tubercle and fill it, increasing by a process of branching and fission. When this stage is reached the invading tubes and their ramifications frequently disappear, leaving the cells full of the bacterioids, as they have been called. When the root dies later such of these as remain are discharged into the soil, and are then ready to infect new plants. In some cases the zoogloea thread or tube has not been seen, the organism consisting entirely of the bacterioids.
