ASSOCIATION OF ORGANISMS-
THE WEB OF LIFE

CHAPTER LX

ASSOCIATION OF ORGANISMS-GENERAL PRINCIPLESANIMALS AND PLANTS

GENERAL PRINCIPLES

The study of natural science during the last half-century has advanced so rapidly that it is no longer possible for one man to grapple seriously even with a single subject, and there is an everincreasing tendency towards specialization. No doubt the sum of our knowledge is thereby constantly being increased at a rate which would otherwise be impossible, but there is another side to the question. For extreme specialization is somewhat apt to lead to a neglect of general principles, and to a more or less complete loss of the sense of proportion. To be unable to see the wood on account of the trees is bad enough, but to have one's vision restricted to a single tree, or perhaps a single branch, is very much worse. In no department of knowledge is the cramping tendency of specialization more apparent than in natural history. There seemed at one time a chance of establishing a science of Biology, designed to deal with both plants and animals, but this has now been merged into botany on the one hand and zoology on the other, and many of the important relations that exist between plants and animals are not given the prominence which they undoubtedly deserve. This cannot altogether be helped, but even under existing circumstances it is both desirable and possible that work of specialist kind should be preceded by studies of a wider and more general nature. This is one of the aims of the new subject of NatureStudy, so far as biology and geology are concerned, another object being to foster that intelligent interest in and accurate observation

of natural objects upon which much of the future happiness of our embryo citizens will depend. If properly taught as a connected whole, and not as a string of isolated facts (to be "learnt "), this subject ought to fare better than physiography and general elementary science, which, though designed with the laudable intention of giving a broad foundation in non-biological science, are now somewhat discredited.

The relations which bind together the innumerable plants and animals now living on the globe are so numerous, and often so complex, that from this point of view the world has been compared to a spider's-web of elaborate texture, in which all the threads are directly or indirectly connected, so that when one is touched the entire structure is thrown into vibration.

It is not within the scope of this work to deal with the complex relations which link together the members of the vegetable kingdom, and students who desire information of this kind are referred to the English edition of Schimper's Plant Geography, to Kerner von Marilaun's altogether admirable book The Natural History of Plants, and also to Scott Elliot's Nature Studies, which gives an excellent account of the leading facts and principles in small compass. But it may be well to attempt here a brief description of the salient features which mark the relations existing between plants and animals. This is much more fully dealt with by the authors just mentioned.

PLANTS AND ANIMALS

It may not be superfluous to remark here that the vegetable world is divided into the following great groups, beginning with the highest-1. SEED PLANTS (Spermaphyta), including most of the large and obvious forms, such as ordinary forest-trees and the inhabitants of our flower-gardens. 2. FERN-LIKE PLANTS (Pteridophyta), comprising not only ferns, but also horse-tails, club-mosses, &c. 3. MOSSES AND LIVERWORTS (Bryophyta). 4. LOWER PLANTS (Thallophyta), in which the body is not divided into stem, root, and leaf, or such a division is only incipient. Multitudes of Thallophytes are minute or microscopic, and in any case they may broadly be assigned to one of three sub-groups: (a) Alga, embracing brown, green, and red sea-weeds (with a smaller number of freshwater weeds), with a host of smaller

forms living not only in water but in most damp places; (b) Fungi, including toad-stools, moulds, mildews, the microscopic yeast-plants, and the still smaller bacteria; and (c) Lichens, which are intimately connected communities of algae and fungi.

All these plants, except fungi (and a few seed-plants), contain leaf-green or chlorophyll, a substance of great biological importance, as elsewhere explained. It is convenient to distinguish forms which possess it as "green plants", though the chlorophyll

Both green plant and animal take in oxygen (o) and give out carbon dioxide (co) in the course of respiration (R). The animal feeds on plants, and by nitrogenous excretion and ultimate death adds to the store of organic matter in the soil. The green plant in the course of feeding (F) takes in carbon dioxide (co) from the air, returning oxygen (0), and also takes up water with dissolved salts from the soil; its dead parts contribute to the store of organic matter in the soil. The groups of bacteria B1-B, respectively produce ammonia compounds, convert these into nitrites, and these again into nitrates. The bacteria B4, and the tubercle-fungi B5, fix the free nitrogen (N) of the air, with production of nitrates. The bacteria Be, in the absence of oxygen, decompose organic matter with liberation of free nitrogen (N).

may be obscured by the presence of other pigments, as, for example, in brown and red sea-weeds.

RELATIONS BETWEEN ORGANISMS AND THE CONSTITUENTS OF the Atmosphere (fig. 1064).—In considering this question it must not be forgotten that the gases which are mixed together in air are also found dissolved in both fresh and salt water, and the relations between these dissolved gases and aquatic organisms are pretty much the same as those subsisting between ordinary air and land organisms. The most important of these gases are carbon dioxide (carbonic acid gas, CO2), oxygen, and nitrogen. It has already been explained in the section on BREATHING (vol. ii, p. 379) that plants and animals respire in the same way, taking in oxygen to facilitate the breaking-down processes which continually go

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on in the body, and giving out carbon dioxide as a product of waste. It is quite a mistake to suppose that plants "breathe in carbon dioxide and breathe out oxygen", as often supposed. If there were not some compensating arrangement, it is clear that the amount of oxygen in air would rapidly diminish and the quantity of carbon dioxide rapidly increase. But it appears that this is not so, for the composition of air remains practically the same, at least for very long periods of time, though probably in earlier periods of the earth's history its constituent gases were in different proportions from what is now the case. This constancy of composition at the present day is intelligible when we remember how green plants feed, and of what their food consists. Such plants, in fact, bridge over in a fashion the gulf between non-living and living matter. For their food consists of carbon dioxide (obtained from the air), and water in which are dissolved certain substances of simple kind, foremost among these being nitrogen-containing compounds known as nitrates, of which saltpetre is a well-known example. In any sort of tree, shrub, or herb the carbon dioxide is taken in by the leaves (and the green part of the stem), while the roots absorb from the soil a watery solution of nitrates, &c., so dilute as to be comparable to the ordinary drinking water of most districts. These simple constituents of the food are converted step by step into the living substance of the plant by the agency of that substance itself. The first step in this series of up-building processes takes place in the leaves (and the green parts of the stem), and consists in a reaction between carbon dioxide and water, giving rise to a substance which is more complex in nature than either of them. This can only go on in daylight and in the presence of chlorophyll, which in some way not clearly understood enables the living substance associated with it to press the energy of sunlight into its service for the purpose of building up a comparatively complex substance from simple ones. And this first step in the manufacture of living matter is accompanied by the liberation of free oxygen into the surrounding air as a by-product. For the carbon dioxide and water, which are the raw materials in this work, contain more oxygen than is required for the purpose, and the surplus passes away to the exterior. It therefore follows that green plants in the course of their feeding (1) take carbon dioxide from the air, and (2) give out free oxygen to the air. And these

gases are respectively taken in and given out in such proportions that the amount of carbon dioxide in the air does not rapidly grow larger, and the amount of free oxygen rapidly get smaller, as would undoubtedly be the case if the results of breathing were not compensated.

The action and reaction between organisms and the air also involve chemical processes which have to do with nitrogen, and in which a leading part is played by various bacteria which live in the soil. Green plants get the nitrogen which they require for feeding purposes in the form of dissolved nitrates, which are derived from more than one source. It is a familiar fact that ordinary earth or soil, such as is to be found in a garden, is more or less dark in colour, largely as the result of the presence of organic matter. This partly consists of the remains of organisms which have died and decayed, and partly of substances derived from the nitrogen-containing excreta of animals. The rotting, decomposition, or breaking down of such materials is the result of chemical changes brought about by certain bacteria in the presence of oxygen, with production of ammonia compounds. Another set of bacteria convert these compounds into salts known as nitrites, from which nitrates are then produced by the action of still another group of bacteria. The nitrates serve as food to green plants, which in their turn are devoured by animals. We thus see that by the death and decay of organisms material is produced which helps to build up the bodies of new generations. This, however, is not the only source of nitrates in the soil, for what are known as nitrifying bacteria are there present, which possess the remarkable power of abstracting free nitrogen from the air, and causing it to enter into combination. There is another arrangement by which, in leguminous and a few other plants, the same end is attained. If, say, a pea- or bean-plant is dug up, and the earth washed away from its roots, these will be found to bear a number of rounded thickenings. Within each such "root-tubercle" live a number of microscopic fungi (possibly bacteria) that appropriate the free nitrogen of the air which circulates in the soil, employing it to build up nitrates. We have here a striking example of Mutualism (symbiosis), i.e. the intimate association of two organisms for their common benefit. The leguminous plant has a supply of nitrates ready to hand, while the tubercle-fungus is sheltered, and no doubt nourished.