The remarkable synthetic process of building up the plasm, to which we give the name of plasmodomism, or carbon-assimilation, usually needs as its first condition the radiant energy of sunlight. Every green plant-cell contains in its chlorophyll-granules so many tiny laboratories, their green plasm being able to form new plasm out of inorganic compounds under the influence of light. The water that is needed for this, besides nitrogenous compounds (nitric acid, ammonia), is drawn from the earth by the roots; the carbonic acid is taken from the atmosphere by the green leaves. The immediate products of the synthesis, due to the separation of the carbonic acid, is, as a rule, a non-nitrogenous starch-flour (amylum). This is further used for the composition of the nitrogenous albumin by an as yet unknown synthetic process, with the aid of nitrogenous mineral compounds. In this process of reduction the separated free oxygen is returned to the atmosphere. The carbohydrates that chiefly co-operate in this are glucoses and maltoses: the mineral substances, especially salts of potassium and magnesium, and compounds of these elements with nitric acid, sulphuric acid, and phosphoric acid. Iron is also found to be an important element in the process, though in a very small quantity. As a rule, the ferruginous chlorophyll can only form new plasm with the help of light-waves. The most important part of the spectrum for this purpose is that containing the red, orange, and yellow waves.
The chief factor in plasma-formation in the organic world is the photosynthesis, or ordinary carbon-assimilation by chlorophyll, the wonderful green matter that amounts to only a very small percentage (about one-tenth) of the weight of the chlorophyll-granules, and can be separated from their plasmatic substance by certain methods. Even when the plant has some other color than green the chlorophyll is still the real plasmodomous substance. Its green color is then masked by some other color—diatomin in the yellow diatomes, phycorhodin in the red rhodophyceæ, phycophæin in the brown phæophyceæ, and phyocyan in the blue-green chromacea or cyanophyceæ. The latter have an especial interest for us, because in the simplest specimens the entire organism is merely a globular bluish-green granule of plasm. Moreover, in the simplest forms of nucleated primitive plants (algariæ)—many of the so-called unicellular algæ—the metabolism is effected by a single grain of chlorophyll. There is usually a large number of them in the plasm of the plant-cells.
Another kind of plasm-synthesis, quite different from the ordinary plasmodomism by chlorophyll and sunlight has lately been discovered in some of the lowest organisms (by Heraeus, Winogradsky, and others). The nitro-bacteria (or nitromonades) are tiny monera (unnucleated cells) that live in complete darkness underground. Their globular colorless plasma-bodies contain neither chlorophyll nor nucleus. They have the remarkable capacity of forming carbohydrates, and from these plasm, by a peculiar synthesis out of purely inorganic compounds—water, carbonic acid, ammonia, and nitric acid. Pfeffer has called this carbon-assimilation, on account of its purely chemical nature, "chemosynthesis," in opposition to the ordinary photosynthesis by means of sunlight. There are also other bacteria (sulphur-bacteria, purple-bacteria, etc.) that show various peculiarities of metabolism. The nitro-bacteria must belong to the oldest monera, and represent a transition from the vegetal chromacea to the animal bacteria.
The extensive class of the fungi (or mycetes) resembles a part of the bacteria in regard to metabolism. These organisms are, it is true, generally regarded as plants, but they have not the capacity of the green, chlorophyll-bearing plants to supply themselves with carbon from the carbonic acid in the atmosphere. They have to take it from organic substances, such as albumin, carbohydrates, etc., like the animals. But while the animals have to derive their nitrogen from the latter, the fungi can obtain it from inorganic matter in the earth. Fungi cannot support life without the addition of organic compounds; but we can make them grow in a food solution consisting of sugar and purely inorganic nitrogenous salts. Thus they are on the border that separates the plasmodomous plants from the plasmophagous animals. Like the latter, the fungi have evolved from the plants through changed food conditions. We find this process even among the unicellular protists in the phycomycetes, which descend from the siphonea. In the same way the real multicellular fungi (ascomycetes and basimycetes) may be traced to the tissue-forming algæ.
All true animals have to derive their food from the plant kingdom, the vegetal feeders directly, and the flesh feeders indirectly, when they consume vegetal feeders. Hence the animals are, in a certain sense, as the older natural philosophy put it four hundred years ago, "parasites of the plant world." From the point of view of phylogeny, the animal kingdom is, therefore, clearly much younger than the plant kingdom. The development of the animals from the plants was determined originally by a change in the method of nutrition which we call metasitism.
The chemical modification of the living matter which is connected with the loss of plasmodomism—in other words, the conversion of the reducing phytoplasm into oxidizing zooplasm—must be regarded as one of the most important changes in the history of organic life. This "reversal of metabolism" is polyphyletic; it has been repeated many times in the course of biological history, and has taken place independently in very different groups of the organic world—whenever a plasmodomous cell or group of cells (=tissue) had occasion to feed directly on ready-made plasm, instead of giving itself the trouble of building it up out of inorganic compounds. We see this particularly among the unicellular protists in the independent ciliated cells. The longer plasmophagous flagellata, which are colorless, and have no chlorophyll (monodina, conoflagellata), closely resemble in form and movement the older plasmadomous and chlorophyll-bearing mastigota, from which they are descended (volvocina, peridinia); they only differ in the manner of nutrition. The colorless flagellata feed on ready-formed plasm, which they obtain either by means of their lashes or by a special cell mouth in their cell body. On the other hand, their ancestors, the green or yellow mastigota, form new plasm by photosynthesis like true cells. But there are also complete intermediate forms between the two groups—for instance, the chrysomonades and the gymnodinia; these may behave alternately as protozoa or protophyta. In the same way we can derive the phycomycetes by metasitism from the siphonea, the fungi from the algæ; and, finally, the process is also found in many of the higher parasitic plants (orchids, orobanches, etc.). (See under "Parasitism.")
As is the case with every other vital function, so for the function of metabolism we find a starting-point in the lowest and simplest group of the protophyta, the chromacea. In their oldest forms, the chroococcacea, the whole body is merely a blue-green, structureless, globular plasma particle, growing by means of its plasmodomous power, and splitting up as soon as it reaches a certain stage of growth. There the miracle of life consists merely of the chemical process of plasmodomism by photosynthesis. The sunlight enables the blue-green phytoplasm to form new plasm of the same kind out of inorganic compounds (water, carbonic acid, ammonia, and nitric acid). We may look upon this process as a special kind of catalysis. In this case there is absolutely nothing to be done by Reinke's "dominants," or conscious and purposive vital forces. There are, as yet, no differentiated physiological functions in these organisms without organs, and no anatomically distinct members; and so their one vital activity, growth, may very well be compared to the simple growth of inorganic crystals.
It has been pointed out repeatedly that the remarkable monera which now play so important a part in biology as bacteria stand, in many respects, quite apart from the ordinary vital phenomena of the higher organisms. This is especially true of their metabolism, which has the most striking peculiarities. Morphologically, many of the bacteria cannot be distinguished from their nearest relatives and direct ancestors, the chromacea, differing from them only in the absence of coloring matter in the plasm. Many of them are simple, globular, ellipsoid, or rod-shaped plasma particles, without any visible organization or movement. Others move about by means of one or more very fine lashes (like the flagellata). No real nucleus can be discovered in the structureless plasma body. The very fine granules which are found in some species, and the vacuole-formation that we see in others, may be regarded as products of metabolism; and the same may be said of the thin membrane or the thicker gelatinous envelope which many of the bacteria secrete. This makes all the more remarkable the peculiarity of their chemical constitution and the metabolism determined thereby. The nitro-bacteria we have mentioned previously are plasmodomous; the anaërobe bacteria (of butyric acid and tetanus) only flourish where oxygen is excluded; the sulphur bacteria (beggiatoa) secrete—by the oxydation of sulphuretted hydrogen—pure regulation sulphur in the form of round granules. The ferruginous bacteria (leptothrix ochrocea) store up oxyhydrate of iron (by the oxydation of carbonic protoxide of iron). The saprogenetic bacteria cause putrefaction, and the zymogenetic fermentation. Finally, we have the very interesting pathogenetic bacteria which cause the most dangerous diseases by the secretion of special poisons—toxins—festering, small-pox, tetanus, diphtheria, typhus, tuberculosis, cholera, etc. On account of their great practical importance, these bacteria have of late been taken over by a special branch of biology, bacteriology. But only a few of the many experts in this department have pointed out the extreme theoretical significance which these zoomonera have for the important questions of general biology. These structureless plasma bodies show unmistakably that their vital activity is a purely chemical phenomenon. Their great variety proves how manifold and complicated must be the molecular composition of the plasm, even in these simplest organisms.
The unicellular protophyta exhibit the same form of metabolism and plasmodomism as the familiar green cells of the tissue-plants; but in most of the protozoa we find special features of nutrition and plasmophagy. The great class of the rhizopods is distinguished by the fact that their naked plasma body can take in ready-formed solid food at any point of its surface. On the other hand, most of the infusoria have a definite mouth-opening in the outer wall of their unicellular body, and sometimes a gullet-tube as well. Besides this cell-mouth (cytostoma) we usually find also a second opening for the discharge of indigestible matter, a cell-anus (cytopyge).
Metabolism in the tissue plants (metaphyta) forms a long gradation from very simple to very complicated arrangements. The lowest and oldest thallophyta, especially the simplest algæ, are not far removed from the communities of protophyta, and, like these, are merely definitely grouped colonies of cells. The social cells which form their most rudimentary tissue are quite homogeneous, with no differentiation beyond that of sex. The thallus or bed-formation consists in the simplest specimens of plain or branched fine threads, consisting of rows or chains of homogeneous cells (so conferva among the green, ectocarpus among the brown, and callithamnion among the red algæ). Other algæ (such as the ulva) form thin leaf-shaped forms of the thallus, a number of homogeneous cells lying side by side along a level. In the larger algæ compact tissue-bodies are formed, in which frequently firmer rows of cells exhibit the rudiments of fibres; and the thallus divides, as in the cormophyta, into root, stalk, and leaves. There is also a trophic differentiation, the fibres undertaking special functions of nutrition (the conduction of the sap). The same must be said of the mosses (bryophyta). Their lowest forms (ricciadinæ) are close akin to the algæ; the highest mosses (the mnium and polytrichum, for instance) approach the cormophyta. Many botanists comprise these lower plants—algæ, fungi, and mosses—under the title of "cell-plants" (cytophyta), and oppose the higher plants—ferns and flowering-plants—to them as "vascular plants" (angiophyta), because they have complex fibres or sap vessels. This distinction has a phylogenetic significance similar to the division between cœlenteria and cœlomaria in the animal kingdom.