ORGANIC PROCESSES

A hint is thus given of the two great processes in nature which are to be interpreted as complementary rather than antagonistic. The first is the synthetic or building up of organic substances out of the raw materials of earth, and the second is the tearing down of the compounds to simpler forms to make the material again available for other organisms. Both are necessary. If the building up process went on without interruption, all the available elements in air, water, and soil would ultimately be incorporated into plant form and life would become impossible. The tearing down, then, of older organisms makes possible the existence of later generations. It is evident that in the actual world some of the substance is carried away by water and buried so deeply in the mud that no immediate use thereof is possible, but long ages later, perhaps, man digs out some of it as coal and starts the process anew. It is obvious that all the building up is done by organisms. It is less obvious but equally true that practically all of the tearing down is done by organisms and that much of both processes is done by these microorganisms. That which becomes clear is that any given type of organism owes its existence not only to the qualities given it by nature but to the various things done by other organisms. If we agree to call that action which helps another organism a "service," and that which is harmful, an "injury," we shall save much time and yet avoid self-deception. Everything turns on the viewpoint. Let us note, then, some of the chemical changes brought about by the microorganisms which may be classed as services.

Bacterial Chemistry

Some of the elements needed by plants, such as phosphorus and potassium, are usually found in the rocks in insoluble form. Bacteria, however, produce carbon dioxide and organic

acids which can dissolve these substances and thus make them available for plants. The same is true of sulphur. Nitrogen is common in the air and a good deal is found in the upper layers of the soil. The Romans knew that the legumes such as peas and beans seem to make the soil richer for other plants. After 1875 there were a number of discoveries showing that some fourteen different sorts of bacteria penetrated the roots of legumes, producing little knots or swellings, which took nitrogen directly from the air and made it available for plants. Alfalfa, if the proper bacteria are present, is said to put at times twenty-five dollars' worth of nitrogen into an acre of soil per year. This is a service the importance of which can scarcely be overestimated. Yet the bacteria may cause also some loss of lime and magnesium to the soil and this may be listed as an injury.

Soil fertility turns in considerable measure on the presence of humus, which is nothing but partially decayed organic matter. This humus has to be broken down into simple forms and this is largely the work of bacteria of many sorts, for no one type can do all the job. The proteins are broken down into ammonia and this into nitrites by bacterial agency. Then the nitrites combine with bases such as calcium and magnesium to form the nitrates on which plants feed. There are also other bacteria which may break down the nitrates and set free some of the nitrogen to the air, thus competing with the plants for foodstuffs. The question of the carbon supply is also very important. Rye straw contains 40 per cent carbon by weight. An acre of beech forest demands nearly a ton of carbon per year. But the carbon in the tree is not much eaten or digested by animals and some of the carbon in the bodies of animals is not available as plant food. The bacteria step in again and by a series of processes make the carbon once more available. Soil fertility is not, as formerly thought, solely a matter of the presence of given amounts of

certain chemicals but turns in large measure on the activity of bacteria.

Soil Formation

The share of certain lowly forms of animal life in the formation of soils is not to be overlooked. Darwin was much impressed by the activity of the earth worms. "It may be doubted whether there are many other animals which have played so important a part in the history of the world as have these lowly organized creatures." He showed that in a single acre there were from 50,000 to 500,000 worms, each of which passed through its body some 20 ounces of earth a year. On two square yards studied by him the amount was 6.75 and 8.38 pounds or at the rate of 141⁄2 to 18 tons per acre per year. Working their way through the earth the worms open roads for roots and rain. The soil passing through their bodies is softened and cast on the surface at a rate of some three inches in fifteen years. In Yorubaland in West Africa, Thomson estimates that every particle of the soil is brought to the surface once in twenty-seven years 1. In as much as the bacteria demand an adequate supply of oxygen, it appears that they have been rendered a service by the worms. Ants and other insects are likewise plowing the soil. Elsewhere as in California observers have called attention to the labors of the hordes of ground squirrels, gophers, prairie dogs and similar animals as soil-builders. Plants thus may profit from the activities of animals as we shall see again later on.

Bacteria in Man

1

The rôle of bacteria as causes of disease is considered in another chapter. The digestive tract (particularly the lower intestines, for the acid conditions of the stomach are unfavorable to bacterial life) of all animals from the birds to man is densely populated by great numbers of bacteria from

a few hours after birth until death. That they prosper there is shown by the fact that they are present in vastly greater numbers than in the food supplies eaten and they cannot be dislodged by starvation. Thirty trillions are said to be excreted daily by the human being.

The alimentary canal of the infant is wholly free from microbes at birth, but within a very few hours thereafter invasion occurs. The first indication of a definite bacterial programme in the alimentary canal occurs about the third day of life. By this time the babe has settled down to a fairly definite life routine. The microbe change consists essentially in a replacement of the adventitious germs of the initial invasion by large numbers of lactic-acid-producing bacteria. . . . They prevent in no small degree the development of putrefactive and even disease-producing microbes.2

In similar vein Herter writes:

There is not the least doubt that in some way-a rather intricate way-these normal and dominant flora of the intestine exert a protective action on the whole body. This protective action is complex in operation, but consists at least in part of a preëmptive effect on the intestinal domain, by which other types of bacteria are in large degree excluded. It is impossible to avoid introducing into the digestive tract many bacteria which would prove undesirable permanent tenants, and these are successfully discouraged by the motile biological activities of the more permanent and better adapted bacterial guests.3

Metchnikoff, one of the most brilliant biologists of the last generation, believed that the displacement of the native flora by "wild" types was a common cause of suffering and premature death. It was this belief which led to the emphasis on various brands of sour milk as valuable foods. Many still question the usefulness of the internal flora and believe it neutral rather than beneficial. Whether the animal profits or not, the arrangement is surely to the advantage of the bacteria.

FORMS OF ASSOCIATION

The wood of trees is eaten by few insects, as they do not seem to be able to digest the cellulose. The white ants or termites, however, feed freely on wood. It has been learned that they have in their digestive tract many little organisms of several varieties of flagellate protozoa. It has been found possible to kill off the flagellates by a change of oxygen pressure, the different sorts dying under different pressures. When the termites have been freed of the flagellates they starve if fed on wood. Let the flagellates be reintroduced and the termites prosper again. Not only do the flagellates do no harm to their hosts but they seem to be a positive benefit. There is reason to believe that this particular association has existed for vast periods of years.

The illustrations just given are described by the scientists under the title "symbiosis," in which the different organisms are occupying the same general place and may be thought of as "roommates," their relations being mutually advantageous, or at least neutral. A second sort of relationship where the partners feed at a common table and where they are “messmates" is called "commensalism," and a third type of association where one lives at the expense of the host is called "parasitism." Emphasis should be laid on the fact that we are here speaking of the relations between different forms of life, for the association of many individuals of one species in communities, such as the swarms of bees or herds of cattle, is of another order.

The difficulty of getting away from our man-centered interpretations is again apparent. The bacteria producing the nodules on the roots of legumes are in a sense parasitic and the nodules pathogenic, yet the final result is beneficial. The disease-producing bacteria are parasitic in man and he calls the results abnormal, but there is nothing abnormal about it from the standpoint of the bacteria. The accurate under