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As our use of a metal depends upon our knowledge of its properties after various treatments and its several alloys, so our handling of organisms will depend upon the intimacy of our knowledge of the fundamental unit, the gene, and of the basic processes of life and heredity.

The studies of Morgan and his students on the way and the degree to which some of these mutally affect each other, and the modification of these effects by external physical forces, constitute a most notable advance in this field. The extension of such studies may be expected to yield a rich harvest of results in elucidation of the mechanism of heredity, and ultimately of the general mode of evolutionary advance. The laboratories of this station may be confidently expected to contribute their full share to this general progress.

ON THEORIES CONCERNING SOILS AS MEDIA FOR PLANT GROWTH*

CHARLES B. LIPMAN

In my humble opinion, one of the common and fortunate characteristics of thinking men is their propensity to theorize. If we accept this assumption it is but natural that in the course of human experience, so vital a topic as the soil and its relations to plant growth should not have been overlooked in the making of man's theories. From early times in human history, there have descended records of direct or implied speculation on the mystery of the developing plant in its cycle from seed to seed. Thus we find in the very early records of the ancients the writings of Hesiod, on agriculture, and the far more extensive writings of Mago, the Carthaginian general. These were employed by Virgil as a basis for the beautiful poetry of his youth, the Georgics, to which I shall make brief reference again. Other famous writings on agriculture in ancient times were the celebrated treatises of Cato, of Varro, and of Columella, which included the prominent speculations of the day concerning plant growth. In so far as the art of husbandry is concerned, it is striking to note that the ancients were nearly as fully informed as we are today. Parenthetically, I may say that perhaps it would be more accurate, in one sense, to say "equally informed," rather

* Address of the retiring president of the California Chapter of the Society of the Sigma Xi, delivered on April 24, 1918.

than "fully informed." Of the truth of this, we have ample evidence in the writings to which I have just referred. We may take as an example the work of Virgil.

The pastoral scenes of ancient Italy, amid which one of the greatest of classic poets was reared, seem to have been replete with evidence of farming technique similar to our own. Moreover, there appears to have been known to Virgil many a tradition respecting agriculture which remains unchanged to this day in most farming communities. Indeed, there seems to be little in our farming methods which was not also known to Virgil, though we think now that we can, in a few cases, give better reasons for our procedures than could the ancients for theirs. I say better, advisedly, because most practices on our farms today are based on empirical tests which are far from being above question, even though they have been studied considerably. In the beautiful lines of the Georgics, we find exhortations on deep plowing, on fallowing, on timeliness of tillage, on the choice of heavy and light soils for certain groups of crops, respectively, and on many other phases of the subject of soil management, as well as on many other agricultural subjects. But even this keen observer, who sang the praises of the humble art of husbandry, was, like all other mortals, prone to theorize. For example, he connects the best times for tillage with the blowing of certain winds, and the presence of snakes in certain lands with the infertility of the latter. Thus early in man's thought on soils and their productiveness, do we have records of his speculations and theories. Such theories, it must be added, however, were arrived at by the deductive, rather than by the inductive method, which we employ and which offers a more rational and more secure basis for the formulation of theory.

So far from improving on the ideas of the ancients, the writers of the Middle Ages actually detracted from their value by a relapse into greater superstition and less reason; and agriculture with general culture suffered from a lack not only of attention but of careful reasoning from a

dearth of rational theories, and of attempts to prove them. Those who would satisfy themselves with regard to the agricultural theory and practice of the Middle Ages, such as it was, are referred to August Meitzen's large tomes on the development of rural settlements and of agricultural practices among the Germanic peoples.

We are obliged, therefore, to continue the narrative of the history of soil fertility theory by bridging the large gap between the end of the era of the ancients, and the end of the sixteenth century. At this point, we begin to see attempts on the part of thinkers to give reasons for the well known effects of certain farm practices. One such early attempt, extraordinary for its keen insight, is described in a statement cited by E. J. Russell from Palissy, a French thinker. This attempt made in 1563 puts forward the idea that manures have for their function the return to the soil of something which has been removed from it by plants; further that all plants contain some kind of salt and that such salts are found in the ash of plants when the latter are burned. Palissy therefore promulgated the hypothesis that the burning of straw in wheat fields returns essential salts to the soil from which they came, hence maintaining its productiveness. He finds in this idea what he considers the reason for the apparent success of the burning of straw and stubble by grain farmers. While in our modern conceptions of the functions of organic and inorganic manures in soils, there is no necessary confirmation of Palissy's speculations, the latter are at least rational and, in part, doubtless in accord with our important facts. The most remarkable feature of the theory, however, is that it is so very far ahead of its time. About two and onehalf centuries were to elapse before so generally sound a hypothesis (in our view) could be fully appreciated and proved. Meanwhile, there was no lack of attempts, often resulting in a relapse into medieval vagueness, to account for the ever fascinating mystery of the plant. Whether such attempts are truly to be regarded as relapses and

retrogressions, or are to be accounted for by tne lack of facilities for the dissemination of such speculations as Palissy's, cannot now be definitely ascertained. To assume the strong probability of the latter view, however, appears to be reasonable.

Whatever the cause of such apparent retrogression may have been, we find in the two centuries following Palissy's statement a considerable number of attempts to find or to guess at the supposed "principle" of vegetation. Most prominent among such attempts is the celebrated experiment of Van Helmont, carried out toward the middle of the seventeenth century. This experiment, though much quoted by all writers on subjects involving plant growth, may perhaps be unknown to many of you and hence I beg the indulgence of the rest for presenting it now. Two hundred pounds of oven-dried soil were placed in an earthen vessel and moistened with water, and a willow shoot weighing five pounds was properly planted in it. The soil was protected against dust and wind by a perforated sheet of iron. Whenever necessary, rain water or distilled water was added to the soil to maintain optimum moisture conditions. Exactly five years after the experiment was started, Van Helmont found that the tree which had grown in that period from the willow shoot weighed about 169 pounds and 3 ounces. The soil was then again weighed and found to have lost about 2 ounces. Van Helmont, therefore, concluded that a little over 164 pounds in weight of wood, bark, and root (the leaves had not been collected and weighed as they fell) were built up of water, since nothing else had been added to the soil during the period of the experiment. This famous experiment is of the greatest value to us, both because it illustrates admirably the seventeenth century theory with regard to soils and plants, and because it serves as an excellent example of the ease with which it is possible to draw erroneous conclusions from sound experiments. If Van Helmont had not disregarded the small but essential loss of two ounces from the soil; if he had thought of the

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