History of biology

by L. C. Miall

Joseph Priestley, who had been proving that air is necessary both to combustion and respiration, made an experiment in 1771 to discover whether plants affected air in the same way that animals do. He put a sprig of mint into a vessel filled with air in which a candle had burned out, and after ten days found that a candle would now burn perfectly well in the same air. Air kept without a plant, in a glass vessel immersed in water, did not regain its power of supporting combustion. Balm, groundsel, and spinach were found to answer just as well as mint. Air vitiated by the respiration of mice was restored by green plants as readily as air which had been vitiated by combustion.

Priestley did not remark that the glass vessels employed in his experiments had been set in a window, and inattention to this point caused some of his attempts to repeat the experiment to fail. He was further perplexed by using vessels which had become coated with a film of "green matter," probably Euglæna. Such vessels restored vitiated air, though no leaves were present, and when placed in the sun, gave off considerable quantities of a gas, Priestley's "dephlogisticated air" (oxygen). Hardly any oxygen was given off when the green matter was screened by brown paper. Water impregnated with carbonic acid was found to favour the production of the green matter. To us, who have been taught at school something about the properties of green plant-tissues, it seems obvious that Priestley ought to have ascertained by microscopic examination whether his "green matter" was not a living plant. But he had always avoided the use of the microscope, his eyes being weak, and after some imperfect attempts in this way he made up his mind that the green matter was neither animal nor vegetable, but a thing sui generis. Neglecting his most instructive experiments, and not waiting till he could devise new ones, or even disentangle his thoughts, he sent to the press a confused explanation, which seemed to teach that vitiated air may be restored by sunlight alone.

A Dutch physician, named John Ingenhousz, who was then living in England, read Priestley's narrative and began to investigate on his own account. Without detailing his numerous experiments, we may give his own clear summary (condensed). "I observed," Ingenhousz says, "that plants have a faculty to correct bad air in a few hours; that this wonderful operation is due to the light of the sun; that it is more or less brisk according to the brightness of the light; that only the green parts of the plant can effect the change; that leaves pour out the greatest quantity of oxygen from their under surfaces; that the sun by itself has no power to change the composition of air." It will be seen that Priestley started the inquiry, devised and executed the most necessary experiments, and got excellent results. Then he lost his way, and bewildered by conflicting observations, which he was too impatient to reconcile, published a barren and misleading conclusion. Nothing was left for him but to acknowledge that Ingenhousz had cleared up all his perplexities.

Nicholas Theodore de Saussure, son of the Alpine explorer, showed in 1804 that when carbon is separated from the carbonic acid of the air by green plants, the elements of water are also assimilated, a result which owes its importance to the fact that starch is a combination of carbon with the elements of water. Saussure also proved that salts derived from the soil are essential ingredients of plant-food, and that green plants are unable to fix the free nitrogen of the air; all the nitrogen which they require is obtained from the ground.

We are unable to follow the history further. Though the main facts were established as early as the beginning of the nineteenth century, experimental results of scientific and practical interest have never ceased to accumulate down to the present time.

The Metamorphoses of Plants.

Speculations concerning the nature of the flower roused at one time an interest far beyond that felt in most botanical questions. The literary eminence of Goethe, who took a leading part in the discussion, heightened the excitement, and to this day often prompts the inquiry: What does modern science think of the Metamorphoses of Plants?

Let us first briefly notice some anticipations of Goethe's famous essay. In the last years of the sixteenth century Cesalpini, taking a hint from Aristotle, tried to establish a relation between certain parts of the flower and the component layers of the stem. Linnæus worked out the same notion more elaborately, and with a confidence which sought little aid from evidence. His wonderful theory of Prolepsis (Anticipation) need not be described, far less discussed, here. He also borrowed and adapted an analogy which had been thrown out by Swammerdam. The bark of a tree, which according to the theory of Prolepsis gives rise to the calyx of the flower, he compared to the skin of a caterpillar, the expansion of the calyx to the casting of the skin, and the act of flowering to the metamorphosis by which the caterpillar is converted into a moth or butterfly. More rational than the speculations just cited, and more suggestive to the morphologists of the future, are his words: "Principium florum et foliorum idem est" (Flower and leaf have a common origin)—which was not, however, a very novel remark in the eighteenth century. Long before Linnæus early botanists had remarked the resemblance of sepals, petals, and seed-leaves to foliage-leaves; Cesalpini has a common name for all (folium).

At the very time when Linnæus was occupied with his fanciful analogies, a young student of medicine named Caspar Friedrich Wolff, who was destined to become a biologist of great note, published a thesis which he called Theoria Generationis (Halle, 1759). This thesis marks an epoch in the history of animal embryology, but what concerns us here is that Wolff examined the growing shoot, and there studied the development of leaf and flower. He found that in early stages foliage-leaves and floral-leaves may be much alike, and thought that he could trace both to a soft or even fluid substance, which is afterwards converted into a mass of cells. It seemed to him possible to resolve the flowering shoot into stem and leaves only. Wolff's thesis, or at least that part of it which dealt with the plant, was little read and soon forgotten; his studies of the development of animals were carried further and became famous.