The scene--or rather the point to which attention is now directed--is the living, working, energy-accumulating plant itself, and not the dead store of materials in the soil. The reason for the change is not far to seek: it is due to the enormous strides made in the study of the physiology of plants during the last quarter of a century, and the subject abounds in examples illustrating the marvellous advances that have been made, and at the same time showing how, in the progress of researches, made for their own sake--_i.e._ in pursuit of satisfaction for the intense curiosity of the scientific man--all kinds of side issues turn up which prove to be of value in practice, and suggestive of further thinking. At the beginning of the nineteenth century--_i.e._ about 1820--the best thinkers were giving up the old ideas that the environment supplied food, as such, to plants, and had recognised that the plant takes up substances from without and rearranges these in its own body. The next twenty years or so form a very dark interval in plant physiology, chiefly owing to the influence of the assumption of a special "vital force," an assumption which was not allowed merely to serve as a hypothesis put forward to stimulate research and suggest better ideas, but which gained a hold over men's powers of reasoning to an extent which now appears monstrous and phenomenal. Many errors crept in during this reign of terror, one of the most fatal of which was De Candolle's revival of the idea of "spongioles"; and another, equally disastrous in many of its effects, was the conception of a sort of vegetable food-extract, humus, existing in the soil in a form peculiarly suitable for direct use by plants. It was during this period that the confusion between the processes of respiration and carbon-dioxide assimilation arose, and exerted its effects for evil into our own day.