The relation between physiology and physics and chemistry suggests similar considerations; it is necessary to make some preliminary remarks in explanation of this relation, because we often meet with the view, especially in modern times, that vegetable physiology is virtually only applied physics and chemistry, as though the phenomena of life could be simply deduced from physical and chemical doctrines. This might perhaps be possible, if physics and chemistry had no further questions to solve in their own domains; but in fact both are still as far distant from this goal, as physiology is from hers. It is true indeed, that modern vegetable physiology would be impossible without modern physics and chemistry, as the earlier science had to rely on the aid of the physics and chemistry of the day, when she was engaged in forming a conception of ascertained vital phenomena as operations of known causes. But it is equally true, that no advance which physics and chemistry have made up to the present time would have produced any system of vegetable physiology, even with the aid of phytotomy; history shows that a series of vital phenomena in plants had been recognised in the 17th and 18th century, at a time when physics and chemistry had little to offer, and were in no condition to supply explanations of any kind to the physiologist. The true foundation of all physiology is the direct observation of vital phenomena; these must be evoked or altered by experiment, and studied in their connection, before they can be referred to physical and chemical causes. It is therefore quite possible for vegetable physiology to have reached a certain stage of development without any explanation of the phenomena of vegetation from physics or chemistry, and even in spite of erroneous theories on those subjects. What Malpighi, Hales, and to some extent Du Hamel produced, was really vegetable physiology, and of a better kind than some moderns are inclined to believe; and their knowledge was derived from observations on living plants, and not from the chemical and physical theories of their time. The discovery even of important facts, for example, that green leaves only can form the food suitable to effect the growth and formation of new organs, was made a hundred years before that of the decomposition of carbon dioxide by the green parts of plants, at a time indeed when chemistry knew nothing of carbon dioxide and oxygen. A whole series of physiological discoveries might be mentioned, which were distinctly opposed to chemical and physical theories, and even served to correct them. We may give as examples, the establishment of the facts that roots absorb water and the materials of food without giving up anything in return, which seemed quite unintelligible on the earlier physical theory of the endosmotic equivalent; and that the so-called chemical rays of the physicists are of subordinate importance in vegetable assimilation, while contrary