The rock on which all are liable to come to grief--the chemist or physicist who requires all his facts in terms of analyses and proportions by weight, and therefore takes too mechanical a view of the subject, or the man who is not scientifically trained at all, and therefore is more liable to go to the other extreme and regard the plant as a mysterious something which grows and has poetical associations and traditions--is the great fact of organised structure, and it is the recognition of this fact and some of its consequences which has altered the whole position of the subject, and brought the study of the plant into the domain of physiology. The living plant, its structure and organisation, the functions of its mechanism, and its relations to the environment, thus forms a subject apart from that which concerns the chemical composition of the plant and its environment, and this distinction designates, in a word, as it were, the change which has been brought about by modern biology. A point to be emphasised to the utmost where agricultural students are concerned is that the essential process of feeding is the same in a green plant, a fungus, and an animal; the greatest confusion still exists with regard to this matter, owing to misconceptions as to the real meaning of the functions of the chlorophyll-corpuscles when supplied with carbon-dioxide and water and the energy of the sun's rays. The plant does not feed on carbon-dioxide, any more than it feeds on oxygen--it feeds on the organic material after it has been constructed, and the chlorophyll-function is merely one mode of obtaining supplies of such organic substance. NOTES TO CHAPTER II. In addition to the references in the last chapter, the student should consult Sachs' _Lectures_, XVII.-XIX., and Pfeffer's _Physiology_, pp. 287-329, for the further development of this subject. An excellent résumé, with new facts and points of view, will be found in Dr. Horace Brown's "Address to the Chemical Section," _British Association Reports_, Dover, 1899; and "Chemistry and Physiology of Foliage Leaves" in _Trans. Chem. Soc._, 1893, p. 604. See also Blackman, "Experimental Researches on Vegetable Assimilation and Respiration," _Phil. Trans._, 1895; and Parkin, "Formation, etc., of Carbohydrates in Monocotyledons," _Phil. Trans._, 1899. CHAPTER III. THE PLANT A LIVING MACHINE. _The plant a machine into which energy and material are taken--Carbon assimilation--Feeding--Accumulation and transformations in the plant. The action of light--The chlorophyll-function._