ASSIM'ILA'TION (Lat. assimilare, assima lare, to make like, from ad, to F sin ills, like, similar) IN PLANTS. The process by which foods are transformed into living substance. It is the final step in nutrition (q.v., where its relation to other processes is shown). The term has long been applied in plant-nutrition to the special process, photosynthesis (q.v.). by which green plants make a particular kind of food with the aid of light, since this is superficially the most striking feature of their nutrition. But when, with more careful analysis, there appeared need of a term to name the process in plants, as defined above. to which the word assimilation had long been applied by animal physiologists, and when there began to arise the confusion un avoidable from using the word in one sense• for plants and in another for animals. some writers sought to escape the difficulty by using in the one case a qualifying word, as in the expression `carbon assimilation' for photosynthesis. But it is better to use the word assimilation only for the conversion of foods into living proto plasm, and to apply other terms to food-getting and food-making.
Plant foods must undergo various changes before they actually form a part of the living protoplasm. Since at present the chemical com position of protoplasm is unknown and that of ninny foods is equally unknown, the chemical details by which the latter are converted into the former could hardly be ascertainable. Pro
teids seem to be most like protoplasm in chemi cal composition, and therefore constitute the most important group of foods for its imme diate nutrition. Because they are so like pro toplasm in many chemical reactions, it is sup posed that the assimilation of proteids consists rather in a change in their molecular structure than in their composition, by which change they become very unstable. But nothing is really known about the matter. The assimilation of carbohydrate foods, which might well be distin guished from assimilation proper. involves pro found chemical changes, with the incorporation of molecules of nitrogen and sulphur, and in some eases of phosphorus. Carbohydrates thus become converted into amides and protei•s of various kinds. These processes go on most vig orously in the leaves of the green plants, prob ably bemuse there is formed by the leaves a good kmpply of available carbohydrates,and to them are brought the nitrates and sulphates from which the other atomic groups needed may lit obtained. But assimilation may occur wherever there is suitable food and active protoplasm. Like all other vital processes, it can go on only limier certain conditions of temperature, moisture, etc. See CELL; PROTOPLASM ; PHOTOSYNTHESIS; NU TRITION ; FOOD or PLANTS. For assimilation in animals, see NUTRITION.