454. But that which builds plant-fabric is not the cellular structure itself; the work is done by the living protoplasm which dwells within the walls. This also has to take and to assimilate its proper food, for its own maintenance and growth. Protoplasm assimilates, along with the other three elements, the nitrogen of the plant's food. This comes primarily from the vast stock in the atmosphere, but mainly through the earth, where it is accumulated through various processes in a fertile soil,--mainly, so far as concerns crops, from the decomposition of former vegetables and animals. This protoplasm, which is formed at the same time as the simpler cellulose, is essentially the same as the flesh of animals, and the source of it. It is the common basis of vegetable and of animal life. 455. _So plant-assimilation produces all the food and fabric of animals._ Starch, sugar, the oils (which are, as it were, these farinaceous matters more deoxidated), chlorophyll, and the like, and even cellulose itself, form the food of herbivorous animals and much of the food of man. When digested they enter into the blood, undergo various transformations, and are at length decomposed into carbonic acid and water, and exhaled from the lungs in respiration,--in other words, are given back to the air by the animal as the very same materials which the plant took from the air as its food,--are given back to the air in the same form that they would have taken if the vegetable matter had been left to decay where it grew, or if it had been set on fire and burned; and with the same result, too, as to the heat,--the heat in this case producing and maintaining the proper temperature of the animal. 456. The protoplasm and other products containing nitrogen (gluten, legumine, etc.), and which are most accumulated in grains and seeds (for the nourishment of their embryos when they germinate), compose the most nutritious vegetable food consumed by animals; they form their proper flesh and sinews, while the earthy constituents of the plant form the earthy matter of the bones, etc. At length decomposed, in the secretions and excretions, these nitrogenous constituents are through successive changes finally resolved into mineral matter, into carbonic acid, water, and ammonia or some nitrates,--into exactly or essentially the same materials which the plants took up and assimilated. Animals depend upon vegetables absolutely and directly for their subsistence; also indirectly, because