Vegetation is constructed of cells or vesicles, and has a cellular tissue. A cell is a living organism. It is that which makes up the tissue of plants. For the whole life of the plant is that of the cells which compose it; in them and by them its products are elaborated, and all its vital processes are carried on. Cell multiplication by division, cell growth, cell modification, exist in plants. Fluids are transferred from cell to cell by a process called endosmose. Absorption takes place by the roots, and the substance absorbed is carried up into the leaves, even to the topmost bough of a tree, passing in its course many millions of apparently water-tight partitions. Plants exchange gases, taking in carbonic acid and giving off oxygen. They evolve heat, have organs of reproduction, and elaborate the material for the final evolution of the seed. This seed, whether of grain, of vegetables, or of fruits, is composed of carbon, hydrogen, and oxygen. And these constitute the starches and sugars which we find have been evolved by the vegetable or plant, and which form the food for animals. Plants, then, convert the elementary substances, the crude material, into food. In doing so, they pass through the processes known as the essentials of life; these are, birth, growth, development, decline, and death. All organic compounds are transitory. They are constantly appearing and disappearing, composing and decomposing, organizing and disorganizing; and they are always dependent upon a certain degree of heat and moisture for their existence or non-existence. The universal constituents of plant life; of organic existence, which are indispensable to vegetation, are carbon, hydrogen, oxygen, and nitrogen. Every vegetable substance is made up of at least eighty-eight to ninety-nine per cent of these elements. The proper vegetable structure, that is, the tissue itself, consists only of three of these elements, carbon, hydrogen, oxygen; while the fourth, nitrogen, is an essential constituent of the protoplasm, which plays so important a part in the formation of the cell, etc. Plants prepare or elaborate out of these chemical elements food-substances composed of those elements--starches and sugars--upon which animals subsist. Animals feeding upon these vegetable substances assimilate, elaborate, them into meat substances, flesh, or proteids. These again are composed of carbon, hydrogen, nitrogen, and oxygen. Nitrogen plays the important role in proteids, being the distinguishing feature, as contrasted with substances of vegetable origin, the carbohydrates. Thus man is provided with two kinds of food: derived from plants, carbohydrates; derived from animals, proteids, or albumens, besides water and mineral salts.