|Standard Cyclopedia of Horticulture|
Plant (Latin, planta). A plant is a living organism consisting of one or more cells, some of which, in most of the higher forms, contain a green substance—chlorophyl—by the aid of which they are able in the light to construct carbohydrate food-matters (as sugar, starch, and the like) from carbon dioxide and water. The cell protoplasm assimilates or uses these carbohydrates and is nourished by them, and from the elements they furnish it is able to make cellulose, the substance which walls it in and gives strength and solidity to the plant. Animals do not (as a rule, at least) have chlorophyl, and cannot construct carbohydrates from carbon dioxide and water, and the same is true of some plants, as explained below.
Green plants absorb carbon dioxide from the air, and in the process of carbohydrate formation they give off a certain quantity of oxygen. However, in the further chemical activities of their cells, oxygen is absorbed and carbon dioxide is given off. In the plants which are not green (as in animals, also) the first process is wanting, while the second takes place. These facts have given rise to the view that plants and animals are quite opposite in their physiological relations to the surrounding air. They should not be contrasted, however, in this way; it is more exact to say that green plants have two important nutritive functions, namely (1) carbon absorption and fixation (technically photosynthesis), and (2) assimilation of food matters. Respiration—the process in which oxygen is absorbed and carbon dioxide is given off—occurs in all plants and animals.
With this general definition of a plant before us we may say that while some lower plants are minute single cells, or rows of cells, and others are flat, expanded and often irregular growths, in all of which there is a marked simplicity of structure, in higher plants we find the plant-body composed of well-defined roots, stems, leaves, flowers, fruits, and seeds. The lower plants referred to perform all the functions necessary for their continued existence, and are not in any sense "imperfect plants," as the older botanists used to denominate them. The difference between lower and higher plants is that the functions of the former are performed by fewer organs, while in the latter there is an approach to one organ for every function. Still it is true that some organs even in the highest plants have more than one function: so that it may be said that plants are theoretically capable of considerably higher development than they have yet attained. Thus while the chief function of the root may be for the absorption of food-matter, it commonly has in addition a holdfast function, and may become an organ of storage also. So, also, while the chief function of the leaf is to supply green cells for carbohydrate making (photosynthesis), it may be used as a storage organ (as in cabbage leaves), or even for making the plant more conspicuous (ornamental), as in many euphorbias. Even the flower usually unites two functions (that of fertilization and of showiness), which in more highly specialized forms are separated, as in the wild snowball where the large marginal flowers are for show but are sterile, while the small inconspicuous central flowers are fertile. One more thing must be included in our general conception of the plant. While it is true that plants are normally, and typically, green in color, there are many plants which have so changed their food habits that they are no longer green. Thus parasitic plants that secure carbohydrates from living organisms, having no need of chlorophyl, are not green, and the same is true of saprophytic plants (those that get their food from dead or decaying organisms), which are also destitute of a green color. This is the explanation of the fungi, lichens, bacteria, and some flowering plants (e.g., dodder, Indian pipe, beech drops, and the like). Such plants are more or less degenerated, and are physiologically like animals, but they still retain enough of the typical plant structure so that one is rarely at a loss where to place them.