When the supply of oxygen gives out, carbon dioxid continues to be produced for a time, at the expense of oxygen, which is in loose combination with the tissues. This is accompanied by the for mation of alcohol. This process is known as intra molecular respiration. Respiration takes place in every living cell, since every such cell has need of energy to perform its work. In plants, each cell absorbs its oxygen for the most part from the air that enters the stomata, lenticels and cracks in the bark, and penetrates everywhere into the spaces between the cells.
(2) The food is used to build tissues, cell-walls and other parts.
(3) It is stored in various special storage or gans, principally as starch, fats, oils and proteids. In the germination of seeds we can see very clearly that the stored food, before it can be used, must be digested just as in the animal body. Starch is changed to sugar and proteids are converted into soluble form. This is accomplished by chemical substances called ferments, the presence of small quantities of which makes possible a large amount of chemical action. They are of the greatest im portance in both constructive and destructive processes.
Plants without chlorophyll (saprophytes, living in decaying matter, and parasites, deriving nour ishment from living organisms) are unable to make elaborated food. Some parasites that have chlo rophyll, as mistletoe, have this power. Insectivo rous plants secure an extra supply of nitrogenous food from the capture of insects. Plants of the pea family secure nitrogen from the air by means of bacteria living in their roots; this relation between two plants is known as symbiosis.
Growth.—Growth may be best illustrated by con sidering the growing tip of a stem. Here we may distinguish three stages : (1) The formative region, where cells are con stantly dividing and new organs are being formed.
(2) The elongating region, where the cells expand by absorbing large quantities of water. This comes next to the formative region.
(3) The maturing region, where the cells no longer expand but assume their characteristic form and markings.
In the first of these stages the cell is filled with protoplasm. As the absorption of water continues, drops are formed in the protoplasm ; these coalesce to form a single large drop (vacuole) that occupies almost the entire cell. This condition persists in the later stages.
Growth depends very much on temperature, increasing rapidly up to about 30° C.; above this
it diminishes. It depends, also, on an adequate supply of water, food and air. Light, especially the blue rays, generally checks the growth.
2IIovement.—Movement in plants may be pro duced by the contraction, or other movement, of the protoplasm, as in animals. It is usually due, however, to unequal growth of opposite sides of an organ (e. g., the opening and closing of flowers). The movements of the leaves of the sensitive plant and of clover are due to changes in the turgidity of cells.
Irritability.—Irritability is the power of re sponding to stimuli. When a leaf folds up at a touch, we say that the touch acts as a stimulus. The amount of energy needed to execute the move ment is much greater than was imparted to the leaf by the act of touching it. The stimulus sets free stored energy, just as a touch on an electric button may explode a powder magazine. Among the stimuli to which plants respond may be mentioned light, gravity, heat, chemical substances, electric ity, strains and contact. In general, the plant responds by bending toward or away from the source of stimulus or by changing the rate of growth.
Reproduction.—The process of fertilization in higher plants has already been described. This is called sexual reproduction, because it results from the union of two nuclei, a male and a female.
In addition, we find asexual reproduction, in which no such fusion takes place. The propaga tion of plants by cuttings, leaves, tubers, roots and bulbs furnishes familiar illustrations of this. Simple division, as in bacteria, or budding, as in yeast, are also methods of asexual reproduction. Asexual reproduction by means of specially modi fied single cells, called spores, is found in ferns, mosses, molds, bacteria and other plants.
In all plants, down to and including the mosses and liverworts, there is a regular alternation of sexual and asexual generations. A sexual genera tion (prothallium) arises from the asexual spore (e. g., of a fern) and bears sexual organs. After fertilization, the egg produces a plant that is called the asexual generation, because it produces no sexual organs, but only asexual spores, which, in turn, give rise to the sexual generation. In the fern, the sexual generation is of microscopic size, while the asexual (spore-bearing) generation is the familiar fern plant.