Page:The American Cyclopædia (1879) Volume XIII.djvu/598

 582 PLANT adapted to the work ; it is so formed as to present the greatest amount of surface to the sun and air, and in its interior structure it is mostly loose cellular tissue with abundant air spaces, which communicate with the external air through innumerable openings (stomata) in the otherwise impervious epidermis. A simi- lar structure is found in other parts which act the same as leaves ; in many cacti and euphor- bias the whole leaf work is done by the green stem, and the young stems of other plants do the same, as do the calyx and some other parts homologous with leaves, such as the develop- ing fruit. The roots absorb from the soil water containing in solution the ash ingre- dients or mineral substances, nitrogenous and other matters, which (without here noticing the means) is carried into the plant and ulti- mately to the leaves ; this crude sap, as it is called, does not, as many suppose, move up- ward indistinct currents through regular chan- nels directly to the leaves to be elaborated, and afterward descend by other channels to contribute to the growth of the plant. There is however an abundant motion of the juices of the plant, and this takes place in any direc- tion required. As soon as the liquid taken up by the soil enters the plant, it meets with elaborated sap in the cells, and in all parts of the plant is a complex liquid. The leaves gather carbonic acid gas from the air ; this is decomposed within them, and an equivalent quantity of oxygen is returned to the atmos- phere; this decomposition of carbonic acid, which in the laboratory can be done only with difficulty, is effected in the delicate tissues of the leaf, but only in direct sunlight ; from the carbon thus obtained and the elements of water carbohydrates are formed; this process takes place apparently within the chlorophyl cells, and in some manner requires the aid of the ash ingredients of the sap. To form albuminoids the nitrates and other nitrogenous compounds taken up from the soil furnish the needed ad- ditional element. The successive steps in as- similation are not known with certainty, as this wonderful chemistry is carried on within the recesses of the leaf, but the general fact of the decomposition of carbonic acid is well estab- lished by experiment. The interesting rela- tions between plants and animals cannot be discussed, but it may be stated in passing that all the food of animals is provided di- rectly or indirectly by plants, and that in pre- paring food for animals plants take from the air carbonic acid, destructive to animal life, and return to it oxygen, essential to the res- piration of all animals ; and that animals on their part return a large portion of the plants they consume as food to the air in the form of carbonic acid; thus these two great di- visions of the organic world, plants and ani- mals, reciprocally act upon the atmosphere, each contributing to the welfare and supplying the wants of the other. The form in which assimilated matter first appears within the plant is probably that of dextrine and glucose or grape sugar, principles readily changed into other soluble carbohydrates or into the in- soluble form of cellulose. Another important carbohydrate is starch, like cellulose insoluble, and the form in which assimilated material is kept in reserve ; it is readily converted into any of the soluble carbohydrates, and ulti- mately into cellulose ; hence we find it wher- ever nutriment is provided for future growth ; it constitutes in some form a large part of most seeds, and is stored near buds, notably in tubers and fleshy roots, to supply the demands of the growth of young shoots. The growth of the plant from reserve material has been already noted in describing germination ; the phenomenon is shown in a striking manner in the potato in spring, which will often make shoots several feet long in a dark cellar, the cells of these shoots being cellulose formed from the starch of the tuber ; but before the starch can contribute to this growth it must be rendered soluble, that it may be moved to the part where growth is going on ; it is well known that a potato in this condition is sweet and " soggy," and unpleasant to use as food, a condition due to the conversion of its starch into glucose (grape sugar), dextrine, and other soluble forms. The change of one form of carbohydrate into another is quite distinct from assimilation, and the name metastasis has been given to it, as the equivalent of the German Stoffwechsel. It is unnecessary here to consider other principles found within plants, most of which are treated in separate articles, such as fats, oils, resin, caoutchouc, special principles like quinine and morphine, acids, and many others which, though of great importance to man as vegetable products, do not, with some exceptions, take part in the growth of the plant, and often result from a conversion of some of the other principles. Movement of Liquids. The motion of liquids within the plant is quite complex. "Within many cells, and probably in all, when young, the protoplasm is in motion; in some cases the whole body of it is seen to rotate very slowly, though under the microscope, the mo- tion being magnified, it appears rapid ; Val- lisneria, the tape grass or eel grass of fresh water, affords a beautiful microscopic object on account of this rotation; in other cases the motion is confined to portions of the pro- toplasm, and is seen in streams or strings run- ning in various directions; this movement in one cell is quite independent of that in ad- joining cells, and has no relation to the pas- sage of liquid from one cell to another ; it is regarded as a vital movement of the proto- plasm. The motion of liquids from place to place within the plant, as from the root up- ward, is due to several causes. In the process of assimilation large quantities of water are evaporated ; a great deal of water is needed as the vehicle by which nutriment is brought from the soil, and to hold the assimilated ma-