Growth

Growth rjin plants) implies the building up of new organic substance from the food, a change of form, and generally a permanent increase in bulk. It may be said, therefore, to begin in the protoplasm. Its pre-requisites are (1) water, to maintain the turgidity of the cells; (2) a favourable temperature; (3) a supply of the chemical constituents of protoplasm, especially carbonaceous and nitrogenous matters; (4) and in the case of aerobiotic plants (q.v.) oxygen for respiration; or, in that of anaerobiotics, fermentable material. The mere stretching of cells from increasing turgiditj' is not growth; but when the stretching of cell walls is accompanied by their thickening it becomes permanent, and may well be considered as growth. The growth of organs as a whole may be mainly considered as either elongation or increase in girth. Within certain limits a rise in temperature increases the rate of elongation both of shoots and roots; but in nature roots are often so deep as to be practically removed from the varying influence of the sun's heat and then grow continuously. Light is not essential to growth, but has in general a retarding influence, as is seen by the arching of the stems of plants in a window towards the light, their illuminated sides growing more slowly. The entire absence of light, however, stops the chlorophyllian action, and so cuts off one of the main sources of plastic or growing material. Growing organs seem to possess an inherent rectipetality or tendency to grow in a straight line, and any heteranxisis or inequality in growth seems to be the result of the varying action of external influences, especially light, gravity, moisture, and contact. From this point of view organs may be divided into those which are cylindrical, with radial symmetry, such as roots, shoots, or the leaves of rushes and onions, those which are bilateral or are flattened vertically, as the leaves of Iris, and those that are dorsiventral, or flattened horizontally, with a contrasting structure on their upper and under surface, as most leaves. The first of these groups, from growth taking place in succession on every side, exhibits circumnutation, a nodding or revolving spiral growth. The second group move their grow-ing points in a zig-zag. Dorsi-ventral organs elongate first on one surface and then on the other; but seldom alternate the side more than once or twice. When their upper surfaces grow faster growth is said to be epinastic; when the lower, hyponastic. Fernleaves, for example, are at first strongly hyponastic, and so become rolled up in the bud; but afterwards, becoming epinastic, the leaves unfold. The directive influence of light is termed heliotropism (q.v.); that of gravity, geotropism (q.v.); that of moisture, hydrotropism. The action of contact is especially seen in tendrils (q.v.) and climbing plants. Of growth in thickness the most important cases are those of roots (q.v.) by means of a pericambium, adding to both xylem and phloem internally and to the cortical tissue externally; of the stems of Aloe Yucca, Draccena, and other arborescent Monocotyledons (q.v.), in which a pericycle, or zone of fundamental tissue, remains merismatic and gives rise to new but closed fibro-vascular bundles; and of the exogenous stems of Gymnosperms (q.v.) and Dicotyledons (q.v.), in which a cambium zone, partly fascicular and partly inter-fascicular in origin, gives rise to rings of xvlem internally, generally annually, and to phloem externally.