xvi
permanent, more or less woody trunk. In exogenous trees, vegetation is chiefly carried on through the inner bark and young wood, the heart-wood and outer bark retaining little vitality, as may be often seen in the case of hollow trees, which, though the trunk and principal branches are reduced to a mere shell, the interior having wholly decayed, still continue to put forth leaves and flowers for many successive years.
All the organs of plants consist of cells of various form and arrangement, either connected in masses, forming what is called cellular tissue, or elongated and joined together end to end so as to present a tubular appearance. Woody fibre is made up of a number of such tubular cells, united by their overlapping extremities. These cells are all originally membranous; and communication is established between the liquids contained within them by the peculiar process of exudation called endosmose. During growth, however, the walls of the cells are thickened by the gradual deposition of earthy and other substances from the liquid, and are sometimes converted in this manner into a solid mass. Growth is the formation of new cells, either by a process of subdivision and extension of those previously existing, or from the development of a minute body called a nucleus, which, formed in the cell-fluid, is afterwards extended through the membrane and becomes a cell itself. In each cell, during growth, a fluid is constantly circulating with more or less rapidity, holding in suspension various matters which are eventually deposited within it. One of the most important of these organic matters is the chlorophyll or colouring-substance of the plant, usually of a green tint. Light is necessary for its production; and consequently, when plants are grown in the dark, they become etiolated or blanched. Starch, gum, and sugar form the principal other substances floating in the cell-fluid; they are present in all plants during growth. Besides these, various resinous and oily secretions are elaborated in certain cells of the plant, principally in those containing that portion of the fluid which has
