Notwithstanding its great variety of forms, this solid congeries of cells, called cellular tissue, is the universal basis of vegetable structure; it forms the principal part of all plants, and the entire mass of many. Though often highly coloured, as in flowers, green leaves and young shoots, it is frequently hyaline and colourless. The dark cells in fig. 4 represent the green part of a leaf, the white ones are those of the colourless skin. Since the primordial cell is the medium in which light and heat act, cellular tissue is present wherever growth is in progress, for all the vital operations take place within its cells. All the organs of plants in their earliest stage consist entirely of cellular tissue, and even in full grown trees the bark and pith of the stem, as well as the soft parts of leaves and flowers, are generally composed of the cells of this tissue, which though assuming a great variety of forms never deviates far from the original type. Every important change in the structure of the cell diminishes or destroys its power of contributing to the nourishment of the plant, as appears in all the tissues derived from it, and which, according to M. von Mohl, is a necessary consequence of the disappearance of the vital part of the primordial cell from those parts of the cellular tissue destined to undergo the change. [Illustration: Fig. 4. Vertical section of the cuticle of Iris germanica:—_a_, cells of the cuticle; _b_, cells at the sides of the stomata; _c_, small green cells placed within these; _d_, openings of the stomata; _e_, lacunæ of the parenchyma; _f_, cells of the parenchyma.]