The higher plants of to-day all consist of very large numbers of cells forming tissues of different kinds, each of which is specialized more or less, some very elaborately, for the performance of certain functions of importance for the plant body as a whole. With the increase in the number of cells forming the solid plant body, the number of those living wholly cut off from the outside becomes increasingly great in comparison with those forming the external layer. Some idea of the complexity and differentiation of this cell mass is given in fig. 19, A, which shows the relative sizes and shapes of the cells composing a small part of the stem of a common flowering plant. The complete section would be circular and the groups V would be repeated round it symmetrically, and the whole would be enclosed by an unbroken layer of the cells marked _e_, as in the diagram B. [Illustration: Fig. 20.—Conducting Cells and Surrounding Tissue seen in fig. 19, A, cut lengthways. _px_, First formed vessels for water conduction; _x_, larger vessel; _b_, food-conducting cells; _ss_, strengthening cells; _p_, general ground tissue.] In the tissues of the higher plants the most important feature is the complex system of conducting tissues, shown in the young condition in V in fig. 19, A. In them the food and water conducting elements are very much elongated and highly specialized cells, which run between the others much like a system of pipes in the brickwork of a house. These cells are shown cut longitudinally in fig. 20, where they are lettered to correspond with the cells in fig. 19, A, with which they should be compared. In such a view the great difference between the highly specialized cells _x_, _px_, _b_, &c., and those of the main mass of ground tissue _p_ becomes apparent. Even in the comparatively simply organized groups of the Equisetales and Lycopodiales the differentiation of tissues is complete. In the mosses, and still more in the liverworts, it is rudimentary; but they grow in very damp situations, where the conduction of water and the protection from too much drying is not a difficult problem for them. As plants grow higher into the air, or inhabit drier situations, the need of specialization of tissues becomes increasingly great, for they are increasingly liable to be dried, and therefore need a better flow of water and a more perfect protective coat. It is needless to point out how the individual cells of a plant, such as that figured in figs. 19 and 20, have specialized away from the simple type of the protococcoid cell in their mature form. In the young growing parts of a plant, however, they are essentially like protococcoid cells of squarish outline, fitting closely to each other to make a solid mass, from which the individual types will differentiate later and take on the form suitable for the special part they have to play in the economy of the whole plant.