Fig. 18.—Diagram of Protococcoid Cell divided into four daughter cells. Walls a are external, and walls b and c in contact with each other.
If the cells of the group all divide again, in the manner shown the mass will become more than one cell thick, and the inner cells will be more completely differentiated, for they will be entirely cut off from the outside and all direct contact with water and food materials, and will depend on what the outer cells transmit to them. The outer cells will become specialized for protection and also for the absorption of the water and salts and air for the whole mass. From such a plastic group of green cells it is probable that the higher and increasingly complex forms of plants have evolved. There are still living plants which correspond with the groups of four, sixteen, &c., cells just now theoretically stipulated.
Fig. 19.—A, Details of Part of the Tissues in a Stem of a Flowering Plant. B, Diagram of the Whole Arrangement of Cross Section of a Stem: e, Outer protecting skin; g, green cells; s, thick-walled strengthening cells; p, general ground tissue cells. V, Groups of special conducting tissues: x, vessels for water carriage; px, first formed of the water vessels; c, growing cells to add to the tissues; b, food-conducting cells; ss, strengthening cells.
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.
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.