If now we imagine that the four cells do not separate, but remain together permanently, we can see the possibility of a beginning of specialization in the different parts of the cell. The single living cell is equally acted on from all sides, and in itself it must perform all the life functions; but where four lie together, each of the four cells is no longer equally acted on from all sides. This shows clearly in the diagram of a divided cell given in fig. 18. Here it is obvious that one side of each of the four cells, viz. that named _a_ in the diagram, is on the outside and in direct contact with the water and external things; but walls _b_ and _c_ touch only the corresponding walls of the neighbouring cells. Through walls _b_ and _c_ no food and water can enter directly, but at the same time they are protected from injury and external stimulus. Hence, in this group of four cells there is a slight differentiation of the sides of the cells. If now we imagine that each of the four cells, still remaining in contact, divides once more into four members, each of which reaches mature size while all remain together, then we have a group of sixteen cells, some of which will be entirely inside, and some of which will have walls exposed to the environment. [Illustration: 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. [Illustration: 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.]