The growth of the wood of exogenous trees takes place thru the ability, already noted, of protoplasmic cells to divide. The cambium cells, which have very thin walls, are rectangular in shape, broader tangentially than radially, and tapering above and below to a chisel edge, Fig. 11. After they have grown somewhat radially, partition walls form across them in the longitudinal, tangential direction, so that in place of one initial cell, there are two daughter cells radially disposed. Each of these small cells grows and re-divides, as in Fig. 12. Finally the innermost cell ceases to divide, and uses its protoplasm to become thick and hard wood. In like manner the outermost cambium cell becomes bast, while the cells between them continue to grow and divide, and so the process goes on. In nearly all stems, there is much more abundant formation of wood than of bast cells. In other words, more cambium cells turn to wood than to bast. [Illustration: Fig. 11. Diagram Showing Grain of Spruce Highly Magnified. PR, pith rays; BP, bordered pits; Sp W, spring wood; SW, summer wood; CC, overlapping of chisel shaped ends.] [Illustration: Fig. 12. Diagram Showing the Mode of Division of the Cambium Cells. The cambium cell is shaded to distinguish it from the cells derived from it. Note in the last division at the right that the inner daughter cell becomes the cambium cell while the outer cell develops into a bast cell. _From Curtis: Nature and Development of Plants._] In the spring when there is comparatively little light and heat, when the roots and leaves are inactive and feeble, and when the bark, split by winter, does not bind very tightly, the inner cambium cells produce radially wide wood cells with relatively thin walls. These constitute the spring wood. But in summer the jacket of bark binds tightly, there is plenty of heat and light, and the leaves and roots are very active, so that the cambium cells produce thicker walled cells, called summer wood. During the winter the trees rest, and no development takes place until spring, when the large thin-walled cells are formed again, making a sharp contrast with those formed at the end of the previous season. It is only at the tips of the branches that the cambium cells grow much in length; so that if a nail were driven into a tree twenty years old at, say, four feet from the ground, it would still be four feet from the ground one hundred years later.