Page:The Oak.djvu/86

 walls, especially the longitudinal walls, are marked either with crowded small pits giving a reticulate appearance, or have sieve-plates; all intermediate stages occur also. The transverse walls are also pitted with sieve-plates.

All the cells of the soft bast contain tannin, and small grains which turn brown in iodine (leucoplasts?). Very little starch is found in them except in winter. Crystals occur in pitted cells here and there (Fig. 18, d and e).

Even in the first year the cambium may produce small groups of thick-walled bast fibers of exactly the same character as those of the primordial groups.

It is obvious that while the wood elements remain fixed in the cylindrical surface where they are developed, the bast elements formed outside the cambium, being driven outward in consequence of growth in thickness, come to lie in a layer of continually increasing radius. If these last elements were unyielding and lignified there would be a solid sheath of elements which refused to extend by mechanical distention, cell division, or growth of cell-walls; this would finally rupture under the pressure from within. This is prevented by the division and growth of the chief phloëm-elements.

In the vascular-bundle system of the stem there are no essential differences in structure as we pass from one region to another; the only variations are in the thickness or breadth of the bundles at different points, such as where other bundles join or leave them. As the leaf-trace passes into the venation of the leaf the ends