68 THE OAK. 



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 ex- 

 tend 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 phloem-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 thick- 

 ness 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 be- 



