68 THE OAK 



the above cells are found to be septate and cut up into 

 parenchyma-like cells irregular bast-parenchyma. The 

 walls, especially the longitudinal walls, are marked 

 either with crowded small pits, giving a reticulate ap- 

 pearance, 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 fibres 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 outwards in consequence of growth in thickness, 

 come to lie in a layer of continually increasing radius. If 

 these bast elements were unyielding and lignified there 

 would be a solid sheath of elements which refused to 

 extend by mechanical distension, 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- 



