32 DIFFERENTIATION OF THE TISSUES 



walls, is to give strength at a time when the bast and wood 

 fibers have not yet made their appearance or arrived at suffi- 

 cient maturity to be effective. It may occur as a continuous 

 zone or in separate strands. In producing the collenchyma the 

 ground meristem cells divide transversely and vertically, the 

 daughter cells enlarge and elongate vertically; the walls gradually 

 thicken at the angles, but the cellulose composing them does not 

 appear to become essentially altered or replaced. The collen- 

 chyma cells usually contain chloroplasts, and they remain living 

 until cut off from the general circulation by cork cells in the 

 formation of borke (see page 55), as frequently happens in 

 woody plants. 



Thin-walled Parenchyma lies next to the collenchyma internally, 

 and as a rule constitutes most of the primary cortex. It has been 

 evolved from the ground meristem by transverse and vertical 

 division of the cells of the latter, and the growth of these cells 

 about equally in all dimensions, or with a slight excess of elonga- 

 tion in the vertical or radial direction (Fig. 14). The cell- walls 

 of this tissue remain thin, and their original cellulose is practically 

 unchanged. The cells frequently contain chloroplasts and 

 remain alive unless involved in the formation of borke, as stated 

 for collenchyma. In virtue of its chloroplasts this tissue, as well 

 as the collenchyma, is able to manufacture food, and it is much 

 used also in the slow conduction and storage of foods that have 

 come to it from the leaves. 



Short Sclerenchyma Cells or Stone Cells are frequently found, 

 singly or in groups, distributed amongst the thin-walled paren- 

 chyma (Fig. 14). These, as a rule, are transformed thin-walled 

 parenchyma cells whose walls have become greatly thickened 

 and more or less lignified. The tubular and often branched pits 

 characteristic of the walls of these cells are thin places left as 

 the thickening of the walls progresses. These cells soon die 

 after the completion, of their walls and seem chiefly to be used in 

 giving strength and protection. 



Long Sclerenchyma Cells or Bast Fibers sometimes occur in the 

 primary cortex or in the pericycle (Fig. 14). They take their 



