THICKENING OF THE CELL-WALL 



29 



During the phase of enlargement the walls undergo no appreciable 

 thickening, but surface growth takes place. This is probably 

 accomplished by stretching of the elastic membrane and inter- 

 calation of new particles of cell-wall substance between those 

 already present. When fully grown more or less marked thicken- 

 ing of the wall occurs, successive layers formed by the agency 

 of the living protoplast being deposited on the inner surface. 

 Subsequently the wall often undergoes considerable chemical 

 changes, as a result of which it may become more or less im- 



FIG. 13. Phloem-structure of the Vegetable Marrow (Cucurbita) in longi- 

 tudinal (left) and transverse (right) sections, c., cambium ; c.c., com- 

 panion cells ; p, phloem parenchyma ; s.p., sieve-plate. 



permeable to water, so that the contained protoplast dies. When 

 the thickening is at all conspicuous, the successive layers gener- 

 ally appear more or less distinct (Fig. 14, D; 16, B). This stratifica- 

 tion is due to the fact that the layers adjacent to one another 

 are of unequal density, so that one is more transparent and 

 consequently brighter than another. 



As a general rule the thickening is not uniform over the 

 whole inner surface of the wall, small areas commonly remaining 

 thin (Fig. 14, B). The intimate relation between adjacent cells 

 is evidenced by the fact that such thin areas, interrupting the 



