THICKEXIXG OF THE CELL-WALL 29 



During the phase of enlargement the walls undergo no appreciable 

 thickening, but surface growth takes place. This is probably 

 accompUshed by stretching of the elastic membrane and inter- 

 calation of new particles of ceU-wall substance between those 

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

 ing of the K-all occurs, successive layers formed by the agency 

 of the li\'ing 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 (Ciiciirbita) in longi- 

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

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



permeable to water, so that the contained protoplast dies. \\'hen 

 the thickening is at all conspicuous, the successive laj'ers gener- 

 all}' appear more or less distinct (Fig. 14, D; 16, B). Jhisstratifica- 

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

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

 consequenth' 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 e^■idenced b}' the fact that such thin areas, interrupting the 



