THE CELLULAR CONSTRUCTION OF THE PLANT 25 



(4) Absorption of Cell-wall. 

 Though in the young cells the wall completely encloses the proto- 

 plasm, it may be partially broken down and absorbed before maturity. 

 This most commonly occurs in those longitudinal rows of cells which 

 are destined to form vessels ; and usually it affects the transverse, 



A B 



Fig. 15. 



A , a longitudinal row of cells from the root of Maize, still with complete septa, 

 nuclei, and cytoplasm, from which a vessel would be formed by absorption of the 

 septa, and disappearance of the protoplasts. B, a mature vessel of Sunflower, with 

 thickened and pitted walls and no protoplasmic contents, cut in slightly oblique 

 longitudinal section. The arrows indicate free passage through holes formed by 

 absorption of the septa. The longitudinal lines on the pitted walls show the limits 

 of the adjoining cells. {A x ioo ; B x 165.) 



but sometimes also the longitudinal walls. The septa between the 

 cells being thus removed, two or more cells may be thrown together 

 so as to form a continuous tube. Such a tube is called a Vessel 

 (Fig. 15). Other cases of absorption of walls may also occur, but 

 that leading to the formation of wood-vessels is the most important, 

 and the most frequent. 



(5) Changes in the Protoplasmic Body of the Cell. 

 The common change of vacuolisation has already been described. 

 Other changes result in the deposit and removal of contained bodies 



