THE CELLULAR CONSTRUCTION OF THE PLANT 29 



which they thus retain. By such changes the cells may be adapted 

 to perform different specific functions. 



(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 



A B 



\\ 



FIG. 18. 



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 100 ; B x 165.) 



are to form vessels ; and usually it affects the transverse, but some- 

 times 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. 18). 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 common. 



(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, 



