THE CELLULAR CONSTRUCTION OF THE PLANT 23 



But in the course of their growth cells may also assume various 

 shapes. Usually there is elongation, and the ends become more or 

 less oblique. This naturally follows from the fact that the part of 

 which they are constituent units, such as stem, leaf, or root, itself 

 grows in length. Such changes often involve a readjustment of the 

 cells among themselves by a sliding process, which is specially obvious 



Fig. 13. 



Various forms of cells, (i.) cell of the parenchyma from the cortex of the. root of 

 a Buttercup, almost spherical ; (ii.) oblong cell of the medullary ray of Lime ; (hi.) 

 stellate parenchyma of pith of the Rush ; (iv.) wood-fibre of the Lime ; (v.) fibrous 

 tracheid of Lime. 



x = intercellular spaces ; n = nucleus; cyt = cytoplasm ; vac = vacuole; st = starch- 

 grains, (i. ii. iii. x 200 ; iv. v. x 75.) 



where the cells become elongated or very wide when mature. They 

 seem then to push the surrounding cells aside, so that the appearance 

 of the mature tissue composed of them may differ strongly from that 

 of the embryonic tissue from which they sprang. Special names used 

 to be applied to all the different characteristic forms which cells 

 might assume. But this is not necessary except in extreme cases ; 

 thus the old name parenchyma is kept for a tissue of roughly spherical 

 or oblong cells with square ends, while long thick-walled cells with 

 pointed ends are called fibres (Fig. 13, i.-v.). 



(2) Changes in the Thickness of the Wall : Pits. 

 During the growth of the cell the wall is stretched, and like a 

 rubber sheet it would become thinner as it yields, were it not for 

 the deposit of new cell-wall substance by the cytoplasm. In ordinary 

 cell-walls this is effected by apposition of successive layers upon the 

 surface of the wall, and so quickly is this carried out that the stretch- 

 ing wall of an enlarging cell actually grows thicker instead of thinner 



