THE CELLULAR CONSTRUCTION OF THE PLANT 27 



quite in accordance with the fact that the cell-wall takes its origin 

 from the cytoplasm. The material required for the growth of cell- 

 wall is deposited from the cytoplasm, as successive layers of cell-wall 

 substance, on the inner surface of the growing wall. Such facts 

 confirm the conclusion that the cell-wall though important, is not 

 a dominating influence in the cell. It is in the protoplasm that the 

 vital activities reside. 



Changes in the Cells during Differentiation. 



It will be well, as a preliminary to the study of the mature tissues 

 to be described in the next chapter, to explain briefly and to tabulate 

 the chief changes which may be traced in the differentiation of tissues 

 at large from the embryonic cells of the apex.^ 



(i) Changes in Size and Shape of the Cells. 

 Practically all cells grow as they mature, and a simple case has 

 been seen in Fig. 12 [A-C). But they may also assume various shapes. 



Fig. 16. 



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 ; (iii.) 

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

 tracheid of Lime. 



AT = intercellular spaces ; « = nucleus; cy/ = cytoplasm ; vac = vacuole ; si = starch- 

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



In the case quoted the change is slight. Usually there is elongation, 

 and the ends become more or less oblique. Such changes ottcn 



•^ This analysis of the changes during differentiation finds its proper place here, and 

 forms the natural foundation for any rational study of mature tissues. But it is open 

 to the student to read it either before or after those tissues have been described, or 

 both before and after. 



