THE CELLULAR CONSTRUCTION OF THE PLANT 21 



It was formerly believed that plastids also arise only by division, but 

 there is now some doubt whether this is always the case, although 

 their multiplication by division is very common. It is different with 

 the cell-wall. In cell-division it appears as a new film deposited from 

 the protoplasm : while, as we shall see, it may be absent altogether 

 from reproductive cells. It is thus a body of secondary importance, 

 as compared with the more constant constituents of the cell. 



Differentiation of Tissues. 



Passing in the examination of the longitudinal section of any 

 bud from the growing point downwards, successively older tissues 



Fig. 12. 



Parenchyma-cells from the cortex of the root of Fritillaria : longitudinal section 

 (X550). A, very young cells, not yet vacuolated. B, older cells with numeious 

 vacuoles containing cell sap. s. each surrounded by the protoplasm, p. C, older cells, 

 with larger vacuoles filled with sap, s. The protoplasm (p) lines the cell-walls 

 internally, and embeds the nuclei (k), which may be suspended centrally, or placed 

 laterally. The large cell to the left has a single large central vacuole, or cell-cavity. 

 (After Sachs.) 



