GROWTH AND CELL DIVISION 



65 



The permanent zone merges gradually and indistinguishably 

 into the elongating zone below it and into the mature portions 

 of the root immediately above it. In general, however, its cells 

 are characterized by their great length and large central 

 vacuoles, or sap cavities. The cyptoplasm in the longer cells 

 is usually in the form of a delicate cyptoplasmic sac, in which 

 the nucleus is embedded. The 

 nuclei appear to be rela- 

 tively smaller in these cells, 

 but accurate measurements 

 indicate that they usually 

 maintain a slow growth as 

 long as the cells increase in 

 size, and that they do not di- 

 minish in volume in the cells 

 in which growth is completed. 



Elongation. In a growing 

 root the meristem layer gives 

 rise to hundreds of new cells. 

 As these new cells are formed 

 part of them remain meristem 

 cells, while others in the upper 

 portion of the meristem begin 

 to elongate and form a part 

 of the elongating zone, as 

 shown in Fig. 34. At the 



FIG. 34. A diagram illustrating the 

 method of elongation of a root 



a, shows the main zones of the root shaded 

 as in Fig. 33, a ; in 6, the same zones are 

 shown after the root has elongated. Note 

 that the meristem and elongating zones 

 remain of the same length in a and 6 as 

 growth proceeds 



same time the cells in the 

 upper part of the elongating 

 zone are reaching their definite size and will add a new disk 

 of cells to the length of the permanent zone of the root. It 

 will therefore be clear that by this process the permanent part 

 of the root becomes continually longer by successive additions 

 from the elongating zone, while the length of the latter zone 

 is kept constant by a similar number of cells derived from the 

 division of the meristem cells. By these two processes namely, 

 cell division and cell elongation the root thus grows in length 

 and its tip advances through the soil. 



