GROWTH 105 



plasm actually docs increase, or decreases but little. In 

 other words the growth of the cell is mainly accomplished 

 by absorbing large amounts of water, the cell wall being 

 increased in area so as to keep pace with the increase in 

 volume. It is possible that in some cases where the 

 growth of the cell is very rapid the total amount of cyto- 

 plasm in the cell may actually be reduced in manu- 

 facturing the additional cell wall substance required. 

 In this growth we can distinguish three phases which can 

 be more or less clearly set off, viz., formative phase, phase 

 of enlargement and phase of differentiation or maturation. 



149. Thus it comes about that at the growing root tip 

 or tip of the stem we can distinguish an area near to the 

 tip where growth is not very rapid but cell division is 

 taking place abundantly (i.e. the cells are in the formative 

 phase of growth), and another area into which the first 

 grades, and a little distance back from it, where the cells 

 are enlarging very rapidly and but little cell division is 

 taking place (i.e. the cells are in the phase of enlarge- 

 ment). This gradually grades off into that portion of 

 the root or stem where growth in size. is no longer oc- 

 curring but where the various tissue differentiations are 

 taking place (i.e. the phase of differentiation). In the 

 root these zones are well marked, while in the stem the 

 elongation may persist for a long while and may become 

 localized in nodes while the internodes cease to grow. 

 In this case the nodes usually retain some meristem and 

 possess the power of producing new cells as well as in- 

 creasing in size. 



150. There are several factors that influence plant 

 growth. There must in the first place be sufficient food 

 stuffs to enal)le the cells to manufacture the necessary 

 new cytoplasm and cell wall. Then there must be 

 sufficient organic substances to produce the osmotic 



