182 ELEMENTS OF STRUCTURAL BOTANY. 



318. Stems and roots, as a rule, exhibit three distinct 

 regions according to tne stage of development at which 

 they have arrived. There is, first, the growing point, 

 the chief characteristic of which is the rapid formation of 

 new cells by division ; secondly, ,the elongating part, 

 chiefly characterized by the growth of the cells in length, 

 there being practically no further division here ; and, 

 thirdly, the fully developed part, in which there is no 

 further division or enlargement, though the cells may 

 continue to discharge various important functions. 



319. Growth, whilst dependent upon an adequate supply 

 of water and of new material, is also largely affected by 

 external conditions, such as temperature and light. Growth 

 may be stopped altogether by either too low or too high a 

 temperature, and between the limits within which any 

 given plant is found to be capable of growth there will be 

 found a particular degree of temperature more favourable 

 to growth than any other either above it or below it. 

 This may be called the optimum. The effect of tempera- 

 ture differs considerably according to the amount of water 

 present in the part affected, dry seeds, for instance, resist- 

 ing a temperature, either high or low, to which soaked 

 seeds would at once succumb. 



320. Light is essential to assimilation, but seeds and 

 tubers, as well as many of the lower plants which are 

 without chlorophyll, such as Mushrooms, will grow in the 

 absence of light as long as the stock of assimilated 

 material upon which they draw is not exhausted. The 

 growth which takes place in the cambium-layer of 

 dicotyledons and in roots is another example of increase 

 in size in the absence of light. The assimilated material 



