738 PART IV. THE PHYSIOLOGY OF PLANTS. 



The growth in length of the plant-body takes place at first 

 throughout its whole extent ; but at a later period it is limited, as 

 a rule, to particular regions (see p. 16). In the growing portion 

 of any member two regions may be distinguished : the formative 

 region, which is the growing-point proper: and the region of 

 elongation adjacent to it. In the formative region the construc- 

 tion of the new tissue from plastic substances takes place, as is 

 specially manifested in the formation of cell-walls accompany- 

 ing the cell-division going on in this region of a multicellular 

 growing-point ; but the amount of elongation is slight. In the 

 region of elongation, the formative processes have ceased : in 

 multicellular plants little or no cell-division takes place in this 

 region ; the cells are here fully formed, and they simply require 

 to increase in bulk, to grow in fact, in order to attain the mature 

 form. Beyond the region of elongation conies the portion of the 

 member which has already ceased to grow. It must be clearly 

 understood that each portion of the growing-point passes through 

 these three phases. For instance, in a multicellular apical grow- 

 ing-point, each cell is produced in the formative region ; and as in 

 consequence of the continued formation of younger cells in front 

 of it at the apex, it comes to lie at an increasing distance from 

 the apex, it passes through the stage of growth, to become an adult 

 tissue-element. 



The movement of growth in length is altogether spontaneous. 

 It may be generally described as the travelling of the organic 

 apex in a line which is the continuation of the longitudinal axis 

 of the growing member. Both the rate and the direction of 

 growth are liable to variation, which may be either spontaneous 

 or induced. The induced variations are discussed in 12, p. 742. 



Variations in the Bate of Growth. The rate of growth of a grow- 

 ing member, as also that of each of its constituent cells, is not 

 uniform. When a member begins to grow, its rate of growth is 

 at first slow ; it then accelerates, until a maximum rapidity is 

 attained ; after which it diminishes until growth ceases altogether. 

 This gradual rise and fall in the rate of growth, extending over 

 the whole of one period of growth, is termed the grand period of 

 growth. 



This periodicity is manifested also in each cell of the growing 

 region. A young cell grows but slowly ; as it becomes older, 

 and is gradually removed from the growing-point, its rate of 

 growth increases up to a maximum ; as it becomes still older 



