CHAPTER III. SPECIAL PHYSIOLOGY OF MOVEMENT. 209 



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 

 and is still more remote, the rate of growth sinks, until finally 

 the adult stage is reached, and growth ceases. 



Careful observation of growing members has shown that, in 

 addition to the spontaneous variation constituting the grand 

 period of growth, small irregular variations are constantly taking 

 place, which, since they are apparently spontaneous, are termed 

 irregular spontaneous variations. 



Another point which must be taken into account is the energy 

 of groicth ; that is, the relative capacity of different members for 

 growth in length. The differences in the energy of growth in 

 growing members manifest themselves in differences either in the 

 length of the grand period, or in the rate of growth ; in other 

 words, members may grow for a longer or shorter time, or they 

 may grow more or less rapidly. In any case the result is that 

 members attain different lengths. Tor instance, it is easy to 

 observe that the lower internodes of most stems remain short; 

 that those above them are longer ; that those of a certain part of 

 the stem are the longest ; and that the upper ones again are short. 

 In the same way the size of the leaves attached to these various 

 parts of the stem increases from below to about the middle, and 

 then diminishes. 



Variations in the Direction of Growth. Although it is true, as 

 stated above, that the result of growth is, generally speaking, that 

 the apex of the growing member is moved onwards in a line which 

 is the continuation of the axis of the growing organ ; yet, during 

 the actual process of growth, this relation of position is not 

 maintained, because the rate of growth is at no time uniform 

 throughout the transverse section of the region of elongation. 

 Suppose a radial stem rising vertically from the soil ; the longi- 

 tudinal axis of the fully grown portion of this stem is vertical, 

 but this is not true of the growing portion. If the apex be looked 

 down upon from above it will be seen to travel in an orbit round 



M.B. P 



