CHAPTER III. SPECIAL PHYSIOLOGY OF MOVEMENT. 



789 



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 growth ; 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. For 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 con- 

 tinuation 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 longitudinal 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 the prolongation of the longitudinal axis of the fully 

 grown portion, in the manner indicated by the following diagram. 



FIG. 477. The growing 

 primary root of the Pea in. 

 two stages. A The root is 

 marked by lines at equal dis- 

 tances. In -B the differences 

 in rapidity of growth are 

 perceptible : the uppermost 

 lines have not been sepa- 

 rated; the root has ceased 

 to grow here. The lowest 

 likewise are still close toge- 

 ther ; at the growing-point 

 elongation has not taken 

 place. In the intermediate 

 zone the elongation has been, 

 very great. 



