290 



INTRODUCTION 



[CH.X 



50 



growth in the tip of the plant do not, however, support this 

 theory, for the protoplasm at this point may go on growing 



for centuries, as we see in the 

 case of trees. Some of the 

 protoplasm at the tip is, how- 

 ever, constantly falling back 

 to form part of the stalk : 

 this part soon ceases to grow, 

 undergoing histological dif- 

 ferentiation. The reason 

 why the animal ceases at 

 length to grow is the same 

 as the reason why the differ- 

 entiated tissue below the tip 

 of the epicotyl ceases to grow 

 not because there is a nec- 

 essary limit to growth force 

 at a certain distance from 

 impregnation, but because it 

 is in the nature of the species 

 that the individual should 

 cease to grow at this point. 

 The indefinite growth of this part, the limited growth of that, 

 are as much group characters as any structural quality. 



To recapitulate briefly : Growth is increase in size, and may 

 result from increment of either the formed substance through 

 secretion, the plasma through assimilation, or the enchylema 

 through imbibition. This increment may be either transitory 

 or permanent ; the latter .class chiefly concerns us here. 

 Growth may be either diffused throughout the entire organism, 

 or local, forming a factor of differentiation. In normal growth 

 the increase is at first slow, then rapidly increases to a maxi- 

 mum, and, finally, in most animals, diminishes to zero. This 

 final cessation is a special quality of certain organisms, to be 

 explained like structural qualities, on special grounds. 



FIG. 82. Curves of growth of Phaseolus 

 multiflorus (continuous line) and Vicia 

 faba (broken line). The ordinates rep- 

 resent actual lengths attained on the 

 respective days by a bit of stem origi- 

 nally 1 mm. long. After SACHS, Lect- 

 ures on Plant Physiology. 



