GROWTH 



213 



Can we not go a step further now and explain the 

 very mechanism of growth — why it is that a cell 

 grows, and how it is that under the influence of 

 external agents growth takes this or that direction ? 

 The fundamental mechanism of the growth of cells is 

 explained by botanists in the following way. Owing 

 to chemical changes in the albuminous substances of 

 the protoplasm, there appear substances which absorb 

 water very greedily ; drops of watery fluid, ^ the so- 

 called vacuoles, appear in the cell. These vacuoles blend 



Growth 



Pr. u. 



Vac. 



Pr. u 



Plasmolysis 



Fig. 63. 



into a general vacuole which drives the whole proto- 

 plasm back against the cell-wall (fig. 63, i. Vac), so that 

 the protoplasm assumes the form of a bladder, called 

 the primordial utricle (fig. 63,i,Pr.M.). Into this vacuole, 

 as into our apparatus in fig. 46 under the influence of 

 substances dissolved in it, there will pass osmotically 

 larger and larger quantities of water ; and since a cell 



nineteenth century by a famous English scientist, Knight, who also 

 discovered the effect upon growth of the force of gravity), as well as the 

 subjection of the growing parts to artificial strain (as has been demon- 

 strated by some contemporary German botanists), lead to an increased 

 development of mechanical tissues. Thus mechanical tissues develop 

 most strongly when they are most needed. 

 ' See chapter ii., fig. 15. 



