116 GROWTH HORMONES IN PLANTS 



"building stones" but as activators, influencing in some way the 

 deposition of materials in cell walls. The primary effect of the hor- 

 mone has been regarded by some investigators as making the 

 cell walls plastic. The stretching that takes place, due to turgor, 

 is accompanied by the incorporation of new wall materials. The 

 result is a permanent increase in size. Other investigators hold 

 that the primary influence of growth hormone is upon the deposi- 

 tion of new wall materials by intussusception and that the forces 

 concerned are so great that turgor pressure is of secondary 

 importance. Whether these or other explanations are valid 

 cannot be decided without more evidence. Growth must be 

 regarded as a function of living protoplasm. The increase in cell- 

 wall boundaries is only one manifestation of the fundamental 

 ability of an organism to build itself out of the materials of its 

 environment. 



