396 THE WONDER OF LIFE 



The strongly refractive, so-called chemical rays, which 

 have httle or no effect on assimilation, have an inhibiting 

 effect on growth. The growth of plants is also dependent 

 on humidity, the amount of oxygen, temperature, electrical 

 conditions, and other influences. The optimum tempera- 

 ture usually lies between 22° and 37° C, and there is a 

 complete cessation of growth in plants at a temperature 

 less than 0° C. or higher than 40°-50° C. 



For animals the general statement may be made that 

 lowering the temperature puts a brake on growth. It does 

 so, in part, by retarding the process of cell-division, and 

 it does this, in part, by retarding the up-building of nuclein 

 compounds in the cells. Growth is much slower in polar 

 than in tropical seas, and the hfe-span is more drawn out. 

 For a developing chick, the temperature above which death 

 occurs is 43° C, the minimum at which growth stops is 

 about 28° C, the normal Umits are between 35° and 39° C. 



Some hght on the difficult question of the limit of growth 

 may be obtained from a simple consideration in regard 

 to cell-growth, which seems to have been made independ- 

 ently by Herbert Spencer, Rudolf Leuckart, and Alex- 

 ander James. Cells may be defined as unit areas or cor- 

 puscles of hving matter, and, as we have aheady noted, 

 the growth of multicellular organisms depends on the 

 growth and division of the component cells. A cell may 

 grow by taking up water, and by accumulation of the by- 

 products of metabohsm, but essentially by having a surplus 

 in the continual recuperation of the hving matter. Now, 

 if we start with a spherical cell and suppose it to grow 

 until it has quadrupled its original volume, it has by no 

 means quadrupled its surface, for the volume increases 

 as the cube of the radius, and the surface only as the square. 



