CYTOST IN THE PLANT WORLD 245 



fronds did not ensue until the ratio of virgin soil to 

 exhausted soil exceeded one half or two thirds. Of 

 itself this observation is of no moment; but inci- 

 dent to the comparison of growth in various mixtures 

 of exhausted and virgin soil it was observed that trans- 

 planted fronds grew more rapidly in a mixture of vir- 

 gin soil with 5 to 10 per cent of the exhausted soil 

 than in the virgin soil alone. This curious observa- 

 tion, which went unexplained for some years, may be 

 readily understood if viewed in the light of our ex- 

 periments with animal cells. 



It will be recalled that in the latter case it was 

 found that small quantities of homologous cytost ap- 

 pear to stimulate the anabolic metabolism and growth 

 of cells, whereas higher concentrations of the same 

 substance exert a distinctly toxic action. Transfer- 

 ring this concept to the plant world, we may reason- 

 ably assume that the accelerated growth of plants 

 in the soil mixtures containing small quantities of the 

 exhausted soil is due to the fact that the latter con- 

 tains homologous cytost, which stimulates the growth 

 of the plant cells. 



Again, as noted above, considerable virgin soil must 

 be added to exhausted soil in order to render it capa- 

 ble of supporting a rich growth. Much smaller quan- 

 tities of the virgin soil should suffice to replenish the 

 exhausted essential elements ; hence we must conclude 

 that the high ratios of virgin soil found necessary to 

 stimulate plant growth play a dual role. Aside from 

 supplying essential elements, it must have acted as a 

 diluent for toxic products present in the exhausted 

 earth. That is, by the addition of the virgin soil, the 

 concentration of cytost present in the exhausted soil 



