246 THE ACTION OF THE LIVING CELL 



must have been lowered to a level compatible with the 

 growth of the plant. 



Recently H. E. Dolk and K. V. Thimann (1932) 

 have been able to isolate a substance capable of accel- 

 erating plant growth by culturing Rhizopus suinus in 

 such manner that a constant stream of culture fluid 

 passes through the vessels in which growth takes place. 

 The continual passage of the culture fluid through 

 the apparatus washes away the "growth hormone," 

 as it is formed. From their experiments the growth- 

 accelerating substance which they obtained appears 

 to be an organic acid easily susceptible to oxidation, 

 whereby it loses its growth-accelerating activity. This 

 easy destructibility by oxidation indicates that this 

 substance differs markedly from the active principle 

 present in the various cytost preparations used in the 

 author's experiments. But, as has been pointed out 

 previously, in the case of animal cytost a marked 

 species specificity appears to exist. Fundamentally 

 such species specificity must depend upon differences 

 in the chemical constitution of the various cytosts. In 

 consequence, there is no reason why the chemical con- 

 stitution of plant cytost or "growth hormones" should 

 not vary markedly from that of the corresponding 

 animal compounds. 



Unfortunately, since the development of the cytost 

 theory the author has not had available the facilities of 

 a greenhouse wherein further experiments with plant 

 cytost could be made. Nevertheless a considerable 

 literature has accumulated in recent years which indi- 

 cates that endocellular products such as cytost are 

 of importance in plant physiology. 



In the first chapter attention was drawn to the fact 



