PLANT GROWTH HORMONES 69 



affected by iodoacetate is presumably of sulfhydryl nature, and there is 

 other evidence to support this. 



At this point it is useful to digress for a moment to consider other 

 organisms. If auxins influence growth through such fundamental sys- 

 tems as the Krebs cycle and the phosphorylation of hexose, we should 

 not expect their action to be limited to higher plants. It is important in 

 this connection that the long disputed question whether they control the 

 growth of algae has recently been answered. Algeus (1946) has shown 

 that indoleacetic acid does in fact promote cell division in green unicellu- 

 lar forms and has given an explanation of why up to now the results 

 have been conflicting. One major factor is the strong influence of pK 

 on entry of auxin acids into the cell, and the other is that the light 

 necessary for continued growth of the algal cultures produces decom- 

 position products of the auxin which are toxic. If these points are 

 recognized and controlled, clear-cut growth promotion is found. Also 

 Dr. Dubos has informed me that there are in some bacteria very marked 

 effects of auxin on growth. This, together with some new evidence 

 (unpublished) about an influence of auxin on muscle tissue in animals, 

 justifies the view that the action is exerted via systems which are of 

 quite general occurrence. 



Lastly we must ask how this purely respiratory process can bring 

 about growth. In the last analysis growth in plants means the intake of 

 water. One way in which this may be brought about is through an effect 

 on the accumulation of solutes. We know from the work of the Califor- 

 nia school that solute accumulation requires oxygen and consumes sugar 

 and that active protoplasmic streaming is characteristic of cells which 

 can accumulate. Following the demonstration by Reinders (1942) that 

 auxin promotes water uptake by potato slices. Commoner and coworkers 

 (1943) have shown that auxin even causes the uptake of water from a 

 solution which is hyper-tonic, i.e. a solution which without auxin would 

 cause wilting. This intake of water closely parallels other effects of 

 auxin on growth, and it is promoted by potassium ions in much the 

 same way as we earlier found the growth of coleoptiles in ordinary 

 auxin solutions to be increased by low concentrations of KCl. 



Very tentatively, therefore, one may propose the following steps for 

 the simplest kind of growth of isolated plant parts immersed in solu- 

 tions : 



I , Auxin enters the cells with great rapidity, accelerating the rate 

 of protoplasmic streaming. 



