PLANT HORMONES AND GROWTH 



great interesl. In the early work it was sufFicicnt to place Ijuds, 

 leaves or other auxui-produciny organs on such media as agar, and after 

 a given diffusion period to apply the agar to standard oat coletjptiles 

 for assay, a procedvu-e which measures the rate of production. How- 

 ever, since for some pur])oses it is necessary to know the amount of auxin 

 actually present, attention has reccndy turned to quaiUitative extrac- 

 tion of the tissues with organic solvents. This led to the discovery 

 that, while in most tissues a certain small amount of auxin can be 

 obtained by direct extraction, there is commonly from ten to fifty times 

 as much present in an inactive form. This inactive material yields 

 auxin slowly when wetted with water, but the process may be greatly 

 accelerated by hydrolysis with some enzymes, notably chymotrypsin, 

 or with alkali. Whether these inactive forms, or "auxin precursors," 

 are the same in different plants and different types of tissues is not clear 

 as yet. At least one of them appears to be an auxin protein. 



The biological significance of these large auxin reserves is worth 

 consideration. It seems probable that growth is commonly controlled, 

 not so much by the rate of transport or even the rate of true formation 

 of auxin, as by its rate of liberation from the stored form. The sudden 

 onset of growth in tree buds in the spring, and its cessation in early 

 summer just when conditions of light, temperatuie, and nutrition would 

 be expected to fa\ or both auxin production and the formation of 

 organic structural iriaterials, may find its explanation in this way. 

 Many types of infection, as by gall and nodule bacteria and certain 

 fungi, give rise to abnormal growth which corresponds well with the 

 assumption of excessive local auxin formation; in the case of legume 

 root nodules, large quantities of auxin are undoubtedly present. These 

 pathological phenomena may be due to liberation of auxin from the 

 inactive precursors just mentioned. It seems safe to predict that fur- 

 ther study of auxin precursors will not only bring to light some interest- 

 ing enzyme systems, but may lead to explanations of a number of 

 normal and abnormal growth reactions. 



(c) Analysis of the Growth Process. The discovery of an active 

 substance, whether vitamin or hormone, is of importance for its own 

 sake and also because the experimental use of the substance can 

 elucidate the physiology of the processes which it controls. The study 

 of growth in plants has made marked progress through the use of 

 auxin as a tool. Before discussing this use of auxin, however, it 



