CHAPTER III 



The Occurrence and Role 

 of Growth hHormones 

 in Plants 



From the experiments on phototropism by Darwin and later workers 

 it is evident that, at least in the seedlings with which they were work- 

 ing, the apex was the primary source of auxin. Removal of the coleop- 

 tile apex deprived the coleoptile of its auxin source until regeneration 

 of the physiological tip. Among plants in general, apices are perhaps 

 the greatest source of free auxin in the plant. The apices are usually 

 associated with meristems, of course, and the apical meristems of 

 shoots, roots, and buds are rich sources of auxin when growth is taking 

 place. Other meristems besides apical ones are also sources of auxin, as 

 for example the meristems in cambium, seeds, and intercalary meri- 

 stems at the nodes of grasses. Besides meristems, enlarging organs are 

 another auxin source. Enlarging leaves, flowers, fruits, nodules and 

 tumors all have been shown to produce large amounts of auxin. In 

 addition to meristems and enlarging tissues, at least one mature type 

 of organ provides an auxin source to the plant, and that is the mature 

 leaf. 



In general it appears that where there is active growth there is 

 auxin production. Growing meristems and enlarging organs of vascu- 

 lar plants have invariably been found to produce auxins. The forma- 

 tion of auxins by a mature organ such as a leaf, however, suggests 

 that growth may not be a prerequisite to auxin production, for leaves 

 produce sizeable quantities of auxin over a considerable length of time 

 during which there is very little growth in size. 



DISTRIBUTION OF AUXINS 



The distribution of auxins over entire plants has been studied in 

 several instances and suggests a general pattern of quantitative auxin 

 relationships within plants. An example of such a distribution study is 

 60 



