GROWTH AND GROWTH HORMONES IN PLANTS 



401 



Elongation 



Cambium 



Activation 



different plants roots have been reported to arise in phloem, in numerous 

 cortical layers, and even in pith. The cell divisions, in any event, continue 

 and involve adjacent cell layers till a meristem of good size is formed; it 

 is only then, one to many weeks after the treatment, that appreciable elonga- 

 tion becomes noticeable. Indeed, as 

 we have seen above, the actual elon- 

 gation of formed roots is inhibited 

 by auxin in all but the most minute 

 concentrations. A number of plants, 

 especially forest gymnosperms and 

 the rosaceous fruit trees, show little 

 or no capacity to form roots in re- 

 sponse to auxin or do it only un- 

 der highly restricted conditions, and 

 there is some reason to believe that 

 here other factors than auxin are 

 limiting. These include carbohy- 

 drates, general nitrogen supply, bi- 

 otin, thiamine, purines (especially 

 adenine and guanine), a factor lib- 

 erated by wounding, and something 

 else (which has not been proven to 

 be a chemical material) which is of- 

 ten present when the tree is very 

 young but decreases rapidly after the 

 third or fourth year. Not all these 

 are involved in any one plant, and 

 there may be others yet undiscov- 

 ered. But it is certainly not surpris- 

 ing that a process so complex as the new formation of an entire root should 

 require the interaction of a multiplicity of factors, and perhaps it is all the 

 more remarkable that auxin should so often be the principal initiator. 



Some of the functions of auxin are summarized in fig. 2. 



Space does not permit detailed description of all the other phenomena on 

 which auxin has been found to act. Clear growth-promoting effects have 

 been reported on certain algae, but not on fungi. The first visible action of 

 auxin is an acceleration of the rate of protoplasmic streaming, a process which 

 requires sugar and depends on oxidative metabolism. Perhaps the most un- 

 expected finding is the action of auxin on flowering. This is exerted most 

 decisively on plants which are close to the border line of the transition from 

 the vegetative to the flowering state. Plants requiring a 16-hour day in order 

 to flower, for instance, were put on a regime of 12- to 14-hour days with a 

 2- to 4-hour supplementary period at very low light intensity; application 



Fig. 2. Diagram of some functions of 

 auxin in the dicotyledonous plant. (From 

 K. V. Thimajin, Anier. Scientist, pp. 

 589-606, 1954.) 



