RESPONSES TO ENVIRONMENT BY PLANTS IN THE VEGETATIVE PHASE 551 



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Figure 15. The interacting effects of light and temperature on the rate of 

 leaf production by Salvinia nutans. 



candles, whereas at 30° C. the rate of leaf production, the ultimate 

 number of cells, and the cell weight are all augmented. 



The interpretation of this complex pattern raises fundamental 

 problems as to what internal phj-siological factors determine the rate 

 at which the leaf primordia are laid down and the ultimate size of the 

 leaves. When new meristems are formed, their further development 

 will depend on supplies of substrates transported from other parts of 

 the plant. From the standpoint of carbon substrates, the developing 

 leaves will not be self-supporting until they are capable of active as- 

 similation. It is well established that meristematic activity is highly 

 correlated with temperature; thus temperature and light may operate 

 by varying the amounts of substrate and the competitive power of the 

 different tissues for the substrates. Again, these external factors may in- 

 fluence the relative levels and distribution of auxins and inhibitors that 

 determine whether new meristems are initiated or what their sub- 

 sequent development is to be. Here I would like to touch on another 

 series of investigations (which are far from complete): studies of the 

 chemical control of leaf expansion in S. nutans. 



