482 



PLANT GROWTH AND PLANT COMMUNITIES 



In dicotyledonous shoots {e.g., of Populus) the cambial region, 

 when isolated from growing leaves and buds, appears as an active 

 region of salt accumulation, in comparison with the dilute solution in 

 the xylem at that level. But when the cambial region is in direct con- 

 tact with growing leaves, via vascular traces in which growth is active 

 in the current year, then the growing leaf, or bud, "calls the time" and 

 accumulates the solute— depleting the cambial region as it does so 

 (Steward, 1954; summary in Steward and Sutcliffe, 1959). 



As each leaf comes successively into its "grand period of growth," 

 it enjoys a period of actiye primary accumulation of solutes. But even 

 so, when new phyllotactic cycles unfold and there are younger leaves 

 above in the same orthostichy, these may require solutes faster than 

 they can be supplied from the roots. Thus the younger leaves may de- 

 plete the older ones, which in turn acquire a vicarious ability to re- 

 absorb salts when the supplies from the root again become available 

 (e.g., as at night). A case in point is the entry of bromide, supplied via 

 the roots, into leaves of the first phyllotactic cycle of Cucurhita pepo 

 seedlings. Growing leaves compete for the bromide they receive via 

 the stem, and they do so in a way which reflects their own intensity of 

 growth, as reflected by the stage of their development relative to 

 "Sachs' Grand Period of Growth" (Figure 13). But if the plants have 

 access via the roots to nutrients and to bromide, concurrently with the 

 daylight conditions that cause the leaves to grow, then tlie young leaves 

 grow better than if they have access to nutrients and to salts only in 



Relative bromide concentration of leaves in 

 an acropetal succession 



Series B 



Figure 13. Relative bromide concentration in leaves of Cucurhita pepo in 

 an acropetal succession. The relative concentration in each leaf equals the 

 concentration in the leaf divided by the average concentration in the leafy 

 shoot as a whole. (Data from F. C. and A. G. Steward, cited in Steward, 

 1954.) 



