68 THE STUDY OF PLANT COMMUNITIES * Chapter IV 



cies, but the periods of root elongation are rarely known. Studies 

 of loblolly pine 205 showed, surprisingly, that it makes some root 

 growth in every month of the year. Even in the winter months, 

 its roots are constantly coming in contact with new supplies of 

 soil water, which fact may partially explain its ability to thrive in 



FlG. 29. Vernal aspect dominance of atamasco lily (Zephyranthes atamas- 

 co) in a low North Carolina meadow where only grasses, rushes, and sedges 

 are visible a few weeks later.— Photo by H. L. Bloniquist. 



the southeastern states where transpiration may at times be fairly 

 high during the winter. 



Periodicity may be controlled by a variety of factors. Length 

 of day affects the time of flowering, some species requiring long 

 days, some short. The fall of leaves in autumn is a response not to 

 temperature but to length of day. Desert vegetation may flower 

 or not depending upon precipitation, and semidesert plants reg- 

 ularly flourish during the brief seasonal rains and exist in an almost 

 dormant condition for the remainder of the year. Arctic and alpine 

 areas usually receive little rain. The melting snow provides the 

 moisture for vegetation. In situations where little snow accumu- 

 lates or where it melts and disappears quickly, the vegetation is 



