56 



ECOLOGY 



haps the ease with which growing 

 roots penetrate sand accounts in part 

 for their great length. While the 

 initial root system of most trees seems 

 scarcely plastic, later development 

 may vary with the habitat. For ex- 

 ample, in swamps the red maple soon 

 loses its tap root, while the lateral roots 

 develop extensively; in dry grounds, 

 however, the tap root persists and 

 most of the lateral roots disappear. 



The advantage of great root develop- 

 ment in xerophytes is clear enough, 

 since the more extensive the root 

 system, the greater is the area of ab- 

 sorptive surface and the greater the 

 likelihood of root contact with remote 

 supplies of water. The advantage of 

 short roots to swamp plants is not 

 clear; indeed, short roots often fail 

 to afford adequate anchorage, though 

 they may be sufficient for absorption. 

 The advantages of long roots to xero- 

 phytes are so obvious that often they 

 have been regarded as sufficiently ex- 

 water and showing luxuriant shoot de- plained thereby ; such an assumption 



velopment; 724 was grown with an . 



under-supply of water, and shows a fails to ^Cognize that plants cannot 



reduced root system and a shoot yet adapt themselves, but must react to 



the stimuli which influence them (p. 

 947). If such reactions happen to be 



724 



725 



FIGS. 723, 724, 725. Seedlings of 

 the creosote bush {Larrea tridentata), 

 illustrating variations in relative root 

 development in a desert plant: 723 is 

 a seedling grown with an excess of 



more reduced; 725 is a natural seed- 

 ling from the desert and shows a much 

 greater root development in proportion 



to the shoot; all show a strong devel- favorable, the plant thrives, but un- 

 favorable reactions result in injury or 

 in death. 



opment of the primary root with but 

 slight development of laterals; 725 is 

 drawn on a much smaller scale than are 

 723 and 724. After SPALDING (drawn 

 from a photographic reproduction). 



Our experimental knowledge of roots is 

 not yet sufficient to enable us to explain 

 the variations in their form and size, but some suggestions may be ventured. In the 

 first place it is not certain that the roots of xerophytes actually are longer l than those 



1 Some recent studies show that many cacti have relatively small root systems 

 which are close to the surface, and extend laterally rather than deeply. 



