5o6 



ECOLOGY 



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 

 water and showing luxuriant shoot de- 

 velopment; 724 was grown with an 

 under-supply of water, and shows a 

 reduced root system and a shoot yet 

 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- 

 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 Spaxding (drawn 

 from a photographic reproduction). 



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- 

 plained thereby ; such an assumption 

 fails to recognize that plants cannot 

 adapt themselves, but must react to 

 the stimuli which influence them (p. 

 947). If such reactions happen to be 

 favorable, the plant thrives, but un- 

 favorable reactions result in injury or 

 in death. 



Our experimental knowledge of roots is 

 not yet sufiScient 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 ' than those 



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

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



