170 



BOTANY: PRINCIPLES AND PROBLEMS 



Fig. 88.- — A Swamp Association. Note tlic contrast between the plants which 

 compose this Plant Association and those in Fig. 89. 



Fig. 89. — A Desert Association, composed of cacti and other extreme xero- 

 phytes. (Courtesy of D, T. MacDougal and Forrest Shreve, Desert Laboratory, 

 Carnegie Institution) , 



