208 EDWARD B. COUCH 



counts seem to be where there is an intermediate stage in compe- 

 tition and stabihzation. It will be noted that the greatest num- 

 ber species occurs in the counts taken on or near the summit of the 

 dune. This is peculiar to dune formations. The combination 

 of plant and vegetative competition is caused by the constant of 

 sand and the tendency toward complete ecesis. The existence 

 of this double condition tends to permit the gathering of repre- 

 sentatives of all the species on the dunes near the summit except- 

 ing Abronia maritiyna which is limited to within about one-quarter 

 mile from the sea. 



The September aspect of the dune is very different from that of 

 the spring. Most of the plants are in a drought resisting condi- 

 tion. Those flourishing in September are: Eremocarpus, Sphaer- 

 ostigma, Eriogonum. The rest of the plants that made such a 

 showing in the spring appear dead but still remain rooted for next 

 season's growth. The leaves and stems of those growing are 

 grayish m color and largely tomentose, a condition peculiar to the 

 drought resisting plants of the dunes. The leaves of the Erio- 

 gonum are more revolute than in the spring, thereby reducing the 

 synthetic and transpiring surface. 



New dune regions where the above plants have not grown show 

 the order of succession to be as given above. In evidence of this 

 the fact may be stated that the owner of the sand spit connecting 

 the mainland with Coronado near San Diego, California, sowed 

 seeds of Abronia umbellata and Gaertneria on the barren spit and 

 after two years all the plants that occur on the dunes are present 

 in the same relations as have been given in the quadrats above. 



