of Mississippi and Louisiana 71 



Yucca gloriosa also appears occasionally as a striking feature 

 of the beach dunes, as was observed on Breton Island. 



Paniaim amanun appears to correspond in its habitat and be- 

 havior to Spinifex squarrosus of New Zealand, as described by 

 Schimper.* 



The upper beach association may at times consist wholly of 



three plants, Paniaim am arum, Ipomoea Pes-caprac and /. acetosae- 



folia (Plate 8). This occurs if the width of the island is not great 



enough to permit the growth of vegetation in the lee of the dunes. . 



If, however, as occurs in the older, wider portions of an island, an 



accumulation of sand of some depth has taken place, or if the 



island has been extended into a sand spit of considerable area, 



other conditions are set up which we describe under the following 

 head. 



The Sand Plain Formation {Plates p and 10) 



Floristically considered there are three leading types of the 

 sand plain. These may be distinguished also by age, in the order 

 of which they may very properly be described. 



The youngest is the sand spit plain. This is usually a low- 

 lying extent of sand, hot, brilliantly lighted and wind swept. It 

 is also subject to inundation by the sea during heavy weather 

 (Hilgard). Being continuous with the older part of the island, 

 their beaches are continuous. The lower beach of the sand spit 

 is less steep and strand pools are of frequent occurrence. The 

 middle beach is in like manner less abrupt, and is coordinately 

 much broader. The dunes of the upper beach may usually be 

 followed for some distance from the older part of the island, but 

 they sooner or later fail. The whole area of the plain is then open 

 to the sea without the protection offered by the dunes, and pro- 

 J ces a scattered vegetation of the beach annuals together with 

 Perennials of similar ecological character. 



du 



The occurrence of driftwood or of plants on the plain causes 

 Regularities in the surface. These are of the form : (i) Of shallow 

 depressions or basins of small extent usually below the general 



level 



°r less level stretches of sand. 



(3) 



I'flanzengeographie auf physiologische Grundlage. 



