I9I9] 



CRIBBS—TILIA AMERICANA 



265 



with it a large part of the accompanying herbaceous vegetation, 

 thus developing a situation of greater openness and more intense 

 exposure. 



Station D (fig. 4) is located on the lee slope of an advancing 

 dune. In most respects this position is more xerophytic than that 

 at C, for there is a complete absence of humus and herbaceous 





Fig. 4 



Fig. s 



Figs. 4, 5. — Fig. 4, station D, with Tilia on lee slope of advancing dune; 

 sand advance rapid at this point, and absence of humus further inhibits her- 

 baceous development; plant associates: Conius stolonifera, Ammophila arenaria, 

 and Primus virglniana; fig. 5, station E, showing Tilia located on crest of high 

 dune and exposed to most desiccating conditions found in dune enviroiunents. 



undergrowth. The incipient rays are strongly reflected from the 

 open sand, giving a very high hght intensity and considerable 

 increase in temperature. Being on a south-facing slope, the great- 

 est exposure occurs in the late forenoon, followed early in the 

 afternoon by shade which is continuous until evening. 



Station E (fig. 5) represents the most exposed habitat to be 

 found on the open sands, as it is situated about 25 m. above the 



