76 



Lloyd and Tracy : The insular Flora 



Or it may be also that the load of shells has not been lifted well 

 upon the muck plain, so that in either case the seaward margin of 

 the dune lies below the water level. In this way a shell beach is 

 formed, such as is represented in the lower figure. A description 



Fig. 2a. Diagram of a shell-marsh island at right angles to the shell dune. 



of the latter condition will serve for the present purpose because, 

 as will be shown, the flora of the two differs but little. 



The conditions of growth on a shell dune are as follows : Or- 

 ganic materials, as well as alluvial detritus are almost entirely ab- 

 sent as they are carried through by the percolating water, which 



Fig. 2b. 



at the same time reduces the salinity if the substratum. The de- 

 gree of salinity may, however, be suddenly increased by the sea 

 water in unusual weather. This absence of salinity is not true of 

 the lower beach, which, however, is devoid of vegetation for other 

 .reasons. Water of capillarity is absent except rarely when the 

 fragments are very small. A fair amount of moisture is, however, 

 present because the shell fragments hold water better than might 

 be suspected. These characters, taken together with the high 

 temperature and intense insolation, produce a decidedly xerophytic 

 condition. The percolating water contains probably an unusual 

 amount of calcium carbonate, making the shell dune not unlike a 

 coral beach. 



In spite of such apparently unfavorable conditions, the small 

 area of the shell dune produces a good number of plants. 



Vegetation is not encountered in the lower beach or the lower 

 zone of the middle beach. The plants of the upper zone of the 

 middle beach are chiefly prostrate annuals, of which Euphorbia 



polygonifolia and 



7ph 



are almost the only example 



