100 HARSHBERGER— COMPARATIVE LEAF STRUCTURE [April 24, 



abundant native plants. Yucca aloifolia forms dumps on low sand 

 dunes at Shelly Bay, on the north shore, associated with Ipomcea pes- 

 caprce, Tonrnefortia gnaphalodes and Opuntia sp. ^ 



EcoLOGic Factors. — The ecologic factors, which have influenced 

 the distribution of the typical sand strand plants of Bermuda, must 

 be referred to briefly. As the plants of the Bermuda sand beaches 

 and sand dunes in general show xerophytic adaptations, we must 

 look upon these adaptive arrangements as a response to the environ- 

 ment. The following environmental factors must be considered as 

 influential in producing the xerophytic structures which the leaves 

 of the Bermuda beach and dune plants especially show : 



1. The intense illumination from above is an important ecologic 

 factor. 



2. .The reflection of light from the white coral sand and the 

 foam-crested breakers beyond is important. 



3. The action of the strong winds that blow across the islands 

 must be considered as modifying plant structure. 



4. The action of the salt spray blown inland by the wind is 

 marked in the case of some plants. 



5. The permeability of the sand to water, so that after a rain the 

 surface layers quickly dry out, has its influence. 



The most potent factor in the modification of leaf structure has 

 been undoubtedly the bright illumination from above and below (by 

 reflection) and the physiologically dry condition of the soil. 



Structural Adaptations. — The leaf adaptations to light are 

 found in the increased number of palisade layers, their presence on 

 the upper and under sides of the leaves, and their arrangement, so 

 that the central part of the leaf becomes palisade tissue throughout, a 

 typical staurophyll. The depression of the stomata below the sur- 

 face, as in Sisyrinchium bcnnudianuin, the distribution of the stomata 

 in pits, as in Nerium oleander and Lantana invohicrata, the develop- 

 ment of hairs as in Tournefortia gnaphalodes, the varnished leaves 

 of Dodon<ra viscosa and thick epidermal layers and cuticle are all 

 arrangements to reduce transpiration. The succulency of the leaves 

 of some of the dune plants is developed perhaps for water storage 

 and the, presence of latex should be mentioned as a means by which 



