i9o8.] OF THE SAND DUNE PLANTS OF BERMUDA. 101 



a dune plant is protected against the untoward influences of its 

 environment. 



Light has been most marked in influencing the development of 

 leaf structure displayed b}^ the typical sand dune plants of Bermuda. 

 The stimuli of light have called forth functional responses which 

 have produced changes in form or structure of the leaves, or in both. 

 The chlorenchyma, composed of chloroplast-bearing cells, is con- 

 verted into two kinds of tissues, palisade and spongy parenchyma, as 

 a direct result of the unequal illumination of the leaf surfaces. 

 Palisade tissue is formed as a response to light, or to low water 

 content, or to both. When both leaf surfaces are equally illumi- 

 nated, the leaf may be termed isophotic, when unequally illuminated 

 diphotic. Diphotic leaves which are unequally illuminated show a 

 division into palisade and spongy parenchyma, and such leaves are 

 called by Clements^ diphotophylls. Isophotic leaves, equally illumi- 

 nated on both surfaces have a more or less uniform chlorenchyma. 

 Clements divides such leaves into three types : ( i ) The palisade leaf, 

 or staurophyll in which the palisade tissue extends from the lower 

 to the upper epidermis. (2) The diplophyll, or double leaf, where 

 the intense light does not penetrate to the middle of the leaf. In 

 consequence, the upper and lower palisade layers are separated by a 

 central loose parenchyma, which is for water storage. (3) The 

 spongophyll, in which the rounded, loose parenchyma cells fill the 

 leaf without palisade tissue. The influence of the light and other 

 environmental conditions on leaf structure is perhaps best shown in 

 the thin and thick leaves of Conocarpus erectus produced on different 

 parts of the same tree differently related to the incident rays of 

 light. A detailed description of these structures for each plant will 

 be given at the end of the paper. The following is a classification 

 of different leaf structures and the plants which illustrate such 

 adaptive arrangements : 



Thick Cuticle. — Nerium oleander, Conocarpus erectus (thin 

 leaf), Sccevola Plnmieri. 



Thick Epidermis. — Canavalia ohtiisifolia, Dodoncea viscosa, Sisy- 

 rinchium hermiidiannm, Stenotaphrum americamim, Ipomcea pes- 



* Clements, F. E., " Research Methods in Ecology," 138-145 ; " Plant 

 Physiology and Ecolog>-," 171-184. 



