PROVISIONS FOR CONSERVING WATER. 281 



taken up and described in systematic order. Tables showing what 

 are believed to be the characters that are most important from an 

 ecological point of view have been prepared for the two groups of 

 sj)ecies. For a number of species the material studied was not 

 obtained upon Ocracoke Island, but from similar situations on the 

 coast of Virginia, and in all such cases the source of the specimens 

 used is mentioned. In some cases comparisons are made with related 

 species, usually from other formations, in order to make clear the 

 differential characters of the strand species. 



In a great majority of the sand strand plants the leaf is bifacial, 

 the two species of Yucca being the only exceptions noted. In some 

 species this specialization is imperfect, as in Oenothera humifusa. In 

 other cases the differentiation of the two sides of the leaf is com- 

 plete, as in Quercus virginiana. In most cases the leaf is thick as 

 compared with the same organ in related nonmaritime species. Good 

 examples are the evergreen, leathery leaves of Quercus virginiana 

 and Ilex vomitoria, as well as the leaves of the two grasses, Uniola 

 paniculata and Panicum amarutn. A strongly thickened cuticle is 

 an almost invariable character, and this is conspicuously wrinkled in 

 a few species. The lateral walls of the epidermis cells are undulate 

 in four species, viz, the comparatively thin-leaved Clitoris petraea, 

 Teucrium nashii, and Physalis viscosu, and the thick-leaved Ilex 

 vomitoria. 



Half of the species have stomata on both leaf surfaces, but in every 

 such case they are especially protected — in the grasses by being sit- 

 uated in furrows; in the species of Yucca by being deeply sunken, 

 and in Physalis, Oenothera, and Croton by a covering of hairs. In 

 the woody species they are always on the dorsal or lower surface only, 

 and in Quercus virginiana they are further protected by a hairy cov- 

 ering (as also in the herbaceous Teucrium nashii). 



Hairs form a dense, protective covering on both leaf surfaces of 

 Oenothera, Croton, and Physalis, which species have stomata on both 

 surfaces: only on the dorsal surface in Quercus and Teucrium, agree- 

 ing with the position of the stomata on that surface only. In Quercus 

 and Croton the hairs are pericellular and stellate; in Physalis they 

 are irregularly branched; in Oenothera and Teucrium they are elon- 

 gated, unbranched, and unicellular. Teucrium also possesses short, 

 glandular, capitate hairs. 



The chlorophyll tissue is homogeneous in the monocotyledons of 

 the sand strand, while in the dicotyledons it is more or less differen- 

 tiated into palisade on the ventral side of the leaf and pneumatic 

 tissue on the dorsal side. The palisade is mostly quite compact, but 

 never of more than 3 and usually of only 1 or 2 layers. 



Colorless parenchyma, which probably performs the function of 

 water-storage tissue, occurs in considerable quantity only in the 

 grasses and the species of Yucca. 



