284 THE PLANT COVERING OF OCRACOKE ISLAM). 



tome bundle is provided with a well-marked mestome sheath and, 



outside that, a parenchyma sheath. 



When we compare the species belonging to the two formal ion 

 classes, sand-strand and salt-marsh, Ave find that a majority of both 

 have several characters in common, all of which are distinctly xero- 

 phytic and are usually interpreted as protecting the leaf against 

 excessive transpiration as well as the effects of too intense light. 

 These are: Thickened leaves, thickened cuticle, and development of 

 the chlorophyll tissue as compact palisade on the most exposed sur- 

 face or surfaces. 



More numerous, however, are the differential characters. The 

 leaves of the sand-strand species are usually bifacial, with stomata 

 only on the dorsal surface or, if on both surfaces, protected by a hairy 

 covering or lying in deep furrows; and the palisade is situated oil 

 the more exposed upper or ventral side of the leaf. The salt-marsh 

 species, on the other hand, have mostly isolateral leaves, vertical or 

 nearly vertical in position, with stomata and palisade on both sur- 

 faces, and (with one exception) lacking the dense hairy covering. 

 Conformably, the most common grass of the salt marsh, Spartina 

 stncta, otherwise so similar in leaf structure to the dune form of 

 S. patens, has no hairs lining its stomatal furrows. The cuticle is 

 wrinkled or warty in many more salt-marsh than sand-strand species. 

 Water-storage parenchyma, which is notably developed in the sand- 

 strand vegetation only in the monocotyledons, is present in a majority 

 of the salt-marsh plants of the most diverse relationship. 



Corresponding to their growth in usually open formation, and con- 

 sequently greater exposure to the wind, the sand-strand plants show 

 a much stronger development of stereome than do the salt-marsh 

 species. On the other hand, the latter are better provided with hypo- 

 dermal collenchyma, or collenchymatic tissue, opposite the veins; but 

 this may be more important as a protection against loss of water 

 than as a mechanical strengthening tissue. 



It should be emphasized that not only the peculiarities common to 

 plants of the two formation classes, but likewise most of their respec- 

 tive differential characters, are really of a xerophytic nature. 1 In 

 some cases, however, a different means has been employed by sand- 

 strand species on the one hand ami by salt-marsh species on the 

 other to secure the same end— protection against excessive transpi- 

 ration and the harmful action of too intense light. 



'In his most recent paper on the subject. Warming (Halofyt-Studier, p. 235) 

 writes: "It is not possible, from the investigations here described, to draw any 

 clear distinction between characters which are truly xerophytic and such as are 

 truly halophytic, if any really exist." Schimper (Pflunzengeogr.. p. 9'J) also holds 

 that haiophytes can not be distinguished as a class from xerophytes, since the 

 principal object of the peculiarities of structure observable in plants growing in 

 saline soil, however moist, is the reduction of transpiration, just as it is in plants 

 surrounded by a physically very dry soil and atmosphere. 



