280 THE PLANT COVERING OF OCRACOKE ISLAND. 



4. Succulency. 



(a) Stem succulents: Opuntia jpes-corvi , Salicornialierhacea. 



(b) Leaf succulents: Fncca spp., Tissa marina, Sesuvium mar itimum, Euphor- 

 bia polygonifolia, Vincefoxicum palustre, Aster subidahis, A. flexuosus, Solidago 

 sempervirens — mostly salt-marsh species. Not only does the increase in thickness 

 of the leaf serve directly as a protection against excessive loss of water, but the 

 thickening tissue consists, in most cases, of colorless, water-storage parenchyma, 

 which is peculiarly tenacious of its water supply. 



5. Structure of the chlorenchyma. 



Nearly all the species, of both sand strand and salt marsh, are characterized by 

 the development of palisade, a compact chlorophyll tissue with cells more or less 

 elongated at a right ang-'.e to the surface and occupying the exposed face or faces — 

 i. e., the ventral face in bifacial leaves, both faces in such as are isolateral. Such 

 tissue is believed to have, among other functions, that of protecting against ex- 

 cessive loss of water the remainder of the leaf (the interior, or the lower face, as 

 the case may be) , which is usually occupied by less compact tissue. 



6. Aromatic, volatile oil. 



An oil of this character is secreted by the species of Myrica. It has been sug- 

 gested, although the idea needs substantiation, that the possession of such oils 

 affords protection againsr excessive loss of water by the formation about the plant 

 of an envelope, which is less pervious to heat rays than is ordinary air. At any 

 rate this is a frequent attribute of plants inhabiting very dry regions.' 



Not to be interpreted as affording protection against excessive 

 transpiration, yet perhaps largely dne to the inflnence of conditions 

 that necessitate such protection, is the develoinnent of short, rigid, 

 almost thornlike branches {IJex vomitoria) and of prickles and spines 

 (Smilax, Biihus trivialis, Opnntia, Zanthoxylum, leaf apices of the 

 species of Yucca). Probably the depauperate form assumed by some 

 of the woody species when growing on the beach is similarly explicable.^ 



Strong thickening of the under-ground parts for storage of reserve 

 food materials does not occur in many si)ecies. The only notable 

 cases detected were Smilax hona-nox (rootstocks with tuberous thick- 

 enings), Yucca spi^. (rootstocks large, fleshy), and KosteJetzkya vir- 

 gin ica (root stout, Avoody, vertical). 



ANATOMY. 



In almost all cases the histological structure of the leaf alone is 

 here considered, that being the organ which shows most plainly 

 adaptations to certain factors of the environment, notably those 

 wiiich affect transpiration. The general peculiarities of leaf anatomy 

 in the vegetation of the sand strand and of the salt marsh, respec- 

 tiveh^, are first enumerated, and the resemblances and differences of 

 plants of the two formation classes are liointed out. Several of the 

 more important species of each category', in all thirty-two, are then 



^Haberlandt, Pflanzenanat., p. 325. Volkens, Fl. ^gypt., p. 46. 



'^A like depressed habit is characteristic of shrubs growing above the limit of 

 trees in high latitudes and altitudes. It is usually attributed to exposure to strong, 

 dry winds, which is probably the chief factor in its development on the beach of 

 Ocracoke Island. 



