282 THE PLANT COVERING OF OCRACOKE ISLAND. 



Stereome occurs subepidermally (especially in the leaf margins) in 

 the Gramineae only. In most of the species of the sand strand, how- 

 ever, it is found as a support to the mestome bundles. These are 

 furthermore reinforced by hj'podermal collenchyma or collenchymatic 

 tissue in most of the dicotyledonous species, this tissue probably serv- 

 ing as a protection against loss of water by evaporation from the 

 vessels. 



The sand-strand grasses deserve further mention with reference to 

 their leaf structure. It belongs to a type of which Chloris petraea 

 exhibits one extreme and Muhlenberg m filipes the other — the type 

 exhibited by most grasses of deserts and steppes. The salt marsh 

 Spartina stricta exhibits a wholly similar arrangement of tissues. 



The margins are more or less conduplicate or involute when the 

 supply of water is small, becoming flat when moisture is plentiful, 

 except in the leaf of Muhlenhergia filipes, which is conduplicate, with- 

 out power to unfold, and appears as if terete. The result of this 

 adaptation is that in dry, sunny weather only the dorsal leaf surface 

 is directly exposed to the air and light. In Panicum amarinn and 

 Chloris ]petraea the movement is effected by true bulliform cells, while 

 in the other grasses (except, of course, Muhlenbergia) the function is 

 probably performed by certain large but otherwise undifferentiated 

 cells of the epidermis, which may be regarded as undeveloped bulli- 

 form cells. 



The stomata lie near the bottom of deep longitudinal furrows and 

 usually occur more abundantly on the protected ventral surface of the 

 leaf, but in Chloris petraea only on the dorsal surface. The walls of 

 these furrows, in Muhlenbergia, Uniola, and Spartina patens, are lined 

 with unicellular, simple, prickle-like hairs, which doubtless hinder the 

 escape of moist air. 



Subepidermal groups of stereome occur in the leaves of all the 

 grasses and in the margins of all except the Muhlenbergia. In this 

 great development of strengthening tissue we have in all probability a 

 protection against the mechanical effects of the wind, to which strand 

 grasses are much exposed. 



The chlorophyll tissue is in every case radially arranged in single 

 layers around or at each side of each mestome bundle. In most cases 

 the adjacent cells of the parenchyma sheath also contain chlorophyll. 



Each mestome bundle is surrounded by a mestome sheath in all the 

 species except Uniola and Muhlenhergia, and this by a large-celled 

 parenchyma sheath. The parenchyma sheath without the mestome 

 sheath occurs in Muhlenbergia. 



Among the species of the salt marsh which were examined, the 

 isolateral type of leaf prevails, Kostelefzkya virginica and Lippia 

 nodiflora being the only exceptions, and in these the leaves are but 

 imperfectly bifacial. In Juncus roemerianus the leaf is terete. 

 Thick leaves are also the rule in this formation class, although less 



