3 ( ,K) BOTANICAL SURVEY OF DISMAL SWAMP REGION. 



glauca are partially, and those of Leaked maritima arc almost per- 

 fectly, isolateral. Tn Galium hispidulum the epidermis is nearly 

 alike on both faces (stomata present, but in relatively small number, 

 on the ventral face also), while the mesophyll is differentiated. lint 

 this is much more commonly the case in the Salt. Marsh formation, the 

 leaves of most plants of the Sand Strand being plagiotropic (more or 

 less nearly horizontal) in position and bifacial or dorsiventral in 

 structure. In two important points leaves of the latter type exhibit 

 adaptations that help to protect against excessive transpiration, first, 

 in the arrangement of the chlorophyll tissue, and second in that of the 

 stomata. 



(a) Chlorenehyma. In by far the greater number of plants of this 

 formal ion whose leaves are strongly bifacial (and such is the case 

 with almost all the dicotyledons) there is a sharp differentiation of the 

 chlorophyll tissue into palisade and pneumatic tissue. The first con- 

 sists normally of cells which are high (their diameter much greater at 

 right- angles to than parallel to the surface) and form a very compact 

 tissue, usually without intercellular spaces. In most of the plants 

 with which we are here concerned the palisade tissue forms a single 

 layer, but in some there are two or even three layers. It is widely 

 believed that this arrangement of the layer or layers of chlorophyll 

 tissue which lie nearest that surface of the leaf (the ventral) which is 

 exposed to the strongest light serves, among other purposes, 1 as a 

 means of diminishing transpiration. This compact, tissue prevents 

 the access to the more open tissue beneath of a large proportion of 

 the light and heat rays which strike the plant. 



The more open pneumatic tissue or spongy parenchyma which is 

 thus protected consists, usually, of nearly isodiametric cells. These 

 are often irregular in form and have their neighboring walls separated 

 by numerous air spaces. It is obvious that if such tissue lay directly 

 beneath the epidermis of the upper surface of the leaf, the loss of 

 water from that organ would be much greater. 



(I>) Stomata. Corresponding to the arrangement of the palisade 

 tissue upon the ventral or upper, and of the open pneumatic tissue 

 upon the dorsal or lower side of the leaf, in most of the strand plants 

 of this region the stomata are either all, or by far the greater number 

 of them, situated in the lower (dorsal) .surface. The pores, as well as 

 the air chambers into which they open, are thus shielded by the entire 

 thickness of the leaf from the direct access of the incident rays. This 

 is conspicuously true of the woody plants of the Sand Strand, e. g., 

 Smilax bona-tiox, Quercus virginiana, Myrica carolineusis, Qelsemium 

 semper vir ens y Lonicera sempervirens, etc. 



1 The controlling fnetor in the strong development of palisade tissue is, however, 

 intense light and the consequent opportunity for a great increase of assimilatory 

 activity. For a brief discussion of this question, which has been ably treated by 

 Stahl, Hinriches, Wagner, and others, see Haberloudt Pnauzenanat., ~te Aufi. 

 253-255. 



