388 BOTANICAL SURVEY OF DISMAL SWAMP REGION. 



superficial layer of sand becomes greatly heated and very dry when 

 exposed to the sun, although the soil beneath remains always cool and 

 moist. 



3. Exposure 1o almost constant and often strong currents of air, 

 which keep the atmospheric envelope of the plant always changing, 

 hence never saturated, and thus stimulate transpiration. 



4. Presence in the soil of sodium chloride, in relatively large quan- 

 tity. As has already been remarked, it is only in that portion of the 

 sand strand which is very near the waves that sodium chloride exists 

 in quantity sufficient greatly to affect vegetation. 



Modifications of structure that probably serve the plant by pro- 

 tecting it against the excessive transpiration that the factors just 

 enumerated tend to induce are rather numerous in the sand-strand 

 vegetation. Some characters are almost certainly adaptations to this 

 end, while the value of others is more doubtful. As a rule it. is the 

 leaf structure rather than the whole form and habit of the plant that 

 is most obviously concerned. Among the most noteworthy peculiar- 

 ities and modifications are: 



(1) Those which effect a reduction of the transpiring surface. The 

 most important of these are: 



(a) Leaves small or narrow, as in Heliantliemum canadense, Lechea 

 maritime,, Salsola kali, Mollugo verticillata, Oenothera humifusa, 

 Diodia teres, Linaria canadensis, and especially Hudson ia tomentosa 

 and Sarothra yeuUanoides. The last two species have scale-like 

 leaves. 



(b) Leaves with the power of becoming conduplicate or involute 

 This character is conspicuous in many of the grasses, notably Panicum 

 amarum, Spar tina patens, Uniola panimdata, and AmmophUa arena- 

 ria, in which most of the stomata lie on the leaf surface thus pro- 

 tected, while the cuticle and epidermis walls are much more strongly 

 thickened on the exposed (dorsal) surface, which is hard and polished. 

 In Panicum and Ammophila the leaves are strongly involute on dry, 

 sunny days, but become nearly plane in wet weather. The leaf 

 margins of Quercus virginiana and of Rabus cuneifolius are some- 

 what revolute when the leaf is exposed to strong sunlight. This 

 serves in some degree to protect the dorsal (under) surface, in which 

 lie the stomata. 



(2) Posit ion of the leaves. These are nearly vertical in many of 

 the grass-like plants. In Smilax glauca, also, when exposed to strong 

 insolation, the leaves assume a nearly vertical position, thus opposing 

 the glaucous under surface to the light and giving the plant a very 

 characteristic appearance. The same phenomenon occurs, but in a 

 less degree, in Rubus cuneifolius, which similarly opposes its tomen- 

 tous lower leaf surface to a strong light. 



(',]) The surface protected by various outgrowths or modifications 

 of the epidermis. 



