430 



BOTANICAL SURVEY OF DISMAL SWAMP REGION. 



sitic forms, is directly subject. Likewise* the high temperatures which 

 prevail for many mouths in the region must affect all plants, but, of 

 course, those that grow in the shade much less than those exposed to the 

 direct sunlight, On the other hand, exposure to strong light is expe- 

 rienced in the deep forest only by the large trees, but in the more open 

 parts also by smaller plants. ■ This exposure to strong light and much 

 heat is partially responsible for ecu-tain characteristics of the leaves 

 of woody plants in the Dismal Swamp, which can be interpreted as 

 affording protection against excessive transpiration (possibly also 

 against, the injurious effect upon the chlorophyll of too intense light). 

 Peculiarities of swamp soil which are well known to cause a lowering 

 of the absorbing action of roots, and hence to reduce the plant's water 

 supply, are probably still more important causes of such modifications, 

 which are thus serviceable in compensating the restricted absorption 

 of water by diminishing the quantity that is transpired by the leaf 

 surfaces. These qualities of the soil are coldness, poverty in oxygen, 

 and acidity, and to them we must ascribe the seeming anomaly that 

 plants growing in shade in wet forests may possess certain peculiari- 

 ties of structure which are generally known as xerophytic, and are 

 likewise developed, albeit to a much higher degree, in plants of the 

 most arid and sun-baked deserts. 2 In other words, as has been well 

 expressed by Nchimpeiy 5 a physically very wet soil is not necessarily 

 physiologically wet, for the amount of water which the roots can take 

 up is by no means always proportionate to the amount present in the 

 substratum. 



ADAPTATIONS TO REDUCE TRANSPIRATION. 



An examination of the leaves of the woody plants which occur in 

 the Dismal Swamp shows that almost every species is in some way 

 equipped to reduce the amount of transpiration. Adaptations of the 

 following kinds may be mentioned: 



1. Position and general structure of the leaf,— In only one species 

 the parasitic Phoradendron fkwescens, were the leaves found to be iso- 

 lateral in structure and orthotopic (approximately vertical, or, in 

 other words, parallel to the direction of the light, rays) in position 

 It goes without, saying that, we have here an excellent protection 

 against excessive loss of water/ All other species examined (except- 



■To the intensity in this region of the light and heat^rays"of~the sun ^iiyone 

 who has been on Lake Drummond in a boat at noon on a midsummer day can 

 testily. Only on the sand dunes does one's skin burn more quickly and fiercely 



The ex 18 tence in the Hygrophile forest of conditions of environment which 

 induce xerophytic characters in vegetation, may account for the presence there of 

 J MW tueda, which is most abundant in the drier parts of the nonhy^rophile for- 

 est, and nourishes even amid the sand dunes. The converse fact that the bald 

 cypress (Taxodium) can be grown successfully when transplanted to a well-drained 

 upland soil is perhaps to be explained in the same way. 



:l Prlanzengeographie, p. t. 



* Moreover, the leaf of Phoradendron is very thick, indeed succulent, and the cuti- 

 cle is quite massive. 



