ADAPTATIONS OF THE AQUATIC PLANTS. 445 



ders, float just beneath the surface of the water, unattached to the 

 bottom, and send up into the air peduncles bearing numerous small 

 purple flowers. U. inflata, on the other hand, inhabits open ponds 

 outside the palustrine forest, where it is often accompanied by Calli- 

 triche het&rophylla. The inflated petioles of its uppermost whorl of 

 leaves form a small buoy, which floats upon the surface and enables 

 the summit of the stem, with its raceme of bright yellow flowers, to 

 develop outside the water. The submersed leaves are provided with 

 numerous bladders. 



The hepatic Rircia fluitans is a common plant of the region, often 

 growing terrestrially, even in moist corn fields, but also occurring in 

 an aquatic form, which floats just beneath the surface of ponds. 



Isnardia (Ludwigia) palustris and Juncus repen-sare likewise often 

 terrestrial marsh plants, but sometimes grow T in shallow water, where 

 only their uppermost leaves reach the surface. The difference between 

 the terrestrial and aquatic forms of Juncus repens is striking. 1 The 

 former has short creeping stems, with short internodes and leaves, 

 often makes a veritable sod, as on the margin of Lake Drummond, 

 and flowers profusely. The water form develops greatly elongated 

 internodes and longer leaves, and apparently does not produce flowers. 

 It sometimes grows in water 3 decimeters (1 foot) deep. 



The species of Sphagnum, <S'. euspidatwm plumosum forma serrata 

 and S. keameyi, already mentioned as associating with Utricularia 

 purpurea in the waters of the ditches in the Dismal Swamp, are there 

 the most abundant of the aquatic plants. Their stems, usually 3 to 

 6 decimeters (I to 2 feet) long, often become detached, and seem to 

 grow equally well when floating freely. Either they are wholly sub- 

 mersed or the uppermost portion isemersed. The foliage is very pale, 

 especially upon the submersed portion. 



Inhabiting the cold water of ditches in the heart of the Dismal 

 Swamp, where the amount of direct sunlight which reaches the surface 

 of the water is small or none, are Potamogeton lvnrJiif.es, with stems 

 rooting in the mud at the bottom and with its firm, rather large, 

 uppermost leaves floating, and Sparganium andrvcladum, likewise 

 rooted in the substratum, while the upper part of tin 1 stem, bearing 

 the uppermost leaves and the inflorescence, rises into the air. 



ADAPTATIONS TO ENVIRONMENT. 



The characteristic aquatic plants of the Dismal Swamp region may 

 be ecologically classified as follows: 3 

 t. Submersed: 



(a) Freely floating (near the surface): Utricularia spp. (flowering peduncles 

 emersed); Riccia fluitans. 



(b) Attached to the bottom: Philotria canadensis (always submersed); Sphag- 



1 See Holm, Bull. Torr. Bot. Club. vol. 26, p. 359 (1890). 



- Following roughly the classification used by Schenck, Biologie der Wasserge- 

 wachse. 



