AQUATIC VEGETATION. 443 



Now, oxygen, as we know, is necessary to the foots, as to all other 

 organs of the plant, furnishing energy for the carrying on of their 

 vital processes. We have seen that the knees of the bald cypress 

 possibly supply the roots of that tree with oxygen which they obtain 

 from the air and transmit through their open, spongy cortex. A 

 similar function is probably performed in various herbaceous marsh 

 plants by the soft, open tissue which often forms a thickening near 

 the base of the stem (or rather, just above the surface of the water 

 or the watery soil) to which Sehenck 1 has given the name aerenchyma. 

 This spongy, aerating tissue, usually bright white in color, is 

 developed in the cortex ami soon ruptures the epidermis, usually 

 first breaking through longitudinal fissures. Although analogous to 

 cork in its point of origin true aerenchyma consists of delicate 

 parenchyma with thin, nonsuberized walls, and contains, when fully 

 developed, numerous large intercellular spaces. It is particularly 

 noticeable in this region in species of the three related orders, 

 Lythraceae (Decodon verticiUatus) , a ]\Ielastomaceae (RTiexia virgin- 

 ica), and Onagraceae (Ludwigia linearis). South of the Dismal 

 Swamp region, in the latitude of Cape Hatteras, occur other species 

 which develop aerenchyma, i. e., Bhe.ria glabella, Ludwigia jnlona, 

 and I j. glandidosa. In Lwhuigia linearis, however, most of the 

 thickening tissue, at least toward the end of the growing season, and 

 in L. pilosa all of it above the very base of the stem, is suberized, 

 and therefore not true aerenchyma. 



AQUATIC VEGETATION. 



- The true water plants of the region were not studied with the same 

 care as was bestowed upon other formations, because with this part 

 of the plant covering the problem which was kept chiefly in view 

 during the progress of the survey does not especially concern itself, 

 and because an exhaustive ecological investigation of the aquatic 

 flora of a region demands much more time than could be devoted to 

 it in this case. Consequently only a superficial description of this 

 interesting formation is possible here, and the lower forms, algae, 

 etc., are not treated. 



This, unlike most of the other formations, presents no clearly defined 

 associations corresponding to topographical conditions. On the con- 

 trary, the distribution of aquatic plants seems to be largely deter- 

 mined by the accidents of dissemination, so that in this pond one 

 species or life form, in that another, may happen to predominate. 

 Theref ore it has seemed most expedient to present a general view of 

 the aquatic vegetation of the entire region without attempting to sub- 



'Uber das Aerenchym, ein dem Kork homologes Gewebe oei Smnpfpflanzen; 

 Jahrb. fur wissenschaft. Botanik, vol. 20, pp. 526 to 57 -t, t. S3 to C J8 (1889). Also 

 Goebel, Wasserpflanzen : Biolog. Schild. Theil 2. pp. 259 to 266 (1891). 



-See J. Schrenk, Bull. Torr. Bot. Club. vol. Hi, pp. 315 to 328, pis. 95 to 97 ( 1889), 



