446 BOTANICAL SURVEY OF DISMAL SWAMP REGION. 



num. keameyi and S. cititpidatinii plmnoxum forma serrata (often submersed): 

 Cctllitrirhe hcterophylla, Jmiciis repenn, and Isnardia palustris (sometimes 

 submersed). 

 2. Floating upon the surface. 



(a) Freely: Sptrodcla polyrhiza. 



(b) Attached to the bottom, uppermost leaves floating: 

 Flowers floating — (Uixtalia odovuta. Xytuplxtftt adreiia (usually). 

 Flowers usually emersed — Xelttmbo luted, Potamogeton Inttchiti'x. 

 Flowers submersed — ('allitriche heteraphyUa (usually). 



;{, Rising above the surface, attached to the bottom: Sparganium androcladum 

 and Myriophyllum heterophyllum (the uppermost flower-bearing portion of the 

 stem emersed). 



The general characteristics of aquatic plants are much the same the 

 world over, and have been so often described that a detailed account 

 of them here would be superfluous. 



The root system is usually comparatively little developed, since 

 water plants absorb most of their fluid nutriment directly through the 

 foliage. In the freely swimming forms the roots reach their minimum 

 development, being entirely aborted in some cases, while in larger 

 plants that are attached to the bottom there is a greater production 

 of mostly simple roots, which are perhaps chiefly used as holdfasts. 



The steins in the latter group usually creep at some distance over 

 the bottom as rhizomes, sending down roots into the soil before rising 

 toward the surface. Thai portion of the stem which ascends through 

 the water is most frequently slender and branching. The rootstock 

 is sometimes thickened and serves for the storage of reserve food, 

 especially in the Nyiuphaeaeeae. 



The submersed leaves are usually either elongated, narrow, and 

 flexuous (Philotria canadensis, J uncus repens, Sparganium andro- 

 cladum), or finely divided with filiform segments {Myriophyllum 

 heterophyllum, Utricularia spp.); and, in rtricularia, provided with 

 insectivorous bladders. They are always thin and delicate, wilting 

 rapidly when exposed to the air. The floating leaves are more often 

 broad (Potamoyeton lonchites, Xymphaea adrena), often orbicular, 

 or nearly so, ami peltate (Nelumbo, Castalia), and with an ombro- 

 phobous (water-shedding) upper surface. Peculiarly specialized are 

 the floating leaves of Utricularia inflata, mentioned above. 



In anatomical structure water plants are chiefly remarkable for the 

 feeble development of certain elements which are strongly developed 

 in most land plants. In the stem the mestome bundles are small, and 

 are grouped together in a central cylinder with the vessels few and 

 but slightly or not at all lignifled. Large laeunos, which probably 

 serve as an aerating apparatus, are present. Mechanical strengthen- 

 ing tissue is in most cases wanting. 



Submersed leaves usually have few or no stomata, chlorophyll tissue 

 of the spongy type (no palisade), epidermis without hairs, its cell 

 walls not thickened, the radial walls usually undulate and the cells 

 often containing chlorophyll. In short, there is a total lack of pro- 

 vision against excessive transpiration, and this explains the rapidity 



