i8o HYDROPHYTES sect, iv 



> 

 sea. The chief distinction in shape concerns the occurrence not only 

 of completely submerged types, but also of species with floating leaves, 

 or with shoots floating on the surface. 



All the species are herbaceous, and nearly all are perennial. 



The Shoot. The construction of the shoot varies widely. In agree- 

 ment with the loose nature of the soil, the vast majority possess creeping 

 axes, and therefore display social growth : such is the case with Pota- 

 mogeton, Hippuris, Nymphaea, Nuphar, all of which have subterranean 

 horizontal stems ; and Myriophyllum, Ranunculus, Callitriche, which 

 have epigeous creeping stems ; the Characeae also belong here mutatis 

 mutandis. Others, such as Littorella and Vallisneria, emit long runners, 

 and on these, at certain distances from the mother-plant, they produce 

 rosette-shoots which become firmly rooted. All such species can give 

 rise to extensive and dense associations which clothe the lake-bed, being 

 rich in individuals though poor in species. A minority of species, including 

 Isoetes and Lobeha, have also vertical rhizomes with the leaves separated 

 by short internodes and ranged in a rosette : but these are devoid of the 

 above-mentioned methods of migration, and are rather represented by 

 isolated individuals. 



Finally, there is a small number of annual species, such as Subularia, 

 Naias, and Trapa, which grow socially only when numerous seeds are 

 strewn over the same ground. 



There are three essentially different forms of assimilatory shoots,, 

 namely : 



A. Rosette-type. The shoots are vertical, short, unbranched, with short 

 internodes ; the leaves are in rosettes, sessile, and mostly submerged. 

 Such is the case in Vallisneria with ribbon-like leaves, Isoetes, Lobeha 

 Dortmanna, Subularia, and Littorella unifiora with more terete leaves. 



B. Nymphaea-type. The shoots assume the form described for the 

 rosette-type, or are horizontal rhizomes creeping in the soil ; there are, 

 however, long-stalked floating leaves. This type is represented by 

 Nymphaeaceae. 



C. Long-stemmed type. From a rhizome or a stem creeping over 

 the ground there arise under water erect stems that have long internodes, 

 and are slender and branched ; the main and lateral axes, as a rule, 

 are of equal thickness, showing no secondary thickening, as is precisely 

 the case with certain plants described in Chapter XL. The stems, which 

 are often very long and slender, are extremely flexible and can yield to 

 movements of the water. Their length depends upon the depth and flow 

 of the water. Terrestrial forms of the same species have shorter inter- 

 nodes. These shoots are of two kinds : 



a. Completely submerged, as in Potamogeton pectinatus, P. perfoliatus, 

 some species of Batrachium, Myriophyllum, Zannichelha, Callitriche autum- 

 naUs, Elodea and Naias (annual). The leaves are linear or oblong (only 

 rarely broad), and in some are very finely segmented. 



b. Possessed of floating leaves in addition to submerged leaves, the 

 former leaves sometimes being arranged more or less in a rosette at the 

 end of a condensed shoot, and then sometimes having tolerably short 

 petioles. As examples may be cited Callitriche verna, Trapa (annual), 

 the majority of species of Batrachium, Potamogeton natans, and EUsma 

 natans. 



