8io ZONES AND REGIONS [Pt. Ill, Sect. V 



deriaceae. Among Pteridophyta, various Salviniaceae, Marsileaceae, and 

 Isoetaceae are represented in fresh water. Among Musci, Fontinalaceae and 

 Hypnaceae for the most part, and Sphagnaceae to a certain extent, display 

 a preference for an aquatic existence, as do Ricciaceae among the Hepa- 

 ticae. This list, however, does not in any way include all the groups of 

 higher plants represented in the fresh-water flora ; with the exception of 

 Gymnospermae, among which however swamp-plants are not lacking, most 

 of the larger groups of Phanerogamae exhibit some species of a more or 

 less aquatic habit. In this respect attention may be drawn to Hottonia, 

 Limnanthemum, Lobelia Dortmanna, Aldrovanda vesiculosa, and Bidens 

 Beckii. 



The structure and mode of life of the higher limnetic aquatic plants are 

 extremely diversified. Common to the majority of them is only the absence 

 of secondary growth in thickness, which occurs in Isoetes alone, and then 

 only in an anomalous form. In other respects we may distinguish according 

 to their structure and mode of life the following oecological types, which 

 are often connected by intermediate forms: — 



i. Isoetes type. Completely submerged rosette-plants, rooted in the 

 ground, usually with cylindrical leaves: Isoetes (Fig. 497, 1, 2), Pilularia, 

 Subularia (Fig. 497, 3), Littorella, and Lobelia Dortmanna. 



2. Nymphaea-Hippuris type. Plants rooted in the ground, reaching the sur- 

 face of the water by means of long-stalked leaves or long shoots, and therefore 

 partly in the air. To these belong the Nymphaeaceae, Limnanthemum 

 Marsilea, Trapa, Batrachium aquatile, Potamogeton natans, and other species 

 with floating leaves, Hippuris, Elatine Alsinastrum, and others, with the 

 ends of their shoots emerging from the water. 



3. Naias type. Plants rooted in the soil, or free-swimming, with long 

 flowing shoots. Not rooted — Ceratophyllum, Aldrovanda, and rootless 

 Utricularia. Rooted — Naias, Zannichellia, various species of Potamogeton 

 and Batrachium, and others. 



4. Hydrocharis type. Free-swimming plants with short; shoots, either 

 quite submerged (Lemna trisulca, Riccia fluitans), or with the greater part 

 submerged (Stratiotes) or half submerged (Salvinia), or with the greater 

 part floating on the surface (Hydrocharis, species of Lemna, Azolla, Riccia 

 natans), or with the greater part projecting above water (Pistia, Eichhornia). 



5. Podostcmon type. Submerged plants fixed to stones in running water 

 (Podostemaceae, various Musci). 



Plants rooted in the soil may be termed semi-aquatic, if they develop 

 no aquatic leaves or only temporary ones, and are otherwise terrestrial 

 plants, as for instance the common reed (Phragmites communis) and other 

 grasses, species of Scirpus, of Sparganium (Fig. 497, 4), of Alisma, and 

 others. Many free-swimming aquatic plants, such as Pistia, are indeed rather 

 subaerial than aquatic plants. As members of formations, however, their role 



