CHAP. XLII BENTHOS OF LOOSE SOIL 183 



the exclusion of transpiration. Finely divided leaves are well suited to 

 the medium, because their surface is thus relatively increased, and 

 consequently the absorption of food-material and of oxygen is facilitated. 

 The movements of the water would hardly permit of larger surface. 



Reproduction. The reproduction of the Cryptogamia takes place under 

 water. But nearly all the Spermophyta thrust their flowers above 

 water ; some, including Hottonia and Nymphaeaceae, are entomo- 

 philous, but others, represented by Hippuris, Myriophyllum and Pota- 

 mogeton, are pollinated by the aid of wind or water, or adopt self-pollina- 

 tion. The pollen is conveyed by the water in Zannichellia, Callitriche, 

 and Naias ; while semi-cleistogamy is enacted under water by Subularia 

 aquatica, Limosella aquatica, Euryale ferox, Elisma natans, and rarely 

 by Batrachium. Peculiar behaviour, paralleled by that of Ruppia 

 spiralis, is displayed by Vallisneria, whose small male flowers break loose, 

 swim on the surface and pollinate the stigmas of the female flowers which 

 rest on the water ; nearest in this respect to Vallisneria stands Elodea. 1 



After pollination, the developing fruits of many species, including 

 Trapa and Ranunculus, are dragged or curled down under water where 

 they ripen. Dispersal of the seed is often accomplished by special means 

 which are suited to the medium : the seeds or fruits of many species, 

 owing to their peculiar structure, are lighter than the water, and are 

 thus conveyed to other habitats. 2 



Vegetative Propagation. Propagation by vegetative means is very 

 widespread among all aquatic plants, and very easily takes place by mere 

 separation of fragments of the shoot ; it is of profound biological signifi- 

 cance, and some species have almost ceased to reproduce sexually. 1 Calla 

 palustris has special buds that easily become detached. 3 The rapid 

 spread of Elodea and the inconceivable number of its individuals in 

 Europe result from vegetative multiplication, as it produces no seed 

 because the female plant rarely occurs in Europe. The great power of 

 vegetative multiplication is to be ascribed to the production of propagative 

 buds, to branching, and to the facile manufacture of adventitious roots. 



Hibernation. 1 Most of the species hibernate as green plants at the 

 bottom of the water, where thermal conditions are not so extreme as in 

 air ; such is the case with Callitriche, Zannichellia, Nymphaeaceae, 

 Vallisneria, and others. Hibernating organs of a special kind, which 

 become detached from the decaying shoot, are represented by the cartila- 

 ginous winter-shoots (' hibernacula ') of Potamogeton crispus and other 

 species, 4 the spherical buds of Myriophyllum and species of Utricularia 

 which include densely packed leaves, also by the gemmae (buds) of 

 Hydrilla and Elodea. 



FORMATIONS, AND DISTRIBUTION 



A great number of associations occur in the limnaea-formation, and 

 some of these, like those composed of Cryptogamia, might perhaps be 

 regarded as being themselves complete formations or sub-formations. The 

 common zonally arranged associations are evidently identical over wide 

 areas in temperate countries. 



In the deepest parts of some lakes in Europe there occurs a zone of 



1 Schenck, 18866. 2 Ravn, 1894. ' J. Erikson, 1895. 



4 Sauvageau, 1888-94; Raunkiar, 1895. 



