!66 HYDROPHYTES 



position and to prevent its being overturned and the same function is 

 performed by the submerged leaves of Salvinia. 



Propagation. The division of the vegetative organs plays an important 

 part in all cases. Not only algae, but also Pteridophyta like Azolla, and 

 Spermophyta such as Lemna, Hydrocharis, Stratiotes, multiply with 

 exceeding rapidity by division, and for this reason they are markedly social 

 and occur in great abundance. Vegetative parts serve as convenient 

 means of dispersal, for instance in Lemna ; the small shoots of Wolffia 

 brasiliensis are distributed by aquatic birds. Accordingly, the produc- 

 tion of spores and seeds is in a number of cases almost unknown or, as in 

 Lemna, very rare. 



Fertilization is necessarily connected with the water in Cryptogamia ; 

 moreover, a few Spermophyta including Ceratophyllum open their flowers 

 under water ; but the flowers of the others are developed in the air, and 

 are mainly entomophilous, as in Utricularia, Hottonia, and Hydrocharis. 

 The fruit is in most cases ripened under water. 



Hibernation and duration of life. Nearly all are perennial, as is true of 

 aquatic plants in general. Salvinia and many algae are annual. Flowering 

 plants often produce special bud-like winter-shoots hibernacula which 

 sink to the bottom in autumn J ; among such plants are Hydrocharis, Utricu- 

 laria, Aldrovanda, and Ceratophyllum ; or after the death of the older 

 shoots the younger ones, which are filled with reserve-food and do not yet 

 contain much air, sink and hibernate without further modification such 

 is the case with Lemna. Certain algae, Cladophora fracta for instance, 

 show similar behaviour, as they sink to the bottom in autumn, hibernate 

 in the form of thick-walled cells which have rich contents, and develop in 

 spring-time into new individuals. 2 



DISTRIBUTION AND ASSOCIATIONS 



As the most prominent species in this formation float about on the 

 surface of water, they are easily transported by wind and currents to quiet 

 spots, where they may be collected together in vast numbers, as may be 

 seen in the case of Lemna. Huber 3 gives some information in regard to 

 the floating islands in calm inlets of the Amazon ; these are often very 

 extensive, and are formed partly of pleuston for example, of Eichhornia 

 azurea but also of half-floating grasses which do not belong to this forma- 

 tion, also of other marsh-plants that have broken loose. In like manner 

 immense multitudes of Eichhornia crassipes occur in North American 

 rivers. 



This formation seems to be strictly confined to fresh water. 



Various associations may be distinguished according to the dominant 

 species (pontederietum, lemnetum, pistietum, and the like). Schroter 4 

 separates the emersed hydrocharids (for example, lemnetum) from submerged 

 types (including the associations, ceratophylletum, scenedesmetum, and 

 zygnemetum) as separate formations. 



1 SeeSchenck, 18866; Raunkiar, 1895^99. 

 * Wille, communicated by letter. 3 J. 



Huber, 1906. 4 Schroter, 1902. 



