i64 HYDROPHYTES sect, iv 



water whose temperature scarcely exceeds o C. In many places the 

 water melted during the day-time freezes every night ; and thus they pass 

 their life in ice and icy water.^ But in yet another respect the snow-alga 

 is remarkably resistant ; it can preserve its existence in a dry condition 

 even though exposed for months to relatively high temperatures.^ The 

 same is true of certain animals living in snow. 



CHAPTER XL. HYDROCHARID-FORMATION OR PLEUSTON 



On the banks of stretches of fresh water, in places protected from 

 currents or from the violence of waves (for example among swamp-plants), 

 and in small ditches and puddles, there occurs a type of vegetation which 

 is not fixed but is free-swimming, or rarely even in part free-floating like 

 true plankton, of whose organisms it may contain an admixture ; never- 

 theless this type of vegetation differs so essentially from plankton that 

 it must be regarded as a special formation {megaplankton). From 

 plankton it is distinguished by two features : 



1. The occurrence of quite other growth-forms, namely megaphytes, 

 including Spermophyta, Hydropterideae, and mosses. 



2. The occurrence of algae belonging to groups entirely different 

 from those in plankton. 



The growth-forms in the two cases are however very different, and 

 for this reason the two formations are at least theoretically to be placed 

 apart. Kirchner in 1896 employed the term ' pleuston ' to denote the 

 real typical representatives of the hydrocharid-formation, as he laid 

 stress on their ' saihng ' character,^ with which is also associated their 

 adaptation to existence in contact with air (transpiration and the like). 

 But in 1902 Schroter adopted the course, which we follow here, of includ- 

 ing in the hydrocharid-vegetation such rootless, free-floating, submerged 

 Spermophyta as Ceratophyllum, Utricularia vulgaris, Lemna trisulca, 

 and other species. 



Moreover, with perfect justice he distinguishes between the constant 

 and the temporary floating-flora. In the temporary flora he includes 

 the masses of algae swimming at spring-time. These are more especially 

 composed of Conjugatae, including Zygnema, Spirogyra, Mougeotia, but 

 also of Oedogonieae, which ascend in great quantities and remain at the 

 surface of the water, whither they are raised by bubbles of gas, as we 

 have already explained. 



FLORA 



The constant representatives of the hydrocharid-formation belong 

 to the following groups : 



Bryophyta, namely Riccia (with both submerged and swimming 

 species), Amblystegium giganteum, and others. In pools on heaths or 

 in pockets on heath-bogs one often finds vegetation very poor in species, 

 and consisting of floating Sphagnum which nearly fills the water. 



See p. 22. ' ^ Wittrock, 1883. 



ttXuv, to sail; see Schroter und Kirchner, 1 896-1902. 



