490 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



though, as it Hves on or near the surface, it is primarily affected by 

 changes occurring there. Thus variations in sah-content and the pH 

 level of the surface snow or ice and of any liquid water will influence 

 the composition of the vegetation, and so will the nature of the 

 surrounding rock from which inorganic salts are obtained. For 

 substances from wind-borne dust, including particles resulting from 

 erosion, dissolve in any surface moisture after these particles alight 

 on snow or ice and serve as the source of minerals for microorganisms. 

 Consequently snow-fields and glaciers in the vicinity of acidic rocks, 

 for example, are apt to support very different vegetation from those 

 in limestone districts. In general, acidic environments support red 

 or pink snows and basic ones yield green snows. Red snows, 

 coloured by various organisms, are found in snow-fields practically 

 the world over ; the much rarer green and yellow snows occur 

 chiefly in the Arctic and in Europe, although they have been re- 

 ported also from the United States. 



It is perhaps best to refer to the plants growing on snow or 

 ice as ' cryophytes ', and the communities they form as cryo- 

 phytic, for they are scarcely planktonic (that is, free-floating) in 

 such ' habitats '. These cryophytes may be usefully classified 

 according to their preferred environments as growing (i) on ice 

 — e.g. Mesotaenium berggrenii ; (2) on snow and neve (Jim) — e.g. 

 Chlamydomonas nivalis ; (3) on both snow and ice — e.g. Cylindrocystis 

 hrehissonii ; and (4) occuring on snow and ice but only after trans- 

 portation from their normal habitats — e.g. various Cyanophyceae in- 

 cluding species of Gloeocapsa. 



Although often a single cryophyte predominates in a particular 

 community, sometimes giving a distinctive colour to the surface of 

 ice as well as snow, usually others are present, the mixture sometimes 

 including a dozen or more species, and, in addition, such animals 

 as Snow-fleas. Dispersal appears to be mainly by wind. Whereas 

 the colour and texture produced vary with the organism and other 

 circumstances, ' red snow ' commonly appears in spots scattered over 

 the surface, often involving wide areas, though sometimes the 

 colonizing is more uniform and extends to a depth of 3 or even 

 5 cm. The ' bloom ' developed by organisms growing on ice may 

 also extend for miles, as in the purplish-brown form on the largely 

 snow-free glaciers of southern Alaska and Greenland. This is char- 

 acterized by filaments of the Alga Ancyclonema nordenskioldii, which 

 form bunches up to 2 mm. in diameter on the surface of the ice 

 but chiefly in small hollows formed by its melting. In addition to 



