FRESHWATER ALG^E OF THE SOUTH ORKNEYS. 101 



kind of fat present in the various members of the yellow snow flora. The occurrence of 

 the fat and the associated yellow pigment is therefore a very characteristic feature of 

 this algal association. 



The enormous prevalence of fat in the members of the yellow snow flora is probably 

 to be regarded as an adaptation against the intense cold of the habitat. It has been 

 shown that in a number of trees (birch, conifers) growing in cold localities the starch is 

 changed into fat in the autumn, 1 and in referring to this fact Warming 2 remarks: 

 ' This is probably of use, in that fatty oil in the form of emulsion prevents sub-cooling, 

 and increases the power of resistance to frost." This view is certainly much strengthened 

 by the discovery of the yellow snow with its striking characteristic of fat-storage. A 

 similar abundance of fat does not appear to have been recorded hitherto in any snow- 

 flora. 3 The fact that the few reproductive stages that were observed in the yellow 

 snow material were generally quite deficient in fat also indicates that the latter possibly 

 functions as a reserve, to be utilised when outside conditions admit of cell-division or 

 other methods of reproduction taking place. 



In most cases the organisms found on snow and ice contain some characteristic 

 pigment (red hgematochrome in the case of Chlamydomonas (formerly Sph&rella) 

 nivalis (Sommerf.), Wille ; violet or purple phycoporphyrin in the case of Ancy- 

 lonema Nordenskioldii, Berggr.), although green snow (due to filamentous algse, 

 desmids, and cyanophycese) has also been described. Similar pigments are found 

 in members of many other types of algal floras, but there is certainly a prevalence 

 of pigmentation in forms occurring on the snow. It has been pointed out that 

 these pigments are capable of absorbing the heat-rays of the sun, 4 and it is also 

 possible that they may serve to screen the chlorophyll during the long summer. 5 

 It is difficult to express an opinion on the function of the yellow pigment colour- 

 ing the organisms of the yellow snow, but it is possible that it acts in one or both 

 of these ways. In support of this it is noticeable that the Protoderma, which 

 presumably forms a sheet on which the other forms settle down, is on the whole 

 very poor in fat and pigment, perhaps because it is screened by the overlying forms 

 from the light. 



We may next proceed to consider the various organisms that are distinguishable in 

 the yellow snow flora. Apart from a number of resting-stages, which could not be 



also took on the typical red stain with tincture of alkanna. On the other hand, even prolonged treatment (36 hours) 

 with equal parts of concentrated caustic potash and ammonia solutions, although it made the fat more transparent, 

 failed to bring about proper saponification. 



1 A. Fischer, " Beitrage zur Physiologic der Holzgewiichse," Pringsheim's Jahrb., xxii., 1891 ; 0. O. Petersen, 

 "Stivelsen hos vore Troer under Vinterhvilen," Danske Vid. SelsL Oversigt, 1896 ; cf. also Schimper, Plant Geography 

 (Eng. trans.), Oxford, 1903, p. 41. 



2 Warming, (Ecology of Plants (Eng. trans, by P. Groom), Oxford, 1909, p. 23. 



3 Ibid., p. 163. 



4 '/. V. B. Wittrock, " Die Flora des Schnees und des Eises besonders in den arktischen Gegenden," Hot. Centralbl., 

 xiv., 1883, p. 159 ; Warming, loc. cit., p. 163. 



6 Cf. Chodat, Algues vertes de la Suisse, Berne, 1902, p. 74. Hsematochrome certainly acts as a screen to the 

 chlorophyll in other algse (e.g. Trentepohlia) ; cf. Oltmanns, Morph. u. Biol. d. Algen., ii., Jena, 1905, p. 200. 



