38 THE FACTORS [Part I 



2. ZERO POINTS OF PLANT-LIFE. 



i. LOWER ZERO POINTS OF PLANT-LIFE. 



The different capacity of different species to withstand low temperatures 

 is well known. Thus Molisch found that a number of tropical plants died 

 of cold at temperatures of + 2° to +5°C, while, on the other hand, the 

 flora of Yakutsk and Verkhoyansk includes a couple of hundred plants that 

 can withstand — 6o° C. In fact, different plants appear capable of supporting 

 the freezing of their cell-sap to very different extents, so that there is 

 a strong selective significance attached to temperatures slightly less than 

 o° C. In the case of tropical plants, freezing generally means freezing 

 to death, whilst the plants of the temperate and the cold zones, at any 

 rate perennials, may be frozen into lumps of ice without dying. In such 

 cases, thawing is more dangerous than freezing, as, if it proceeds too 

 rapidly, it kills more plants or plant-parts than the most intense cold. 



Only in a few areas are the climatic conditions such that the vegetation must be 

 able to endure without injury a repeated succession of frosts and rapid thaws. 

 Thus Kihlman described 'the extraordinary power of withstanding severe and 

 rapid oscillations of temperature and thus passing the freezing-point several times 

 within twenty-four hours, as the chief characteristic ' of the stunted vegetation of 

 the tundras in Russian Lapland. Similar conditions prevail in very high mountain 

 regions. So alpine plants above the level of perpetual snow in the Alps, for instance 

 Ranunculus glacialis and Gentiana nivalis, whilst in full blossom, pass the nights 

 in a completely frozen state, and during daytime are exposed to the most intense 

 insolation. 



Microscopic observation of frozen plant-parts shows that the intercellular 

 spaces, normally filled with air, contain ice-crystals, which were formed at 

 the expense of the cell-sap of neighbouring cells. The loss of water thus 

 occasioned, in very many cases, may be regarded as the cause of death, since, 

 as Muller-Thurgau has shown, it would also cause death if the tempera- 

 ture were favourable. But apart from this, as may be inferred from Molisch's 

 observations, cold exerts a directly injurious influence on the protoplasm. 



Those parts of plants that contain little water, as well as plants that 

 can endure great drought without injury, are particularly endowed with 

 the power of withstanding cold. Thus, in some experiments made by 

 C. de Candolle and R. Pictet, certain dry seeds were exposed to a tempera- 

 ture of — 8o°C. without any injury to their germinating power, whereas seeds 

 swollen in water were killed by temperatures much nearer to the freezing- 

 point. Spores of fungi and of other cryptogams also show a similar power 

 of resistance to cold ; so do plants whose vegetative organs can withstand 

 a high degree of desiccation without injury. 



Death from cold is undoubtedly in very many cases a result of the 



