FREEZING AND BURNING. 543 



The same thing occurs iu land-plants and lithophytes as with animals and 

 aquatic plants. Plants which closely resemble each other externally and show 

 great similarity in their anatomy may yet behave quite differently in the matter of 

 freezing. While the Stone Pine and the Shore Pine (Pinus Pinea and Halepensis) 

 cannot bear the frost of winter, the Arolla Pine and Bhotan Pine {Pinus Cembra 

 and excelsa) flourish in regions where the trunks and acicular leaves of all the 

 trees are cooled down for weeks to —20'. Rhododendron Pontieum freezes at 



— 2°, but Rhododendron Lapponicum survives the severest cold of the northern 

 wintei". If Eeheverias are brought out of the greenhouse on a cold autumn night 

 into an open place where the temperature falls to —1°, they will be irretrievably 

 lost; while most of the European succulent plants closely allied to the Eeheverias, 

 and agreeing with theni in the structure of the fleshy leaves, endure the same 

 degree of cold without injury — not only for a night but even for weeks. The 

 northem Sedwm Rhodiola and several Alpine species of house-leek growing on the 

 narrow ledges of rock faces in the high Alps (e.g. Sempervimum montanum and 

 Widfenii) are exposed for weeks to a temperature of —10°, and yet the protoplasm 

 of their fleshy leaves does not freeze. There are also a number of biennial and 

 perennial plants which cannot actually be called succulents, but which nevertheless 

 form smooth, turgid leaves in the autumn arranged in rosettes lying on the ground, 

 outwardly in no way protected against loss of heat. The leaves of these rosettes 

 are exposed to the greatest cold in regions where the winter is severe, especially 

 when little or no snow has fallen, and the temperature of the succulent tissue is 

 often cooled down to —20°, and yet the protoplasm is not killed. The Scurvy 

 Grass (Cochlearia officinalis) is, in this matter, particularly worthy of notice. 

 It would naturally be expected that its smooth, turgid, dark-green leaves would 

 be killed with the first hoar-frost, while in reality they endure a very consider- 

 able cold without the slightest injury. There are few places on the earth where 

 such a severe winter climate prevails as on the shores of Pitlekaj on the northem 

 coast of Siberia, where the Vega expedition passed the winter of 1878-79. In 

 November the mean temperature amounted to — 16"58°, in December to — 2280°, 

 in January to -26-06°, in February to -25-09°, in March to -21 65°, in April to 



— 18-93°. But these were only the averages; on many days the temperature fell to 



— 30°, and-40^ and once the minimum even reached —46° C. On the summit of 

 a fairly high sand-hill over which the icy north and north-east wind swept almost 

 uninterruptedly, a plant of Scurvy Grass {Cochlearia fenestralis) was observed. 

 This plant had begun to bloom in the summer of 1878, and had also partly 

 developed fruit. When the winter began, however, tliis Cochlearia still possessed 

 unripe fruits, flowers, and flower-buds as well as succulent green foliage-leaves; 

 and it was to be expected that the delicate succulent tissue would be completely 

 destroyed during the long winter under the influence of the continuous cold. But 

 in the summer of 1879 the plant, whose tissue had undoubtedly been cooled down 

 for a long time to — 30°, and frozen, began again to grow, and continued its growth 

 where it had been interrupted at the beginning of winter. The leaves resumed 



