Researches in Vegetable Physiology . 153 



Witli regard to the degree of cold which living plants are 

 able to support, M. Goeppert of Breslau* calls attention to the 

 fact that the lowest temperatures ascertained in the polar 

 regions ( — 40° to — 52°"6 F.) only relate to a very restricted 

 number of plants. Those whose stem is not sufficiently high 

 to pass the layer of snow are under very different conditions. 

 Sheltered under a screen which is a bad conductor of heat, 

 these plants are subjected to a temperature which hardly falls 

 below 28°'4 F. But if the snow protects them from too sharp 

 a cold, and becomes the indispensable preserver of plants in 

 high latitudes and on the mountains, their development is 

 none the less arrested. The plants best known as flowering 

 in winter, Hellehorus foetidits and niger and Bellis lyerenyiis^ 

 cease growing as soon as the temperature is low ; they merely 

 do not suffer from frost : a half-opened flower may be com- 

 pletely stiffened by the cold for several days ; but as soon as 

 the thaw comes, it resumes its development. 



In our latitudes, the heat of sunnner, by heating the soil, 

 may exert a certain influence upon winter vegetation. In the 

 arctic regions this is not the case : the soil, always frozen, 

 does not retain any heat ; all must come from the sun ; and 

 it is thus that we sometimes see plants (willows, rhododen- 

 drons) frozen in their lower parts, and bearing at the extremi- 

 ties of their branches leaves and expanded flowers. 



It must not be supposed that a plant because it is frozen is 

 for this reason protected from the deleterious influence of a 

 sharper cold. Each species can bear a certain diminution of 

 temperature : some may, without injury, be completely frozen 

 and afterwards thawed; but for each there exists a certain 

 minimum which cannot be passed without producing fatal 

 consequences. However, M. Goeppert, who has devoted him- 

 self for many years to the study of the relations of temperature 

 and vegetation, gives us hopes of more ample details upon 

 this curious subject. 



It has often been asked, at Avhat moment do frozen cells 

 perish ? at their freezing or their thawing ? It is difficult to 

 give an answer to this ; and direct experiments are almost im- 

 possible. It is evident that all the cells which may freeze or 

 thaw several times without injury only perish when the thaw- 

 ing takes place under unfavourable circumstances ; it is a 

 well-known matter of experience that if, after a cold night, the 

 temperature rises gradually and the sky remains cloudy, many 

 plants, even young delicate shoots, recover perfectly. If, on 

 the contrary, the sun causes too rapid a thaw, the evil acquires 



* Botan. Zeit. 1871, Ts^os. 4 & o. 

 Ann. & Mag. N. Hist. Ser. 4. Vol. ix. 11 



