THE NATURAL CONDITIONS OF PLANTS. 67 
fall to 70° below zero ; but that, if wind arise 
although the thermometer rise rapidly with it, 
the cold then becomes insupportable. These 
same voyagers acquaint us with an interesting 
fact, illustrating the truth of the old saying, that 
there is nothing new under the sun. Even the 
closed cases are as old as the creation. We are 
told, that the snow itself affords shelter to the 
productions of those inhospitable regions against 
the piercing winds that sweep over fields of ever- 
lasting ice. Under the cold defence of the snow 
plants spring up, dissolve the snow a few inches 
round, and the part above being again quickly 
frozen into a transparent sheet of ice, admits 
the sun’s rays which warm and cherish the 
plant in this natural hothouse, until the returning 
summer renders such protection unnecessary. 
I need not, however, go to the Pole for illus- 
trations of the effect of disturbed atmosphere in 
cold weather. One of our poets has said, — 
64 And, with east winds, will teach you how to shave.” 
All are fa m iliar with our cutting March winds, 
which are so injurious and destructive to vegeta- 
tion in the open air, but have no effect upon 
enclosed plants. With respect to change of 
air, the plants obtain all the change which they 
require, by virtue of the diffusion law already 
