PROTECTION OF PLANT-HOUSES DURING COLD NIGHTS. 319 



or earth preserves vegetable matters in the fields from the inju- 

 rious influence of cold during severe winters. ^ 



When frames and such places are covered with snow, it 

 should be allowed to remain on till it melts away by the influ- 

 ence of the atmosphere. In like manner, trees and shrubs 

 should never have the snow drawn from their branches, during 

 snow storms, except where the branches are likely to be broken 

 down by the weight of snow lying upon them. Snow is not 

 only the best, but also the most natural, covering during the 

 winter months. 



* That the warmth of the soil acts as a protection to plants may be 

 easily understood. A plant is penetrated in all directions by innumera- 

 ble microscopic air-passages and chambers, so that there is a free com- 

 munication between its extremities. It may, therefore, be conceived 

 that, if, as necessarily happens, the air inside the plant is in motion, the 

 effect of warming the air in the roots will be to raise the temperature 

 of the whole individual, and the same is true of its fluids. Now, when 

 the temperature of the soil is raised to 50^ at noonda)'-, by the force of 

 the solar rays, it will retain a considerable part of that \varmth during 

 the night ; but the temperature of the air may fall to such a degree, that 

 the excitability of a plant would be too much and too suddenly impaired, 

 if it acquired the coldness of the medium surrounding it. This is pre- 

 vented by the warmth communicated to the general system, from the 

 soil through the roots, so that the lowering of the temperature of the 

 air by radiation during the night, is unable to affect plants injuriously 

 in consequence of the antagonist force exercised by the heated soil. 



