84 PLANT LIFE ON THE FARM. 



same effects as actual frost would do. The cause of these 

 movements is due to a swelling or turgescence and a con- 

 sequent growth first on one side and then on another side. 



Action of Heat and Moisture upon Leaves. But 



little beyond what has already been mentioned need be said 

 upon the relation of heat and moisture to leaves. A few 

 words upon the influence of excessive temperatures may, 

 however, here be appropriately given. 



If the temperature fall below a given point, variable for 

 each species, and also for each individual plant, the 

 functions of the leaf are held in abeyance, chlorophyll is 

 only imperfectly formed (hence the yellow tinge of frosted 

 wheat) ; and if the temperature be still further depressed 

 death results. 



Action of Frost When a leaf is frozen the fluid con- 

 tents escape from the cells by permeation through their mem- 

 brane, and freeze on the outside of the cell, so that the 

 spaces between them are full of ice. It rarely happens 

 that the juices of the cells freeze in the interior of the cells 

 if they do, rupture of the cell wall and death are the most 

 probable results. Under ordinary circumstances the cells 

 lose that turgescence which, as has been stated, is necessary 

 for their activity. All the functions of life are arrested, 

 not necessarily never to be resumed, for, in some cases, 

 when the ice in the tissues of the plant melts, the water is 

 re-absorbed by the membrane, and life action is resumed. 

 Winter wheat must frequently become frozen in this 

 manner, but it is comparatively rarely that the plant is 

 killed outright, farmers wisely choosing those varieties 

 which experience has shown to be the hardiest. If the 



