G-i PLANT LIFE ON THE FAKM.- 



spectrum manifest similar movements. Some leaves are 

 raised, others depressed, some fold upwards, some down- 

 wards, but the object in all cases is probably the same 

 namely, to shield the leaves from the cooling effect of 

 radiation from the surface during the night, a process 

 which produces the same effects as actual frost would do. 

 The cause of these movements is due to a swelling or 

 turgescence and a consequent 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 

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

 the spaces between them are full of ice. It rarely hap- 

 pens 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 circum- 

 stances 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 fre- 

 quently become frozen in this manner, but it is com- 



