64 PLANT LIFE ON THE FARM, 
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- 
