260 LEAVES 
thin cloth while sweeping. Also for this reason it is well to spray 
with clean water or even wash the leaves of plants with clean 
rags, so as to open any stomata that may be clogged. Plants 
are often much injured by the clogging of their stomata, as in 
case of hedges along roadsides or plants around cement factories. 
As for other plant processes, there is an optimum temperature 
at which photosynthesis is most active, and above or below this 
temperature photosynthesis diminishes. The optimum tempera- 
ture, although varying considerably for different plants, is not 
far from 80° (Fahrenheit) for most plants in our region. Tem- 
peratures unfavorable for photosynthesis not only affect the 
yield of crops but also may lengthen the time required for 
maturity, as in case of Corn when the summer is cool. 
Since water is one of the materials for making sugar, it must 
be present in sufficient quantities to supply this demand. Fur- 
thermore, the lack of water tends to cause the stomata to close 
and may thereby diminish the amount of carbon dioxide entering 
the leaf. In some cases, as in Corn, the lack of water causes the 
leaves to roll, in which case there is not a good exposure to light. 
For the most active photosynthesis an abundance of chloro- 
plasts well supplied with chlorophyll is also necessary. As farmers 
know, Corn pale in color does not grow so rapidly as Corn that 
is dark green. 
Transpiration from Plants 
Transpiration is the loss of water in the form of vapor from 
living plants. Transpiration, although similar in many ways to 
ordinary evaporation, differs from the latter process in that it is 
modified by the structures and vital activities of the plant. By 
transpiration plants are almost constantly losing water to the air. 
It is for this reason that shoots quickly wilt when their connec- 
tions with roots are severed, so that they receive no water from 
the soil to compensate for the loss of water to the air. The 
rapidity with which green grass or weeds wilt when mowed on a 
hot day is a matter of common observation. Transpiration is 
not limited to leaves; but all parts of plants above ground are 
exposed to transpiration. Fruits and seeds, although usually 
jacketed in a rather heavy covering, lose water during storage. 
Even during winter, the buds, twigs, and branches of trees are 
continuously losing water to the air. However, the leaves, on 
