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 



