98 PLANT BIOLOGY 



work and grow, and the final products of this action are 

 carbon dioxid and water. As a result of the use of this 

 oxygen by night and by day, plants give off carbon dioxid. 

 Plants respire ; but since they are stationary \ and more or 

 less inactive, they do not need as much oxygen as animals, 

 and they do not give off so much carbon dioxid. A few plants 

 in a sleeping room need not disturb one more than a family 

 of mice. It should be noted, however, that germinating 

 seeds respire vigorously, hence they consume much oxy- 

 gen ; and opening buds and flowers are likewise active. 



Transpiration. Much more water is absorbed by the 

 roots than is used in growth, and tliis surplus water passes 

 from the leaves into tlie atmosphere by an evaporation process 

 known as transpiration. Transpiration takes place more 

 abundantly from the under surfaces of leaves, and through 

 the pores or stomates. A sunflower plant of the height 

 of a man, during an active period of growth, gives off a 

 quart of water per day. A large oak tree may transpire 

 1 50 gallons per day during the summer. For every ounce 

 of dry matter produced, it is estimated that 15 to 25 pounds 

 of water usually passes through the plant. 



When the roots fail to supply to the plant sufficient water 

 to equalize that transpired by the leaves, the plant wilts. 

 Transpiration from the leaves and delicate shoots is in- 

 creased by all of the conditions which increase evapora- 

 tion, such as higher temperature, dry air, or wind. The 

 stomata open and close, tending to regulate transpiration 

 as the varying conditions of the atmosphere affect the 

 moisture content of the plant. However, in periods of 

 drought or of very hot weather, and especially during a 

 hot wind, the closing of these stomates cannot sufficiently 

 prevent evaporation. The roots may be very active and 

 yet fail to absorb sufficient moisture to equalize that given 



