TRANSPIRATION 



171 



day and night and different seasons of the year. It is usually 

 very low in desert plants and high in land plants which have 

 an abundant water supply ; low at low temperature and high at 

 high temperature; low at night and 

 high during the day. It has been esti- 

 mated that it is necessary on the average 

 for 300 pounds of water to pass through 

 a plant in the production of cue pound 

 of dry material. Of course, it is higher 

 for some plants and lower for others, 

 being about 272 pounds in the case of 

 com and about 55Y pounds in the case 

 of oats, depending upon the aerial and 

 sub-aerial environment of the plants. 

 Leaves are the organs of greatest im- 

 poirtance in transpiration, but this pro- 

 cess also takes place through stems, 

 flowers, fruits, seeds and practically all 

 parts exposed to the air. The transpira- 

 tion of water from stored fruits and 

 vegetables results in -their wilting. 



Demonstration of Transpiration. — 

 This process may be demonstrated by 

 wrapping a potted plant with a rubber 

 doth so as to prei^'ent evaporation from 

 the surface of the pot and soil. Weigh 

 carefully and place under a bell jar. In the course of a 

 few hours moisture will be observed on the inner surface of the 

 bell jar. "Weigh again and subtract from the first weight and 

 thus determine the amount of water that has been transpired 



Fig. 98. — Potometer used in 

 demonstrating' transpiration. 



