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THE POPULAR SCIENCE MONTHLY. 



found to transpire hourly per metre of foliage sixteen grammes. It 

 was then inserted into a tube of water, and subjected to the pressure 

 of a column of water two and a half metres high. Under these con- 

 ditions the evaporation mounted to fifty-five grammes per hour, and the 

 branch at the end of five hours weighed more than at the commence- 

 ment. 



The general result of these experiments shows the mutual working 

 of the various parts of the plant with reference to the phenomena of 

 transpiration. The roots, absorbing water from the soil by endosmose, 

 direct it toward the stem. Whether the motive force here is injection 

 by the roots or absorption resulting from the transpiration in the green 

 parts of the plant, or a union of both, is a question still unsettled. The 



Fig. 1. 



Fio. 2. 



stem serves not only as a passage for the water to reach the leaves, but 

 also as a reservoir to be drawn on during rapid evaporation. In the 

 leaves the sap is concentrated by the transpiration, and the matters in 

 solution enter into the cell formation, or, changed by the action of light, 

 are distributed throughout the plant by the descending sap. The cir- 

 culation would be quite similar to that in an animal, were it not for the 

 irregularity. While the supply of water from the roots varies but 

 slightly, the loss by evaporation from the leaves is subject to the great- 

 est fluctuations, according to the temperature and hygroscopic condi- 

 tion of the surrounding air. During these periods the leaves draw on 

 their stock of constitution water and the supply in the stem ; and when 

 both fail, the phenomenon of wilting ensues. 



