76 ON SAP. 



Since, therefore, you have deprived them of the power of 

 absorption, you must diminish, at least, that of exhalation, 

 and, by keeping them in some degree of obscurity, en- 

 deavor to preserve the sap which they already contain. 



Emily. } should be curious to know what quantity 

 of water a plant exhales in a day. 



Mrs. B. It has been ascertained by Mr. Hales, that a 

 full-blown helianthus, or sunflower, placed under advan- 

 lageous circumstances in regard to light and temperature, 

 evaporated twenty ounces of water per day, which is seven- 

 teen times more than that evaporated by a man, supposing 

 their surfaces equal. This experiment was made by 

 weighing, first, the water in the vase in which the sun- 

 flower was placed, then the plant itself; and, after due 

 time being given to the experiment, the water and the 

 plant were again weighed. The plant had absorbed as 

 much water as the vase had lost ; but it was not found 

 to have increased in weight so much as the water in the 

 vase had diminished by twenty ounces, which affords a 

 conclusive proof that these twenty ounces had been 

 evaporated. Of course, suitable precautions had been 

 taken in order to prevent any immediate evaporation 

 from the water contained in the vase. Fleshy fruits, 

 such as apples, plums, peaches, &c. have few or no 

 pores : they therefore retain the moisture they receive 

 from the sap, which enables them to remain long on the 

 tree, after coming to a state of maturity, without drying 

 up and withering. Whilst dry fruits, such as peas o 

 beans, wither in consequence of the number of their 

 pores by which they exhale moisture. There is the same 

 difference between thick fleshy leaves, such as those of 

 the cactus and other succulent plants, and dry leaves, 

 such as those of the pine and the fir, which are at the 

 opposite extremity of the scale ; common leaves bear a 

 medium, between the two, but, in the same space in 

 which a common leaf contains six or seven stomas. the 

 leaf of a pine has sixty or seventy. 



414. Why may they be preserved longer in the shade 1 ? 415. How 

 much water was a full-blown helianthus, or sunflower known to evaporate 

 in a day! 416. How was the experiment made 1 ? 417. Why do 

 apples, plums, peaches and other fleshy fruits remain a long time on the 

 tree without withering, after having come to maturity'? 418. And 

 why do peas, beans &c. dry so soon 1 ? 419. What is said of the dying 

 of leaves 1 



