ON SAP. 73 



and the instant your instrument reaches the pith, you will 

 hear the sap gush, and see it follow the instrument as you 

 draw it out. 



We must not, however, bestow the whole of our at- 

 tention on this nursling sap, but return to that which ris- 

 es through the alburnum to feed the mature plant. This 

 sap reaches the leaves without having undergone any 

 change ; but as soon as it arrives there, a considerable 

 portion of its water exhales by the stomas, leaving the nutri- 

 tive particles which it held in solution deposited in the leaf. 



Emily. Arid pray, what is the proportion of the quan- 

 tity of water evaporated to the whole quantity absorbed 

 by the roots ! 



Mrs. B. It varies exceedingly, according as circum- 

 stances are more or less favorable to evaporation. A 

 plant can evaporate only in proportion to its absorption : 

 the quantity, therefore, depends not only on the abun- 

 dance or deficiency supplied from the soil, but also on 

 the number of ramifications of the roots ; that is to say, 

 of mouths to suck up water. On the other hand, these 

 mouths, however numerous and abundantly supplied, 

 can continue to receive water only in proportion as the 

 exhalation by the leaves carries off what has already been 

 taken in, so as to make room for more. Thus while 

 water enters at one extremity of the plant, it must find 

 its way out at the other. 



Emily. Were you to poor water into a tube closed at 

 the opposite end, it would soon be filled, and, though 

 you continue to pour, it would receive no more, and the 

 water would flow over; but if you opened the closed end, 

 you might pour in at one end as fast as it flowed out at 

 the other. But what is it that promotes the flowing out, or, 

 in other words, the evaporation, of the water by the leaves? 



Mrs. B. The most essential circumstance is light. 



Caroline. You surprise me : I should have thought 

 that heat would have been more necessary than light to 

 produce evaporation. 



394. Wnat will take place if the instrument with which the incision 

 is made reaches the centre! 395. What takes place with the sap, that 

 lises through the alburnum, on reaching the leaves'? 396. On what 

 does the quantity of water evaporated depend! 397. On the other 

 hand what is said in relation to this subject! 398. What illustration 

 does Emily give of the tube filled with water! 399. What causes the 

 evaporation of water by the leaves! 



