74 ON SAP. 



Mrs. B. Heat augments it mechanically : but with- 

 out light no exhalation from the leaves will take place ; 

 and it will even be inconsiderable, unless the sun's 

 rays fall upon the plant. 



Caroline. Is it not very singular that light should be 

 most favorable to the ascension of the sap which passes 

 through the alburnum, whilst heat is most congenial to 

 that which rises through the centre of the stem ? What 

 is the reason of this difference ? For both saps, I con- 

 clude, must be of the same nature, since the spongioles 

 cannot choose, but must suck up whatever is suffi- 

 ciently fluid to enter their pores ? .r 



Mrs. B. Being derived from the same source, they 

 were, no doubt, originally of the same nature ; but when 

 separated into different channels a difference arises : the 

 nursling sap, we have concluded, undergoes a prepara- 

 tion in its passage towards the buds, and their expansion, 

 produced by heat, is alone required to call it up. 



While the sap which passes through the alburnum 

 must not only throw off a considerable quantity of its 

 water by the leaves, but also undergo a chemical change, 

 for both of which processes you will find that the aid oi 

 the solar rays is absolutely required. 



Let us first consider the simple evaporation by the 

 leaves. The quantity of water exhaled by plants, is to 

 that which they absorb generally in the proportion of two 

 to three ; one third only, therefore, remains in the plant, 

 and becomes a part of its substance ; the rest may be 

 considered simply as a vehicle which Nature had employed 

 to convey a due quantity of nourishment into the plant, 

 and which, after having deposited its cargo, disappears. 



Emily. Is the water then which is evaporated per- 

 fectly pure ? 



Mrs. B. It does not contain above a ten-millionth 

 part of the foreign matter which it held in solution when 

 absorbed, a very trifling per centage for the expense of 

 freight. 



400 What explanation does Mrs. B. give of the agency of light and 

 heat in this work 1 ? 401. What question is asked by Caroline as to a 

 singular phenomenon in the ascension of sap 7 402. What does Mrs. 

 B. in reply say of the nursling sap 1 ? 403. What does she say of the 

 sap which rises through the alburnum'? 404. The quantity of water 

 exhaled by plants is in what proportion to that absorbed 1 ! 405. Is the 

 water thus evaporated pure 1 ? 



