72 ON SAP. 



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Emily. How much this sap, destined for the nourish- 

 ment of the young buds, resembles the milk of animals, 

 a provision which Nature has made for a similar purpose, 

 and which is secreted from the common stock of nour- 

 ishment only when there are young to feed on it. 



And pray what is the cause that produces the rising of 

 the sap in spring ? 



Mrs. B. Heat is the circumstance most favorable to 

 the absorption of this nursling sap, as it is heat which first 

 expands the buds, and makes room for it. An experi 

 ment has been made by placing two pieces of vine in two 

 similar vases of water, and then introducing the stem and 

 branches of one of them, through a hole in the wall, into 

 a hothouse : the buds of this plant were rapidly devel- 

 oped, and the water in the vase as rapidly absorbed ; 

 whilst the buds of the other plant made only the usual 

 progress, and the water in the vase diminished in the same 

 slow proportion. 



If plants are pruned in the spring, the sap will rush out 

 often with violence : in vineyards, this flowing of the sap, 

 ' when plants are cut, is called the tears of the vine. Mr. 

 Hales made an experiment by cutting off the upper end of 

 the branch of a vine, and enclosing the wounded extremity 

 of the lower part (which remained on the stem) in a tube ; 

 the sap flowed from it with such violence, and in such 

 abundance, as to rise to the height of forty-three feet in 

 the tube, thus sustaining the weight of one atmosphere 

 and a half. 



Caroline. What a prodigious force ! Of course, if 

 you make an incision into the stem of a tree in the spring, 

 the sap will flow out. 



Mrs. B. No, not at least with violence, for the spring 

 sap rises with force, only in shoots of one year's growth, 

 and will consequently flow with velocity from none but 

 these. In making the incision, you must penetrate to the 

 centre in order to reach the full channel of the spring sap, 



387. To what does Emily compare sap! 388. What causes sap 

 lo rise in the Spring! 389. In what experiment was it made mani- 

 fest that heat was a principal agent! 390. If plants are pruned 

 in the spring what is said of the sap! 391. The tears of plants, are 

 what! 392. What experiment was made by Mr. Hales, and what was 

 the result! 393. Why will not the spring sap flow out with violence 

 if an incision be made in the tree! 



