122 CIRCULATION OF THE SAP. 



Section 5. — Circulation of the Sap. 



223. There are three kinds of circulation recognized by most 

 Botanists. 1. A general circulation from the roots to the leaves 

 and back again. 2. Cyclosis, or the circulation in the lactifer- 

 ous tissue. 3. Circulation of rotation in the individual cells. 



That there is a general circulation from the roots to the 

 leaves, is plainly indicated by the rapid evaporation which is 

 constantly going on from the surface of those organs. How 

 soon does a vigorous plant wither and diminish sensibly in 

 weight when cut in the mid-day sun ! and plainly for no other 

 reason than that the source of its supply of fluid is cut off. 

 Hales, many years ago, made some interesting experiments, not 

 only proving this general circulation, but determining the force 

 with which the fluid moved forward. 



224. By the aid of a glass tube, containing mercury, at- 

 tached to the stalk of a vine cut off two feet and nine inches 

 from the ground, the force of the sap at its maximum raised 

 the mercury 32 J inches, which was on the twelfth day after the 

 experiment commenced, April 18, at 7 A. M., which force was 

 sufficient to raise water thirty-six feet. 



" In another like mercurial gauge, fixed near the bottom of a 

 vine which ran 20 feet high, the mercury was raised by the 

 force of the sap 38 inches, equal to 43 feet 3 inches of water ; 

 which force is. more than five times greater than the force of 

 the blood in the great crural artery of the horse, seven times 

 greater than the force of the blood in the like artery of the dog, 

 and eight times greater than the blood's force in the same ar- 

 tery of a fallow doe." 



225. These experiments show not only circulation, but that 

 it is carried on with great force. The force with which the 

 sap moves in vegetables varies with the seasons and the hours 

 of the day. It is most powerful in the spring, and in the morn- 

 ing of the day, and under the direct action of the sun after a 

 rain. The course which the sap takes in its general circulation 

 is from the roots through the alburnum to the leaves, and down- 

 ward through the bark, and laterally by the medullary pro- 

 cesses. These facts may be shown by cutting in early spring 

 into the sugar-maple, and we shall find the sap running from 

 the alburnum only, and mostly from the lower surface of the 

 wound, showing the upward course of the sap is through this 



223. How many kinds of circulation ? What are they ? What proves 

 the general circulation ? — 224. What was Hales' first experiment ? Second ? 

 — 225. What do these experiments show ? How does the force vary f 

 What is the course of the sap ? What proofs ? 



