NATURE AND SOURCES OF FOOD. 99 



again refilled by the flow of the sap from the subjacent tissues, 

 and in this manner the sap is gradually set in motion from the 

 extremities of the branches to the roots through the entire sys- 

 tem of the plant. When at length the young branches have 

 developed themselves from the buds, and the leaves are spread 

 abroad in the atmosphere, the ascent of the sap becomes 

 powerfully accelerated by the evaporation which takes place 

 from their surface. 



The height to which the sap rises in forest trees is very 

 great, and the force with which it ascends is very considerable. 

 The force with which the sap ascends in the stem of the vine 

 was measured by Hales, a celebrated English .physician. In 

 the early part of the month of April, he fitted a bent tube to 

 one extremity of the stem of a grape-vine, which he had cut 

 down to about two and a half feet above the ground. This 

 tube was graduated and its curve filled with mercury. In a 

 few days he found that the ascending force of the sap had raised 

 the mercury upwards of 38 inches. Now, since the pressure 

 of the atmosphere supports a column of mercury varying from 

 28 to 30 inches in height, it follows that the ascending force 

 of the sap is greater than the pressure of the atmosphere. In 

 some of his experiments. Hales calculated that the ascending 

 force of the sap in the stem of the vine was five times greater 

 than that which impels the blood through the principal artery 

 of the horse. A piece of bladder tied over the stump of another 

 vine, from which a piece had been cut off early in May, was 

 torn into shreds by the rising of the sap. 



As the sap rising in the stem attains a greater distance from 

 the root, it becomes less watery and more thick and mucilagi- 

 nous. It finds, in effect, amassed in the tissues which it tra- 

 verses, portions of gum, sugar, starch, &c., left in them by the 



