EXHALATION BY THE LEAF. 181 



height, and with a leaf-surface of thirty-nine square feet, and 

 covered the top of the pot with sheet-lead, into which he in- 

 serted a narrow glass tube, to admit air, and a wider one 

 stopped with a cork, through which he watered the plant. 

 This pot he weighed every morning and evening for fifteen 

 days, and as there was no way of escape for the water poured 

 into it, except through the absorption of the roots and the 

 exhalation from the leaves, he learned that the average 

 amount exhaled per diem was one pound and four ounces, or 

 about one ounce of water for two square feet of leaf-surfiice. 



Similar experiments with other plants showed that a cab- 

 bage exhaled in proportion to its surface nearly twice as much 

 as the sun-flower, or one ounce for each square foot, and that 

 a grape-vine exhaled less than the sun-flower, and less than 

 most other plants. Hence the vine rarely suffers from drought. 



He then fastened a branch of spearmint into an inverted 

 syphon, and poured in water so that the cut end of the 

 branch was immersed in it. He found that exhalation and 

 absorption proceeded so as to lower the fluid very percept- 

 ibly in the open arm of the tube. 



Another experiment had for its object the determination of 

 the force with which the root of a growing tree would absorb 

 water. For this purpose he attached a glass tube to the end 

 of an amputated root half an inch in diameter and, having 

 filled it with water, placed the open end of the tube in a dish 

 of mercury. In six minutes so much water had been ab- 

 sorbed into the tree as to raise the mercury to the height of 

 eight inches. 



In order to determine whether water would enter and per- 

 meate an inverted branch through the small end, Mr. Hales, 

 having sealed the large end of an apple branch with foliage 

 upon it, cut off the tip and immersed it in water. The fluid 

 passed freely to all parts of the branch, so that the leaves 

 remained fresh, and in three days exhaled more than four 

 pounds of water. 



The fact that sap would flow freely either way through a 

 root was shown by laying bare a large root of a tree having 

 a sucker growing upon it, and removing that portion of it 

 between the sucker and the earth, preserving the portion 

 between it and the tree. The sucker continued to flourish. 



