Public Parks. 45 



summer ; and this effect in modifying the range of temperature, as 

 indicated by repeated experiments, is far from being insignificant. 

 In summer, plants and trees, in addition to their conducting powers, 

 render the atmosphere cooler by the great quantity of water that is 

 exhaled from the leaves during foliation. Hales found that a sun- 

 flower tliree and one-half feet high, with a surface of 5.616 square 

 inches, exposed to the air, perspired at the rate of twenty to thii'ty 

 ounces avoirdupois every twelve hours, or seventeen times more than 

 a man.* A vine with twelve square feet of foliage exhales at the rate 

 of five or six ounces a day ; and a seedling apple-tree, with eleven 

 square feet of foliage, lost nine ounces a day. 



An experiment, performed by Bishop Watson, will assist in 

 giving an idea of the extraordinary amount of change effected by 

 this function in plants. He placed an inverted glass vessel, of the 

 capacit}'' of twenty cubic inches, on grass v\^hich had been cut during 

 a very intense heat of the sun, and after many weeks had passed 

 without rain ; in two minutes it was filled with vapor which 

 trickled down its sides. He collected these drops on a piece of 

 muslin, which he carefully weighed ; and repeating the experiment 

 for several days, between twelve and three o'clock, he estimated as 

 the results of his inquiries, that an acre of grass transpires in 

 twenty-four hours, not less than 6,400 quarts of vv^ater. This is 

 probably an exaggerated statement, as the amount transpired during 

 the period of the day in which the experiment was tried, is far 

 greater than any other, f 



When we consider the vast perspiring surface presented by 

 a large tree in full leaf, it is evident that the watery vapor it 

 exhales is immense. " The Washington Elm," at Cambridge, a tree 

 of no extraordinary size, was some years ago estimated to produce 

 a crop of seven millions of leaves, exposing a surface of 200,000 

 square feet, or about five acres of foliage. % 



The refreshing coolness, then, of a grove on a hot summer day is 

 not to be wondered at ; and how often have we, while enjoying it, 

 inquired what was the cause, simply supposing it was the result of 

 shade. This exhalation is dependent on the capacity of the air for 

 moisture, at the time, and the presence of the sun, while frequently it 

 is scarcely perceptible at night. 



* Vegetable Statics. 



t Carpenter's Comparative Physiology. 



X Gray, — "How Plants Grow.'' 



