250 THE SECOND BOOK OF BOTANY. 



haled five or six ounces a day ; and a young apple- 

 tree, with eleven square feet of foliage, lost nine 

 ounces a day. Hales calculated that the force which 

 impels the sap in a vine^in summer-time is five times 

 as great as that which drives the blood through the 

 large arteries of a horse ; but the rate of evaporation 

 has a large share in determining the force of the flow. 



The influence of evaporation in starting the flow 

 of sap is seen when a plant, with a certain time of 

 leafing, is grafted upon a stock which puts forth its 

 own foliage at a later period. The sap starts with 

 the expansion of the leaves upon the grafted stem, 

 and, of course, earlier than usual. Again, when the 

 branches of a tree are enclosed and warmed in win- 

 ter, so that the buds swell, the sap of the trunk is set 

 in motion to supply the demand. 



It is chiefly through the stomata that evaporation 

 takes place. Situated in the epidermis, directly over 

 the intercellular spaces, they permit the process when 

 water is abundant, and arrest it when the supply fails. 

 Their agency is of the utmost importance, for, unless 

 the surplus water of the ascending sap is got rid of, 

 the plant cannot digest its food ; and, unless the ac- 

 tion of the sun and air is checked when the supply is 

 limited, it would wither and perish. In dry weather, 

 from lack of moisture, the stomata shorten, straighten, 

 and so close the orifice, and put a stop to evapora- 

 tion ; but, when full of water, they lengthen, curve 

 outward, and open a free passage for the escape of the 

 abundant moisture. 



In some plants, as the cactus, the skin is so thick 

 and dense that, succulent as they are, they yet live 

 and flourish in dry, hot climates. 



