PLANTS. 71 



The Flow of the Sap. The sap is the moisture of the 

 soil which has passed upward, through the roots, into 

 the plant. The plant is entirely filled, or saturated, 

 with sap. 



By the principle of osmose and the force of capillary 

 attraction the sap moves partly along the tubes formed 

 by the union of long cells, and partly through the mem- 

 branes from cell to cell, until every part of the plant is 

 filled. 



When a plant has become saturated, there can be no 

 more flow of sap until room has been made for more. 

 The flow is kept up, partly by the growth or enlarge- 

 ment of the plant, forming new cells which need to be 

 filled with moisture, but mostly by the evaporation of 

 the moisture or sap from the leaves. 



Leaves contain a great number of minute openings, 

 particularly on the under side, which bring the air in 

 immediate contact with the sap within. Through these 

 openings the moisture of the sap is continually escaping 

 into the air by evaporation. 



In damp weather evaporation is slow, and hence the 

 upward flow of sap is likewise slow ; but in dry weather, 

 and especially under the influence of the warm sunlight, 

 it goes on more rapidly. 



The quantity of water conveyed in this way into the 

 atmosphere is very large, amounting during the season 

 to many times the weight of the full-grown plant. Soil 

 occupied with crops is thus dried much faster than that 

 upon which no vegetation is growing. 



The abundant flow of sap from a maple in the spring 

 is due to the fact that the tree is not only filled with sap, 

 but this is placed under some pressure by the force of 

 capillary attraction. As the tree contains no leaves at 



