PLANTS. 71 



The Flow of the Sap. — The s;i}) is the moisture of the 

 soil which has passed uj)ward, through the roots, into 

 the jthint. The plant is entirely lilled, or saturated, 

 with sap. 



By the j)rinciplc 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 l)een made for more. 

 The flow is kei)t u)>, partly by the growth or enlarge- 

 ment of the })lant, 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 l)ring 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 weathei-, 

 and esi)ecially under the influence of the warm sunlight, 

 it goes on more rapidly. 



The rpiantity 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 sa]i from a ma])le in the spring 

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

 but this is placed under some pressure by the force of 

 capillary attraction. As the tree contains no leaves at 



