238 PLANT AND ORGANIC CHEMISTRY 



quired, or to the seeds or other organs in which organic stores 

 are being laid up. 



If the stems or plants are cut in the spring, a flow of sap pro- 

 ceeds from the cut surface of that portion of the stem which is 

 connected with the roots. This fact was investigated by Hales. 1 

 He concluded that there is " a considerable energy in the root to 

 push up sap in the bleeding season." This force is termed the 

 root pressure, and is the measure of the absorbent activity of 

 the root hairs. The root pressure is not only manifested by caus- 

 ing the flow of sap; it also may cause the exudation of drops 

 of sap on the surface. There is a marked periodicity in the flow 

 of sap, which is not due to the immediate result of variations 

 in external conditions, but is inherent in the absorbent cells 

 themselves. 



The current travels from the roots to the leaves through the 

 lignified cell walls of the wood of the plant. The activity of the 

 exhalation of watery vapor from the plant is not the same from 

 its surfaces. The refreshing effect of a shower on withered 

 leaves is due to the moisture penetrating the soil and being 

 absorbed by the root hairs. From experiments 2 it has been 

 shown that if the air is very moist, and the leaves dry, the leaf 

 surfaces may absorb a little water. 



The cuticle offers a certain amount of resistance to the pas- 

 sage through it of vapor ; this is due to resinous or waxy sub- 

 stances contained in it. The Mexican ocotilla 3 offers a striking 

 example. It grows in very dry and exposed parts of the country, 

 where rainfalls are infrequent. The bark is chiefly composed 

 of wax and resinous substances. 



Other substances, as well as water, can be absorbed by 

 leaves, 4 experiments having shown that if a drop of calcium sul- 

 phate solution be placed on a leaf, it will have disappeared in 

 the course of a few hours. This is more rapid when placed on 

 the under surface. Though it seems that leaves may absorb 

 water and substances in solution under certain circumstances, 



1 Statical Essays, i, 1769 (4th edition). 



2 Detmer and Boussingault. 



3 H. C. De S. Abbott, Proc. A. A. A. S., xxxiii. See p. 117. 



4 Boussingault, Ann. Chem. et Phys., ser. V, xiii; also Agronomic, VI, 1878. 

 Mayer, Landwirthschajtl. Versuchs-Stat., xvii, 1874. 



