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 Agronomie , VI, 1878. 
Mayer, Landwirthschajtl. Versnchs-Stat., xvii, 1874. 
