172 ABSORPTION BY FOLIAGE. 
When the buds are ready to burst, and the green leaves 
begin to show themselves beneath their scaly covering, the 
ground has become drier, the thirst of the roots is quenched, 
and the flow of sap from them to the stem is greatly dimin¬ 
ished* 
Absorption and Exhalation of Moisture. 
The leaves now commence the process of absorption, and 
imbibe both uncombined gases and an unascertained but per¬ 
haps considerable quantity of watery vapor from the humid 
atmosphere of spring which bathes them. 
The organic action of the tree, as thus far described, tends 
to the desiccation of air and earth; but when we consider 
what volumes of water are daily absorbed by a large tree, and 
how small a proportion of the weight of this fluid consists of 
matter which enters into new combinations, and becomes a 
/ I have thought it probable that the irregular flow of sap on different days 
in the same season is connected with the variation in atmospheric pressure; 
for the atmospheric conditions above mentioned as those most favorable 
to a free flow of sap are also those in which the barometer usually indi¬ 
cates pressure considerably above the mean. With a south or southeast 
wind, and in lowering weather, which causes a fall in the barometer, the 
flow generally ceases, though the sap sometimes runs till after the begin¬ 
ning of the storm. With a gentle wind, south of west, maples sometimes 
run all night. When this occurs, it is oftenest shortly before a storm. 
Last spring, the sap of a sugar orchard in a neighboring town flowed the 
greater part of the time for two days and two nights successively, and did 
not cease till after the commencement of a rain storm.” 
The cessation of the flow of sap at night is perhaps in part to be as¬ 
cribed to the nocturnal frost, which checks the melting of the snow, of 
course diminishing the supply of moisture in the ground, and sometimes 
congeals the strata from which the rootlets suck in water. From the facts 
already mentioned, however, and from other well-known circumstances— 
such, for example, as the more liberal flow of sap from incisions on the 
south side of the trunk—it is evident that the withdrawal of the stimu¬ 
lating influences of the sun’s light and heat is the principal cause of the 
suspension of the circulation in the night. 
* “ The flow ceases altogether soon after the buds begin to swell.”— 
Letter before quoted. 
