Note 167 — Chapter III

"The best state of weather for a good run," says my correspondent, "is clear days, thawing fast in the daytime and freezing well at night, with a gentle west or northwest wind; though we sometimes have clear, fine, thawing days followed by frosty nights, without a good run of sap, 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 indicates 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 beginning 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 ascribed 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 stimulating influences of the sun's light and heat is the principal cause of the suspension of the circulation in the night.