MOTION OP THE JUICES. 373 



flow from a cut when the trunk itself is warmed to a cer- 

 tain point and, in general, the flow appears to be the 

 more rapid the warmer the trunk. During warm, clear 

 days, the radiant heat of the sun is absorbed by the dark, 

 rough surface of the tree most abundantly ; then the 

 temperature of the latter rises most speedily and acquires 

 the greatest elevation even surpasses that of the atmos- 

 phere by several degrees ; then, too, the yield of sap is 

 most copious. On clear nights, cooling of the tree takes 

 place with corresponding rapidity ; then the snow or 

 surface of the ground is frozen, and the flow of sap is 

 checked altogether. From trees that have a sunny ex- 

 posure, sap runs earlier and faster than from those hav- 

 ing a cold northern aspect. Sap starts sooner from the 

 spiles on the south side of a tree than from those towards 

 the north. 



Duchartre (Comptes Rendus, IX, 754) passed a vine 

 situated in a grapery, out of doors, and back again, 

 through holes, so that a middle portion of the stem was 

 exposed to a steady winter temperature ranging from 18 

 to 10 F., while the remainder of the vine, in the house, 

 was surrounded by an atmosphere of 70 F. Under 

 these circumstances the buds within developed vigor- 

 ously, but those without remained dormant and opened 

 not a day sooner than buds upon an adjacent vine whose 

 stem was all out of doors. That sap passed through the 

 cold part of the stem was shown by the fact that the 

 interior shoots sometimes wilted, but again recovered 

 their turgor, which could only happen from the partial 

 suppression and renewal of a supply of water through the 

 stem. Pay en examined the wood of the vine at the con- 

 clusion of the experiment, and found the starch which it 

 originally contained to have been equally removed from 

 the warm and the exposed parts. 



That the rate at which sap passed through the stem 

 was influenced by its temperature is a plain deduction 



