334 HOW CROPS GROW. 



is closely connected with the changes of temperature that 

 take place above ground. The sap begins to flow from a 

 cut when the trunk itself is warmed to a certain 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 eleva 

 tion even surpasses that of the atmosphere by several 

 degrees ; then, too, the yield of sap is most copious. On 

 clear nights, cooling of the tree takes place with corre 

 sponding 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 exposure, sap runs earlier 

 and faster than from those having 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 vigorously, 

 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 conclusion of the 

 experiment, and found the starch which it originally con- 

 i allied to have been equally removed from the warm and 

 the exposed p trls. 



That the rate at which sap passed through the stem was 



