MOTION OF THE JUICES. 375 



becomes heated again, the gaseous and liquid contents of 

 the ducts expand, the flow of sap is renewed, and pro- 

 ceeds with increased rapidity until the internal pressure 

 passes its maximum. 



As the season advances and the soil becomes heated, 

 the root-power undoubtedly acts with increased vigor 

 and larger quantities of water are forced into the trunk, 

 but at a certain time the escape of sap from a wound 

 suddenly ceases. At this period a new phenomenon 

 supervenes. The buds which were formed the previous 

 summer begin to expand as the vessels are distended with 

 sap, and finally, when the temperature attains the proper 

 range, they unfold into leaves. At this point we have 

 a proper motion of sap in the tree, whereas before there 

 was little motion at all in the sound trunk, and in the 

 tapped stem the motion was towards the orifice and 

 thence out of the tree. 



The cessation of flow from a cut results from two cir- 

 cumstances : first, the vigorous cambial growth, where- 

 by incisions in the bark and wood rapidly heal up ; and, 

 second, the extensive evaporation that goes on from 

 foliage. 



That evaporation of water from the leaves often pro- 

 ceeds more rapidly than it can be supplied by the roots 

 is shown by the facts that the delicate leaves of many 

 plants wilt when the soil about their roots becomes dry, 

 that water is often rapidly sucked into wounds on the 

 stems of trees which are covered with foliage, and that 

 the proportion of water in the wood of the trees of tem- 

 perate latitudes is least in the months of May, June, and 

 July. 



Evergreens do not bleed in the spring-time. The oak 

 loses little or no sap, and among other trees great diver- 

 sity is noticed as to the amount of water that escapes at 

 a wound on the stem. In case of evergreens we have a 

 stem destitute of all proper vascular tissue, and admit- 



