CIRCULATION OF THE SAP. 23 



ral interesting cases in illustration of this principle, will be 

 found in the marginal note on page 32. 



The perspiration of leaves, or the evaporation of the sap 

 from their surfaces, has been already alluded to. The de- 

 gree of this perspiration may be very readily shown by cut- 

 ting off two small branches from a tree at midsummer, and 

 throwing them down in the sun's rays after having stripped 

 one of its leaves. The branch furnished with foliage will 

 become sensibly shrivelled in a few minutes, while the other 

 will remain unchanged for a long time. For this reason 

 watering should be given to trees but sparingly while yet 

 only in bud, but copiously when in full leaf and in vigorous 

 growth. For the same reason a tree losing most of its roots 

 in digging up for transplanting, may continue apparently 

 fresh and healthy till in full leaf, when the few remaining 

 roots cannot supply with moisture the entire crop of leaves, 

 which soon drop and wither, and the tree perishes. A cor- 

 responding reduction of the buds when the tree is set out 

 would prevent this disaster. 



After the fall of the leaf in autumn, all action in the tree 

 is nearly suspended, and every part stationary. On the 

 approach of warm weather, however, the roots begin to 

 drink in the moisture from the earth ; but having yet no 

 escape through the leaves, every part is filled. While in 

 this state, cutting into the wood causes an instant flow of 

 the sap from the distended vessels. But as soon as the 

 leaves open, they form a rapid drain on this superabundant 

 water, and the regular circulation as before explained com 

 mences. 



