300 BERTRAMS VISIT TO HER 



oak trees were still well covered, though they had 

 changed colour. 



" Yes," said he, " but the leaves of the young 

 beech, though they have become brown and dry, 

 will not fall till spring ; and the fibres of the 

 oak are so tough, that the leaf does not easily 

 separate from the branch. You may also per- 

 ceive that the apple and peach trees remain 

 green, very often till the beginning of December. 

 Some botanists attribute the defoliation of trees 

 to the drying up of the vessels which connect 

 the leaf with the stem ; and others to the swell- 

 ing of the young buds for the succeeding year. 

 This, they say, deprives the old leaf of its ac- 

 customed supply of sap, and as they enlarge, 

 they push it out of their way ; but there is a 

 material objection to this theory, that the leafits 

 of pinnated leaves fall in the same manner, 

 though there are no buds to push them off. 



" It is also supposed that the vessels of the 

 petiole gradually become woody, and incapable 

 of freely transmitting the sap ; it therefore stag- 

 nates, the vessels become overloaded, and the 

 parts which connect the stem and the leaf crack 

 at the insertion of the petiole. The vessels 

 being thus interrupted, the leaf is no longer 

 supplied with proper nourishment ; it loses its 

 elasticity, and becomes dry and brittle ; and 

 the least shock, whether the effect of frost or of 

 wind, detaches it. 



