470 FROST-CLEFTS. 



limbs, accidents, or the work of man. These inju- 

 ries cause a mechanical weakness in that part. He 

 had at first supposed that a thaw caused the clefts 

 by the contraction of the outer layers, which were 

 then too small to contain the inner, and therefore 

 burst. But, upon observation, he found that they 

 occurred in the severest cold, and closed upon the 

 approach of a thaw. 



That these clefts were occasioned by freezing and 

 thawing, was the theory of Gaudichaud. He exam- 

 ines the conjecture of Goppert, that those that occur 

 in the axils of the branches are to be attributed to 

 the influence of wind. But he found that clefts 

 seldom, if ever, occur here ; but are beneath the 

 branch, in the toughest and strongest place : and 

 that the wind, at the time of the clefts which he 

 observed, was very gentle. 



They may have occurred from the outer layers be- 

 ing reduced in volume by evaporation. Thus they 

 were unable to endure the expansion of the inner 

 layers not aifected by this evaporation. This theory 

 exploded on discovering that the moisture of the 

 atmosphere, when the cleft took place, was greater 

 than for some time preceding. The author states 

 that De Vriese, in his " Principles of Vegetable 

 Physiology," expresses the opinion that the sap 

 rises in the winter, and that the cold splits the stem, 

 and allows the juice to run out. But, as the tree is 

 frozen to its centrfe, there can be no ascension of the 



