THOMAS ANDREW KNIGHT, ESQ. 27 



in the trunk (owing to its greater bulk and the temperature of 

 the fluid it receives from the earth), and last at the root. If 

 we suppose these laminae to contract first in the smaller 

 branches during the decreasing temperature of the evening, 

 the resistance the rising fluid will meet with in those branches 

 will be less than the pressure exerted in the trunk and large 

 boughs, and the sap will, in consequence, flow with greater 

 freedom during the evening and night (as my experience 

 induces me to believe it does) ; and during this time plants 

 ought, according to this theory, to grow most, as a few experi- 

 ments I have made incline me to believe they do. In the 

 morning the increasing temperature of the air would put the 

 sap in the smaller branches in motion, and thus supply the 

 progress of vegetation during the day. No kind of weather 

 appears so well calculated to produce the expansion and con- 

 traction I have supposed to exist, as that in which there are 

 frequent hot gleams of sun with intervening clouds and showers ; 

 and in such weather I think plants usually make the most rapid 

 progress. 



" When trees are burst by frost, it is, I believe, usually sup- 

 posed to arise from the congelation and consequent expansion 

 of the fluid remaining in the sap vessels ; but this opinion I 

 think must be erroneous, for the sap vessels (in the common 

 kind of fracture) are not ruptured, nor does the fracture follow 

 their direction it follows that of the silver grain ; and I believe 

 that the internal part of the tree is cleft by the expansion of 

 the external part, owing to the sudden change of temperature 

 in the end of long and hard frosts, as frequently happens to 

 other hard and brittle bodies. The silver grain is here 

 extremely well placed to produce this effect, and I have little 

 doubt does produce it. But there is another species of rupture, 

 common in pollard trees, which follows the circular line of the 

 sap vessels ; and this is probably occasioned by the freezing of 

 the sap. 



" My letter has grown to a most immoderate length, and I 

 therefore will not at present trouble you with further observa- 



