THOMAS ANDREW KNIGHT, ESQ. 25 



subject false ; that of capillary attraction is surely without 

 foundation, not being any way equal to propel the sap in the 

 manner described by Hales. Dr. Hunter's opinion, that the sap 

 is raised by the expanded air-vessels pressing on the sap- 

 vessels, does not agree with the fact, that the sap flows with 

 great force when the temperature of the surrounding air is 

 declining ; nor do I see a force here adequate to the effect pro- 

 duced. Dr. Darwin's imagination is generally too strong for his 

 judgment ; and it has, I suspect, created more in this case than 

 nature has done. My theory may perhaps be more absurd 

 than either; but such as it is, I will profit by the permission you 

 have given me to lay it before you. 



" There are two kinds of grain in wood ; the one usually called 

 the false or bastard, the other the true or silver grain. The 

 former consists of those concentric circles which mark the 

 annual increase of the tree ; and the latter is formed of polished 

 lamina? diverging in every direction, from the centre towards 

 the bark of the tree, slightly adhering to each other at all 

 times, and scarcely at all during the spring and summer, whence 

 the increased brittleness of wood at these seasons. If you will 

 examine a piece of English oak, you will find the laminae I 

 describe, and that every sap tube is touched by it at short dis- 

 tances, and is slightly diverted by it from its course. If these 

 laminae be expansible by increase of temperature, I conceive 

 that they are placed as well as possible to impel the sap to the 

 extremities ; and that they are expansible by change of tempe- 

 rature I am led to suspect, by their being much affected and 

 put in motion by the state of the atmosphere long after the tree 

 has ceased to live. I shall at present confine my observations 

 to the English oak, though the same observations are applicable 

 in a greater or less degree to every other kind of tree, and even 

 to the cabbage-stalk. In sawing oak into boards, it is usual to 

 cut it, as much as is possible, into what are called quarter 

 boards; being so named from the tree being first cut into 

 quarters. In a true quarter board the laminae of the silver 

 grain lie exactly parallel with the surface of the board, and a 



