ON THE DIFFERENT SORTS OF WOOD. 1^5 



decay could cause the disappearance of the wood vessels ; 

 and am more con6rraed in this idea, as I have always found 

 that it is the constant consequence of the stoppage of the 

 sap, whenever it takes place during the life of the plant. 

 This I before showed, when I endeavoured to prove how- 

 mistaken physiologists were in supposing that part of the 

 stem of the tree was void of sap, and was only filled in the 

 newest part. 



On summing up the evidence I should conclude, that the The ground 

 gwund is too cold and moist for these vegetables. On dis- j°°j*° ''" 

 secting turnips and potatoes growing in the same ground, I 

 find it exactly the same disorder as that in the brocoli, that 

 is a total disappearance of the ligneous parts, both in stem 

 and root. The tubercles of water mentioned in the brocoli, 

 turnip, &c. are always full of stinking water, as the putridity 

 i^ just beginning. May not the disorder of these plants in 

 some measure serve to^prove, with the rest of the facts al- 

 ready adduced in my former letters, that the sap is con- Sap conveyed' 

 veyed by the wood alone for the nourishment of the plant ? Ir,l^^^^^° 

 I think the body of evidence I have at different times given 

 now nearly amounts to certainty, that the sap runs in the 

 wood alone, and does not circulate. This disorder must not 

 be mistaken for that called the scab in potatoes, which is a Scab in 

 malady that proceeds from a cryptogaraian plant, first P^^^'"*^** 

 growing on the outside of the potato, in which insects fix 

 and lay their eggs, to insure food to their hatching young ; 

 which soon dive into the interior, and cover the root with 

 blotches. 



The next part which belongs to the wood, and which I 

 wish thoroughly to explain, is that which appertains to the 

 balls. I have shown, that there are in plants two sorts, 

 one which generally regulates the mechanism of plants, 

 (the account of which I gave in my last letter on mechanical 

 botany): the other the ball found in the wood of trees. Ball found ia 

 When a bud, formed of the knot of the line of life, and ^^'^ "^"""^ °*" 

 passing through its covered way to the exterior, is by any 

 accident severed from that line, the bud stops, and can 

 proceed no farther, though the wood vessel joins to it. Iii 

 time the albumen, which surrounds it, changes to wood, 

 and the bud, though its interior never grows, yet continues 



to 



trees. 



