PLANTS. 
9 
the rest in the spring, and adds a new circumfe- 
rence to the wood through its whole length. Trees, 
like insects and reptiles, have several skins folded 
over one another ; but with this difference, that 
reptiles and insects divest themselves of these first 
skins, and entirely quit them, to appear, from time 
to time, in a new form, and with additional splen- 
dour; whereas trees have annually a new habit, hut 
then it is cast over the preceding, and merely serves 
to form an additional ring. It is evident that the 
fine bark furnishes the tree with the rounds of fibres 
that yearly enlarge its bulk, because when the large 
bark, with that which is inward, is cut off in any 
part, leaving the wood exposed to view, the wood 
will never receive any augmentation there; both the 
bark and the wood continue their growth in the ad- 
joining parts, but the aperture remains as it was 
first made, and can only be closed in process of time, 
by the lengthening of the protuberances formed by 
the neighbouring fibres. 
It is easy to distinguish the annual accretions in 
trees. It is only necessary to cut a trunk, or a large 
branch, horizontally, to discover the several circles, 
or different degrees of thickness round the heart ; 
and we may infallibly determine the years of the 
tree’s age, by the number of circles visible in the 
wood : the last revolutions are always of a lighter 
consistence ; they are called the sappy parts of the 
wood, and are rejected by workmen, as too light 
to be any way serviceable to their purpose. These 
soft parts contract a solidity in the succeeding years; 
