Qf the Wood of frees. 265 
the black fpots in the wood, in all the figures before re¬ 
ferred to. 
Their fizes are as different as the trees to which they 
belong, being at leaft twenty times bigger in elm or oak, 
than in holly or pear, &c. 
Their fituation is alfo different: in apple, fig. 482. 
and in walnut, fig, &c. they are fpread abroad in every 
annual ring; in others they keep more in the compafs of 
fome line or lines, either diametrical or peripherical. In 
holly, &c. they are radiated or run in even diametrical 
lines between the pith and bark. 
Whether the air veffels are irregular or radiated, nature 
hath fo difpofed them, as that many of them ftand always 
near the infertions. 
In afh the air veffels ftand in circles on the inner 
margin of every annual ring. Thefe circles are in fome 
very thick, as in afh and barberry, in fome thin, as 
elm, &c. 
Their form is fuch that they are never ramified, but 
continued from one end of a plant fmall or great, quite 
through to the other end thereof. 
As to their texture they oftentimes appear to be un¬ 
wreathed in form of a very fmall plate, which alfo is not 
only of different breadths in different plants, and ufually 
broader in the root than in the trunk j but alfo the faid 
veffels are oftentimes unvvreathed, not in the form of a 
plate but of a round thread. The caufes of which diver- 
fity are principally three, the weftage of the fibres of 
which the air veffels confift; the difference between the 
faid fibres, or between the warp and woof, and the dif¬ 
ferent kinds of woof. 
By the weftage of the fibres it is, that the veffels often¬ 
times untwift in the form of a plate; as if a fine narrow 
ribband be wound fpirally, and edge to edge, about 
ftick, 
