90 
WOOD AND BARX OF TREES. 
Endeavour to 
discover it. 
Acacia and 
chesnut good 
examples. 
Difference of 
the wood in 
the root, and 
stem. 
Substance of 
niv next letter. 
plete) is measured, it is much narrower than when the size is 
taken a few years after ; some addition, therefore, must have 
been made. This is of consequence to discover, as it all helps 
the growth and motion, and teaches us how much more a tree 
is a moving machine, than we have ever considered it to be. 
The coils that appear round the sap vessels will not shew them- 
selves in a vegetable cutting taken with the instrument, as that 
tears them all off; but when cut with a sharp knife or razor, 
and left a little time, they will rise up, and the smallest 
magnifier will shew the roughness they create, and the large 
coils that pass from one sap vessel to another, especially in the 
chesnut and acacia. In the latter they can be seen with the 
naked eye, and they are so tight in the layers of wood, that the 
cutter, can not divest them of them — many of the sap vessels 
appear to have spirals across them, in 2 or 3 places ; but I 
rather think it is many apertures broken into one, which before 
holding tight at the interior of the circle, are, when released, 
drawn across the holes from the contracting power of the spiral 
wire. 
There is no very great difference in the figure of the wood 
of the root and stem 5 except that the latter possess not so 
much spiral wire ; that the coils from one sap vessel to another, 
are not so large and strong, that the folds in the stem are hardly 
observable , indeed, except in Jirs, I never found any there, and 
even in them infinitely less than in the root ; nor are they, 
I believe, marked by any spirals, as I see no cross ones sticking 
from one side to another, which immediately obtrudes on the 
sight when dissecting the wood of roots 5 but the stem is 
perpetually marked by the shooting of the buds, which in trees 
and shrubs (especially at certain times of the year) is almost 
constant. These points, therefore, make the principal 
difference, and balance the more regular motion the folds of 
the root produce. I shall leave the description of the rising of 
the bud in the middle of the wood in those plants that shoot 
each year from the earth, till my next letter, when I give the 
root of those plants. It is, indeed, one of the most beautiful 
effects arising from the spiral formed wood, and its consequent 
motion $ it admirably accounts for the different manner th® 
buds shoot, in running up the wood instead hf across it, as in 
trees j since in the one the spirals are in their cases and confined, 
but 
