74 
COURSE OF THE SAP. 
[P4RT II. 
Medullary rays 
longitudinal 
plates, only 
bounded in 
length by the 
height of trees 
and length of 
branches and 
roots. 
The shake, and 
cup-shake. 
to these, intermediate medullary rays are deve¬ 
loped from each new concentrical pith, which 
run from that new concentrical pith to the 
bark, and are annually prolonged. Indeed, as 
very few medullary rays could be developed in a 
seedling of perhaps half an inch in girthing, it 
seems only natural that the number of rays 
should increase with the growth of the tree. 
Otherwise, when the girthing of the tree had 
increased from half an inch to thirty feet, the 
medullary rays would stand very far apart at 
their outward ends; and in the bark of thirty 
feet circumference there would be only the same 
number of rays as in the bark of half an inch 
circumference. The new rays, however, have 
no right strictly to the name of medullary , since 
they do not originate in the central pith or me¬ 
dulla. The medullary rays, which appear like 
the spokes of a wheel when the stem is cut 
across, are, in fact, thin plates running the 
whole length of the stem, roots, and branches. 
In width they increase every year by the width 
of the new layer of wood across which they ex¬ 
tend to the bark, and in length they increase 
every year by the length of the new shoot of the 
branches and roots. 
The medullary rays and the concentrical 
