1.] MEDULLARY RAYS. 7 
necessary, for many purposes in carpentry, to regard this 
arrangement of medullary rays, to insure that the work 
shall remain, when finished, free from warp or twist 
upon the surface. The timber should be cut as nearly 
as possible in the direction of these rays, the shrinkage 
in seasoning being, for the most part, angular to them. 
Workmen in general, and modellers in wood in par 
ticular, endeavour to embrace the greatest length of 
medullary figure in their work to guard against warping, 
well knowing that if they do so it will stand satisfactorily 
the test of time and wear.) Others, who are engaged i in: 
the cleaving of posts, rails, . palings for park and other_ 
fences, know that they can only successfully do the by 
rending the piece in the direction of these rays. (It is’ 
by a careful study of this that we obtain our best and- 
most beautifully figured wainscot from the slow-growing 
Oaks found in the North of Europe, Austria, Asia Minor, 
and in some districts of North America. 
By the contact of these medullary rays with the 
annual layers, and chiefly in the newly-formed wood, 
a means is afforded for the ascent of sap from the root. 
The sap is believed to be drawn upwards every spring 
by capillary attraction, and continues for a time to flow 
through the pores and fibres of the tree until it reaches 
the upper side of the leaves; thence it returns, by the 
under side of the leaf downwards, between the outer 
circle or zone of ligneous layers and the bark, per- 
meating in its course the whole body of the tree, and 
contributing to form annually a new layer. During 
this progress the sap undergoes some very important 
chemical change, and, becoming gradually elaborated, 
tends to the formation of a substance called cambium, 
between the liber, or bark, and the alburnum. The 
stem is thus enlarged by a new layer on the outside of 
