i.] MED ULLAR Y RA YS. 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 in 

 the cleaving of posts, rails, or palings for park and other 

 fences, know that they can only successfully do this 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 



