SECT. I. ORGANOGRAFHY AND GLOSSOLOGY. 45 



which are called the "Alburnum," remain soft and 

 pale, and are rejected by workmen as being unsuited to 

 economic purposes. The variously coloured fancy woods 

 employed by the turner consist of the heart only, the 

 alburnum in the ebony, even, being quite white. 



Each zone is principally composed of cellular tissue 

 towards its inner, and of vascular tissue towards its 

 outer parts : and each is supposed to be as a repetition 

 of the parts formed during the first year's growth. In 

 the common sumach (Rhus typhinum), especially, the 

 cellular or inner part of each zone has precisely the 

 same appearance as the pith, which is here of a pecu- 

 liar brown colour and easily recognised. But as there 

 are no tracheae among the vessels in the outer part of 

 the zones, whilst these are abundant in the medullary 

 sheath, the analogy alluded to is not perfect. 



Some woods contain scarcely any ducts, as many Coni- 

 ferae ; and the delicate material of which rice-paper (as 

 it is called) is composed, consists entirely of cellular 

 tissue. This curious substance is procured from the 

 herbaceous stems of a species of ./Eschynomene, growing 

 in China. The whole stem is about an inch thick, and 

 resembles a mass of pith covered by a very thin epi- 

 dermis. There is, however, a 

 central column of real pith run- 

 ning through it. By means of 

 some sharp instrument, the stem 

 is cut spirally round the axis into 

 a thin lamina (fg. 36.), which is 

 then unrolled, and may be made 

 up into sheets containing about a 

 foot square of surface. 



(51.) Medullary Rays (see fig. 24.). These form 

 what carpenters term the " silver grain " in wood, and 

 are generally distinctly traceable in dicotyledonous trees. 

 They may be seen passing in straight lines from the centre 

 to the circumference, but cannot be traced continuously 

 to any great extent in a vertical direction. They ap- 



