THE ANA.TOMY OF STEMS. 263 



microscope, consist of longitudinal fibres apparently 

 not solid, but narrow tubes or oblong cells, tbe sides 

 of which are thick and nearly opaque, and of vessels 

 of different kinds. These are arranged parallel to 

 each other, except where they are separated by the 

 divergent layers, as may be seen in a thin tangental 

 section of any stem placed under the microscope. In 

 the alburnum, the walls of the concentric tubes are 

 tender and transparent ; but by the deposition of lig- 

 neous matter in the membrane of which they consist, 

 and in the tubes themselves, they become opaque and 

 firm ; and according to the degree of this, the wood 

 is more or less dense, hard, and tenaceous. Other 

 matters, also, are deposited in this part of the woody 

 texture ; such for example as Guiac in that of the 

 Guiacum officinale, coloring matter in the Logwood 

 (Hazmatoxylon Campechianum), and even silex, which 

 has been extracted from the Teak wood (Tectonia 

 grandis) by Dr. Wollaston. The vessels of the con- 

 centric layers are chiefly porous and annular, and 

 their sections produce the openings observed in the 

 transverse section of any stem ; but besides these, 

 in the circle of the wood of the first year's growth, a 

 circle of spiral vessels surrounds the pith. These 

 are, however, justly regarded by Mirbel not as vessels 

 of the wood ; but of a distinct sheath lining the wood, 

 which he has denominated Vetui medullaire. 



2. The Divergent layers consist of flattened mas- 

 ses of cellular substance, which cross the concentric 

 layers at different parts, and, separating the bundles 

 of longitudinal tubes of which they consist from each 

 other, produce the reticulated arrangement seen in 

 the tangental section of any stem ; the oblong tubes 

 and vessels forming the tissue of the net-work, the 

 meshes of which are filled up by the cells of the di- 



