ILLUSTRATED BY FIGURES. 25 
portions of concentric woody layers of various trees belonging to the family 
of Conifere. 
This family consists of trees and shrubs, the internal structure of whose 
stems presents the following arrangement. There is a central column of 
cellular texture named the pith, around which is developed a cylinder of 
woody texture, which, in its transverse section, shews divergent lines, with 
intervening cells. The divergent lines are named medullary rays, or in- 
sertions. According to the age of the stem, branch or twig, there are fewer 
or more of these woody layers, one being formed annually. The woody 
layers are produced by the liber or inner layer of the bark, and the increase 
thus takes place from without, and not from the centre or pith. Irregular 
vacuities are frequently formed in the woody texture, partially filled with ir- 
regular cellules, and sometimes containing concrete juices. 'They are named 
Lacune, and are seen in Figs. 1, 2, and 3. I have not observed any in 
the Yew or American Cedar. It will be observed in the figures generally, 
that the texture becomes denser towards the outer margin of each concen- 
tric layer. 
Fig. 6. Cycas revoluta. Transverse section, shewing a small portion 
near the centre of a young stem. The cellular tissue is very irregular. A 
portion of the double circle of fibres is seen, as well as three lacune, and 
part of a fasciculus of spiral vessels. As no fossil plants of this genus have 
occurred to me, I shall not further insist on the organization of the Cycadee. 
Fig. 7. Mahogany, Swietenia Mahagoni. ‘Transverse section. The 
layers here are not well defined, a circumstance frequently observable in 
trees which grow in warm climates. With reference to the specimen here 
figured, it is necessary to remark, that the portions of it which have been cut — 
very thin, present a cellular appearance, while those which are thicker pre- 
sent a more opaque aspect, with minute apertures of a circular form. 
Fig. 8. Common Oak, Quercus Robur. Transverse section, shewing 
part of three layers. The layers here are well defined, and are moreover 
marked towards one of their margins by a denser texture. 
Fig. 9. Grey Poplar, Populus canescens. Transverse section, shew- 
ing part of two layers. 
D 
