OF THE DIFFERENT TRIBES OF PLANTS. 19 



be more particularly directed, I shall, in the first place, mention a few more 

 particulars respecting the appearance which they present in the transverse 

 section. 



Fig. 4. American Fir. Shews two complete annual or concentric lay- 

 ers, and portions of other two. 



Fig. 5. Scotch Fir, Pinus sylvestris. One concentric layer, with part 

 of other two. 



Fig. 6. Norway Fir, Pinus Abies. A concentric layer, and portions 

 of other two. 



Now, in these it will be observed, that the apertures of the elongated 

 cells are generally contracted, sometimes entirely closed, towards the outer 

 or distal margin of each annual layer ; that the medullary rays are placed 

 at irregular distances, and vary in breadth ; and that a few large rounded 

 apertures occur, which are those of vessels or lacunae. The latter, however, 

 are always few and irregularly scattered in the Coniferae, and in these re- 

 spects bear no resemblance to those of the other Dicotyledonous stems. 



Fig. 7- Common Yew, Taxus baccata. Part of four layers. 



Fig. 8. American Cedar, from Mobile. Part of two layers. In these 

 trees I have not observed any circular apertures. 



Fig. 9. Cycas revoluta. Transverse section of 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 great spiral vessels, 

 which are filled with cellules, and part of a fasciculus of vessels proceeding 

 to a leaf. The Cycadeae belong to the same class as the Coniferae ; but I 

 have not succeeded in procuring proper specimens for examination. 



Fig. 10. Mahogany, Swietenia Mahogani. The annual layers here are 

 not well defined, a circumstance frequently observable in trees which grow 

 in warm climates. 



Fig. 11. Common Oak, Quercus Robur. Part of three layers, which 

 are well defined, and marked towards one of their margins by a denser tex- 

 ture. 



Fig. 12. Grey Poplar, Populus canescens. Part of two layers. In 

 this figure the apertures of the cellules are open ; but in the others, in 



