ture is uniform ihroiighout the whole tribe of Coniferae, the only 

 perceptible difference coosisting of the dimensions of the meshes, 

 trees of slow growth, as Taxus Baccata, having the finest tex- 

 ture. It may be right to mention, that in some of the Coni- 

 ferse, (not in all), there are occasionally circular open'mgs to be 

 seen, known to botanists under the name of Lacunae. These, 

 however, are very irregular in their distribution, sometimes oc- 

 curring frequently, at other times not at all. 



The structure of the true Dicotyledons consists of a system of 

 vessels separated from one another by masses of cellular matter. 

 The vessels or pores are always bounded by curve lines. In 

 some trees they are circular, in others they are elliptical, and the 

 degree of eccentricity of the ellipses in different trees is remark* 

 ably different. The elliptical pores are sometimes divided by 

 one or two transverse partitions. In some trees the vessels are 

 empty, and in other trees they are filled with a resinous or 

 gummy- resinous matter. The size, form, number and arrange- 

 ment of the vessels or pores differ so widely in different trees, 

 that one species may be as clearly distinguished from another 

 by the organic structure as by the shape of the leaves or the 

 florification. As in the Coniferae, the vessels generally become 

 smaller as they approach the outer edge of the annual layers. 

 In some kinds of trees the vessels are numerous, in other kinds 

 they are sparingly bestowed ; and it is worthy of note, that, in 

 some kinds of wood of great strength and durability, as the oak, 

 they are not only numerous, but also of very large dimensions. 



The cells constituting the cellular portion of Dicotyledonous 

 trees, have different forms and different dimensions in different 

 kinds of wood. In the coarser kinds of mahogany for instance, 

 the cells have a rhomboidal form. In some trees they are sphe- 

 rical, and in others the form is very irregular ; but however the 

 form may differ, it will in general be found that the smaller the 

 cells the greater will be the strength of the timber. This is 

 very strikingly the case with regard to the diflerent kinds of ' 

 ebDito In the Scotch Elm the cells are smaller than in the 

 English Elm, and these, again, are smaller than in the Dutch 

 Ehii ; and it is generally known the timber of the Scotch Elm 

 is better than that of the English, and that tlic Dutch Elm is 

 good for nothing. 



A a2 



