94 SPECIES OF ABIES DETERMINED BY 



the transverse section are the following. First, carefully examine 

 the central midrib or vein of the leaf. It consists, in Abies, of a 

 pair of fibro- vascular bundles, placed in general very close together, 

 and exhibiting distinctly two wood portions, or groups of wood 

 prosenchyma, one belonging to each of the separate bundles. Abies 

 can be at once separated from Pseudotsuga and Tsuga by this 

 character, as in both of these the bundle is single, only one wood 

 portion being visible. Second, observe the sheath of delicate paren- 

 chymatous cells surrounding the single or double bundle. It is a 

 single layer of cells, very different from the tissue of the mesophyll 

 or ground tissue of the leaf on the one side, and from the cells of 

 the fibro-vascular bundles on the other. Thirdly, the general 

 parenchyma of the leaf is to be scrutinised. If stomata are 

 developed on one side of the leaf only, that is, on the under 

 side, then the chlorophyll -bearing cellular tissue is loose, with 

 many intercellular spaces communicating with the stomata of 

 the under surface. On the upper side the chlorophyll -bear- 

 ing cells are generally placed so as to form two zones or more 

 of the pallisade tissue. If, however, stomata be present on 

 the upper side, then the pallisade tissue is interrupted, and the 

 loose tissue, with the usual intercellular spaces will be noticed. 

 Fourthly, in the mesophyll, two resin canals will be found, their 

 position varying in different species. The resin canal is an inter- 

 cellular canal, surrounded by a zone of small rather thick-walled 

 cells. The canals are, in one series, placed in the middle of the 

 parenchyma, and run from base to apex of the leaf parallel to the 

 midrib, but about equidistant from the midrib and margin, and 

 also nearly equidistant from the upper and under sides of the leaf. 

 In another series the resin canals are placed very close to the 

 epidermis of the under side of the leaf, running parallel and close 

 to the margin of the leaf. The resin canals are very conspicuous 

 objects in the transverse section, and are of great importance in 

 separating the different forms. Fifthly, the epidermis of the upper 

 and lower surfaces of the leaf, and the cells immediately beneath 

 it, have to be carefully examined. The epidermis consists of a 

 single layer of cells with thick walls, and externally provided with 

 a well developed cuticle. It also bears the stomata in rows, the 

 rows forming very definite white bands on the under surface, one 

 on each side of the midrib. When stomata are present on the 

 epidermis of the upper side, they generally occur in long rows, 

 sometimes over the whole surface, or limited to the upper half or 



