34 



ANATOMICAL STRUCTURE OF THE LEAVES. 



Fig. 17. Transverse section of leaf of Abies pectinata. 



are very distinctly shown near the inferior epiderm at a short distance 

 from each margin. This position of the resin-canals is characteristic of 

 Abies throughout, differing in some of the species only by being wholly 

 within the parenchyma (mesophyll) or close to the epidermis of the 

 under side, and by their distance from the margin. Comparing Fig. 17 

 with that preceding it, and also with Fig. 19, the structural difference 



between the leaf of an 

 Abies and a Tsuga is 

 manifest, whilst the close 

 similarity of Abies to 

 Abietia is scarcely less 

 evident. For further 

 illustration, Fig. 18 shows 

 a transverse section of a 

 leaf of Abies i-owolor. 



Fig. 19 shows a transverse section of a leaf of the Douglas Fir, Abietia 

 Douglasii, magnified fifty diameters. In the leaves of this Fir the 

 hypoderm, according to the late Professor McXab,* is very variable ; in 

 the section here given, it occurs only along the region of the midrib 

 under the epiderm of the upper surface. The resin-canals are two and 

 traverse the mesophyll near the epiderm of the lower surface as in Abies. 

 Fig. 20 represents a transverse section of a leaf of Pinus Laririo 

 enlarged thirty diameters. The hypoderm is here a very narrow band of 

 thick walled cells, following the contour of the leaf, whilst the mesophyll 

 consists of a broad band of cells containing chlorophyll between it and 



the parallelogram enclosing the 

 two fibrovascular bundles / and 

 marked off in the centre by a 

 layer of smaller cells. Traversing 

 the mesophyll about midway 

 between the hypoderm and the 

 central parallelogram are eight 

 resin-canals r r lined with thin- 

 Fig. IS. Transverse section of leaf of Abies concolor. walled epithelium cells. Ill the 



triquetral (three-sided) leaves of 



the Pines, with three and five leaves in a sheath, there are fewer 

 resin-canals, and where there are but three, there is usually one in or 

 near each of the three angles. 



The STOMATA or minute pore-like perforations of the epidermis of adult 



leaves should also be 

 mentioned here on account 

 of the important part they 

 play in the economy of 

 the plant. The chief 

 points observable in their 

 relation to the species, 

 irrespective of their phy- 

 siological significance, are 

 their position, distribu- 

 tion or arrangement, and 

 their number. As regards 



their position, they occur on the under surface chiefly, on the upper 

 * Proceedings of the Royal Irish Academy, 2nd ser. II. 703. 



Fig. 10. Transverse section of leaf of Abietia Doiiglasil. 



