6 GENUS PmUS 



with equally thin or thick walls — or biform, with very thin walls in the outer row of cells and very 

 thick walls in the inner row or rows of cells — or multiform, with cell-walls gradually thicker 

 toward the centre of the leaf. These conditions may be tabulated as follows — 



Cells of epiderm and hypoderm similar fig. 19. 



Cells of epiderm and hypoderm distinct. 



Cells of hypoderm uniform, thin or thick figs. 20, 21. 



Cells of hypoderm biform fig. 22. 



Cells of hypoderm multiform fig. 23. 



The biform hypoderm is not always obvious (clausa, Banksiana, etc.,) where in some leaves there 

 is but one row of cells. But with the examination of other leaves one or more cells of a second row 

 will be found with very thick walls. Among Hard Pines there is no Old World species with a biform 

 hypoderm. But there are a few American species with uniform hypoderm (resinosa, tropicalis, 

 patula and Greggii) ; while, in some leaves of the few American Hard Pines with multiform hypoderm, 

 the uniform hypoderm is a variation. 



THE GREEN TISSUE. 



In this tissue are the resin-ducts, each with a border of cells, corresponding in appearance and in 

 chemical reaction with the cells of the hypoderm and with thinner or thicker walls. With reference 

 to the green tissue the foliar duct may be in one of four positions. 



1. External . . against the hypoderm fig. 24. 



2. Internal . . against the endoderm fig. 28. 



3. Medial . . .in the green tissue, touching neither hypoderm nor endoderm . . . .fig. 26. 



4. Septal .... touching both endoderm and hypoderm, forming a septum fig. 30. 



Among the Soft Pines the external duct is invariable in the subsection Paracembra. It is also 

 characteristic of the Strobi, where it is sometimes associated with a medial duct. In the Cembrae 

 and the Flexiles, however, the ducts are external in some species, or medial or both in others, without 

 regard to the aflBnities of these species. 



Among the Hard Pines the external duct is characteristic of the Old World, there being but two 

 American Pines with this character (resinosa and tropicalis). The internal duct is peculiar to Hard 

 Pines of the New World, its presence in Old World species being extremely rare. The medial duct 

 is common to species of both hemispheres, either alone or in association with ducts in other positions 

 (figs. 25, 27). The septal duct is peculiar to a few species (oocarpa, tropicalis, and less frequently 

 Pringlei and Merkusii). I have also seen it in a leaf of P. canariensis. The internal and septal ducts 

 appear to be confined to the species of warm-temperate or tropical countries. 



The number of resin-ducts of a single leaf may be limited to two or three (strobus, koraiensis, etc.), 

 but in many species it is exceedingly variable and often large (pinaster, sylvestris, etc.). Eighteen 

 or more ducts in a single leaf have been recorded. Such large numbers are peculiar to Pinus. Occa- 

 sionally a single leaf, possibly the leaves of a single tree, may be without ducts, but this is never 

 true of all the leaves of a species. 



THE STELAB TISSUES. 



The walls of the endoderm are, in most species, uniform, but, with P. albicaulis and some species 

 of western North America, the outer walls of the cells are conspicuously thickened (fig. 32). Both 

 thin and thick walls may be found among the leaves of the group Macrocarpae and of the species 

 longifolia. 



The fibro-vascular bundle of the leaf is single in Soft Pines, double in Hard Pines. This distinc- 

 tion is employed by Koehne as the basis of his two sections, Haploxylon and Diploxylon. The double 

 bundle is usually obvious even when the two parts are contiguous, but they are sometimes com- 



