THE DERMAL TISSUES OF THE LEAF.
The stomata of Pine leaves are depressed below the surface and interrupt the continuity of epiderm and hypoderm. They are wanting on the dorsal surface of the leaves of several Soft Pines, constantly in some species, irregularly in others. In Hard Pines, however, all surfaces of the leaf are stomatiferous. In several species of the Soft Pines the longitudinal lines of stomata are very conspicuous from the white bloom which modifies materially the general color of the foliage.
Under the action of hydrochloric acid the hypoderm is sharply differentiated from the epiderm by a distinct reddish tint, but without the aid of a reagent the two tissues do not always differ in appearance. The cells of epiderm and hypoderm may be so similar that they appear to form a single tissue. In most species, however, the epiderm is distinct, while the cells of the hypoderm are either uniform, 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 affinities 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.