The Secondary Leaf. Plate II
Secondary leaves, the permanent foliage of Pines, are borne on dwarf-shoots in the axils of primary leaves. They form cylindrical fascicles, rarely monophyllous, prevalently of 2, 3 or 5 leaves, occasionally of 4, 6, 7, or 8 leaves. The scales of the fascicle-bud elongate into a basal sheath, deciduous (fig. 15) in all Soft Pines except P. Nelsonii, persistent (fig. 16) in all Hard Pines except P. leiophylla and Lumholtzii. Inasmuch as these three species are easily recognized, the fascicle-sheath is useful for sectional distinctions.
External Characters
The number of leaves in the fascicle is virtually constant in most species, the variations being too rare to be worthy of consideration. With some species, however, heteromerous fascicles are normal. The influences that cause this variation are not always apparent (echinata, etc.), but with P. ponderosa, leiophylla, sinensis and others, the number of leaves in the fascicle is, in some degree, dependent on climatic conditions, the smaller number occurring in colder regions. In Mexico, for example, where snow-capped mountains lie on subtropical table-lands and extremes of temperature are in juxtaposition, the conditions are favorable for the production of species with heteromerous fascicles, and the number of leaves in the fascicle possesses often climatic rather than specific significance.
Among conifers, the leaf of Pinus attains extraordinary length with great variation, from 5 cm. or less to 50 cm. or more, the maximum for each species being usually much more than twice the minimum. Climate is the predominating influence; for the shortest leaves occur on alpine and boreal species, the longest leaves on species in or near the tropics.
The length of the leaf is complicated by the peculiarities of individual trees and by pathological influences; as a general rule, however, the length of leaves is less or greater according to unfavorable or favorable conditions of temperature, moisture, soil and exposure. Therefore the dimensions of the leaf may be misleading. It can be said, however, that certain species always produce short leaves, others leaves of medium length, and others very long leaves.
Persistence of the leaf varies with the species and with the individual tree. But it is noteworthy that the longest persistence is associated with short leaves (Balfouriana, albicaulis, montana, etc.).
Internal Characters
Since the leaf-fascicle is cylindrical, the cross-section of a leaf is a sector, its proportional part, of a circle. Theoretically the leaf, in section, should indicate the number of leaves composing its fascicle. This is absolutely true for fascicles of two leaves only. No fascicle of five leaves, that I have examined, is equally apportioned among its five members. It may be divided in various ways, one of which is shown in fig. 18, where the leaf (a) might be mistaken for one of a fascicle of 3, and the leaf (b) for one of a fascicle of 6. Therefore if absolute certainty is required, a fascicle of triquetral leaves is best determined by actual count.
The transverse section of a leaf may be conveniently divided into three distinct parts—1, the dermal tissues, epiderm, hypoderm and stomata (fig. 17-a)—2, the green tissue, containing the resin-ducts (fig. 17-b)—3, the stelar tissues, enclosed by the endoderm and containing the fibro-vascular bundle (fig. 17-c).
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.
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. Occasionally 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 Stelar 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 distinction 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 completely merged into an apparently single bundle. This condition, however, is never constant in a Hard Pine, and a little investigation will discover a leaf with a true double bundle.
Some cells about the fibro-vascular bundle acquire thick walls with the appearance and chemical reaction of the hypoderm cells. Among the Soft Pines this condition is most obvious in the group Cembroides. Among the Hard Pines it appears in all degrees of development, being absent (figs. 24, 25), sometimes in irregular lines above and below the bundle (figs. 26, 27, 30, 31), or forming a conspicuous tissue between and partly enclosing the two parts of the bundle (figs. 28, 29).
The leaf-section furnishes sectional and other lesser distinctions. It is often decisive in separating species otherwise difficult to distinguish (nigra and resinosa or Thunbergii and sinensis, etc.). Sometimes it is sufficiently distinct to determine a species without recourse to other characters (tropicalis, oocarpa, Merkusii, etc.). An intimate knowledge of the leaf-section, with an understanding of the limits of its variation, is a valuable equipment for recognizing species.