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).