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.
