4 GENUS PINUS 



uous (fig. 15) in all Soft Pines except P. Nelsonii, persistent (fig. 16) in all Hard Pines except P. leio- 

 phylla 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. pon- 

 derosa, 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 tablelands and extremes of temperature are in juxtaposi- 

 tion, 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 mini- 

 mum. 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, moistiu*e, 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 fas- 

 cicle. 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 stomatifer- 

 ous. 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 diflFerentiated from the epiderm by 

 a distinct reddish tint, but without the aid of a reagent the two tissues do not always differ in appear- 

 ance. 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. 



