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Invisible or latent buds are present at the nodes and at the apex of dwarf shoots. The former are the origin of the numerous shoots that cover the trunk and branches of P. rigida, leiophylla and a few other species . The latter develop into shoots in the centre of a leaf-fascicle when the branchlet, bearing the fascicle, has been injured.

The size, color and form of buds, the presence of resin in quantity, etc., assist in the diagnosis of species. Occasionally a peculiar bud, like that of P. palustris, may be recognized at once.

THE BRANCHLET. Plate I, figs. 12-14.

The branchlet, as here understood, is the whole of a season's growth from a single bud, and may consist of a single internode or of two or more internodes , each internode being defined by a leafless base and a terminal node of buds.

The spring-shoot is uninodal in all Soft Pines and in many Hard Pines, but, in P. taeda and its allies and in species with serotinous cones, it is more or less prevalently multinodal.

The uninodal spring-shoot may remain so throughout the growing season and become a uninodal branchlet. Or a summer-shoot may appear on vigorous branches of any species with the result of converting a uninodal spring-shoot into an imperfect multinodal branchlet. The summer-shoot may be recognized, during growth, by its green, not scarious bracts and, at the end of the season, by the imperfect growth of its wood and foliage .

The perfect multinodal branchlet is formed in the winter-bud and the spring-shoot is multinodal. It is gradually evolved among the Hard Pines, where it may be absent, rare, frequent or prevalent, according to the species. In fact there is, in Pinus, an evolutionary tendency toward multinodal growth, with its beginnings in the summer-shoot and its culmination in the multinodal winter-bud, most prevalent among the serotinous Pines.

The multinodal shoot is never invariable in a species, but is rare, common or prevalent. This condition prevents its employment for grouping species. For Pines are not sharply divided into multinodal and uninodal species, and no exact segregation of them, based on this difference, is possible. In fact the character is unequally developed among closely related species, such as P. palustris and caribaea. Both produce multinodal shoots, but the former so rarely that it should be classed as a uninodal species, while the latter is characteristically multinodal. The multinodal spring-shoot, however, has a certain correlative value in its relation to other evolutionary processes that are obvious in the genus.

The length of the branchlet is much influenced by different soils and climates. In species able to adapt themselves to great changes, the length of the internode may vary from 50 cm. or more to 1 cm. or less. In the latter case the branch is a series of very short leafless joints terminated by a crowded penicillate tuft of leaves . Such a growth may be seen on any species that can survive exposure and poor nourishment.

The presence of wax, as a bloom on the branchlet, is associated with trees in arid localities, especially Mexico, where it is very common. With several species the character is inconstant, apparently dependent on environment, and is a provision against too rapid transpiration.

The branchlet furnishes evidence of the section to which the species belongs, for the bract-bases persist after the bracts have fallen away. The color of the branchlet, its lustre, the presence of minute hairs, etc., are often suggestions for determining species.

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 in all Soft Pines except P. Nelsonii, persistent 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 , 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 .

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 might be mistaken for one of a fascicle of 3, and the leaf 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 --2, the green tissue, containing the resin-ducts --3, the stelar tissues, enclosed by the endoderm and containing the fibro-vascular bundle .

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 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 ; 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.

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 . 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 . The septal duct is peculiar to a few species . 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 , but in many species it is exceedingly variable and often large . 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 . 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 , sometimes in irregular lines above and below the bundle , or forming a conspicuous tissue between and partly enclosing the two parts of the bundle .

The leaf-section furnishes sectional and other lesser distinctions. It is often decisive in separating species otherwise difficult to distinguish . Sometimes it is sufficiently distinct to determine a species without recourse to other characters . An intimate knowledge of the leaf-section, with an understanding of the limits of its variation, is a valuable equipment for recognizing species.

The flowers in Pinus are monoecious, the pistillate in the position of a long shoot, taking the place of a subterminal or lateral bud, the staminate in the position of a dwarf-shoot, taking the place of a leaf-fascicle but confined to the basal part of the internode.

Pistillate flowers are single or verticillate. On multinodal shoots they are often multiserial, appearing on two or more nodes of the same spring-shoot . On uninodal shoots they are necessarily subterminal , the lateral pistillate flower being possible only on multinodal shoots where it is often associated with the subterminal flower . Like the multinodal shoot, on which its existence depends, the lateral pistillate flower cannot be employed for grouping the species. It is merely the frequent, but not the essential, evidence of condition of growth that is more perfectly characterized by the shoot itself.

Staminate catkins are in crowded clusters, capitate or elongate , but with much variation in the number of catkins in each cluster. In P. rigida I have found single catkins or clusters of all numbers from two to seventy or more. In P. Massoniana and P. densiflora a cluster attains such unusual length that this character becomes a valuable distinction between these species and P. sinensis, which has short-capitate clusters. The catkins differ much in size, the largest being found among the Hard Pines.

In the connective of the binate pollen-sacs there is a notable difference , the smaller form being characteristic of the Soft Pines. But this is not invariable , and the absence of complete data does not permit an accurate estimate of its importance.

After pollination the pistillate flower closes and becomes the conelet, the staminate flowers withering and falling away. The conelet makes no appreciable growth until the following year. Like the pistillate flower it may be subterminal or lateral, but a subterminal pistillate flower may become a pseudolateral conelet by reason of a summer-growth . Such a condition may be recognized on the branchlets of the present, and of the previous year , by the very short internode and short leaves beyond the fruit.

The conelet offers some distinctions of form, of color, and of length of peduncle, while in some species its reflexed position is an important specific character. The most important distinctions, however, are found in its scales, which may be

The cone of Pinus shows great differences of color, form and tissue; these are useful for specific and sectional distinctions, while the gradual change from the primitive conditions of the Cembrae to the elaborate form, structure and mode of dissemination of some serotinous species are obvious evidence of an evolution among the species of remarkable taxonomic range. A form new among Coniferae appears, the oblique cone, and a new condition, the serotinous cone, both appearing at first alone and, finally, in constant association.

COLOR OF THE CONE.

With few exceptions the color of the ripe cone may be classified under one of the following shades of brown or yellow.

Nut-brown The stain of the walnut-husk. Rufous brown A pronounced reddish nut-brown. Fulvous brown A yellowish nut-brown. Tawny yellow The color of the lion. Orange Ochre-yellow to red-orange.

These colors may be paler or deeper. They may be obscured by a fuscous shade or may be modified by a dull or lustrous surface. The presence of two or more of these shades in a single species and the inherent difficulties of color description lessen the value of the character. Nevertheless certain allied species, such as P. nigra and Thunbergii, or P. densiflora and Massoniana, may be distinguished by the prevalent difference in the color of their cones.

DIMENSIONS OF THE CONE.

The cone is small, medium or large in different species, but varies greatly under the influences of environment or of individual peculiarities. The character possesses relative value only, for great variation is possible in the same locality and even on the same tree.

THE PEDUNCLE.

All conelets are pedunculate, but in some species the peduncle, even when long , may become overgrown and concealed by the basal scales of the ripe cone. Articulation usually takes place between the peduncle and the branch, sometimes with the loss of a few basal scales which remain temporarily on the tree . With P. Nelsonii, and to a less degree with P. Armandi, there is articulation between the cone and its peduncle.

There are several species bearing persistent cones with no articulation. This condition appears in other genera, such as Larix and Picea, but without obvious significance. In Pinus, however, the gradual appearance of the persistent cone, for it is rare, common, prevalent or invariable in different species, and its essential association with the serotinous cone, suggest an evolution toward a definite end.

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