ar 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.

[Illustration: PLATE II. SECONDARY LEAVES]

THE FLOWERS. Plate III, figs. 33-39.

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 (fig. 33). On uninodal