THE WOOD. [Plate VII].
With the exception of the medullary rays, a very small proportion of the whole, the wood of Pinus, as seen in cross-section (fig. 82), is a homogeneous tissue of wood-tracheids with interspersed resin-ducts. In tangential section the medullary rays appear in two forms, linear, without a resin-duct, and fusiform, with a central resin-duct. In radial section the cells of the linear rays are of two kinds, ray-tracheids, forming the upper and lower limits of the ray, characterized by small bordered pits, and ray-cells, between the tracheids, characterized by simple pits.
The walls of the ray-tracheids may be smooth or dentate; the pits of the ray-cells may be large or small. These conditions admit of four combinations, all of which appear in the medullary rays of Pinus, and of which a schematic representation is given in [Plate VII]. These combinations are
| Ray-tracheids with smooth walls. Soft Pines. | ||
| Ray-cells with large pits | Subsection Cembra | fig. 80. |
| Ray-cells with small pits | Subsection Paracembra | fig. 81. |
| Ray-tracheids with dentate walls. Hard Pines. | ||
| Ray-cells with large pits | Group Lariciones | fig. 83. |
| Ray-cells with small pits | Other Hard Pines | fig. 84. |
This, the simplest classification of Pine-wood, is not without exceptions. P. pinea of the Hard Pines resembles, in its wood-characters, P. Gerardiana and P. Bungeana of the Soft Pines. The dentate ray-tracheids of P. longifolia are not always obvious. The tracheids of P. luchuensis, according to Bergerstein (Wiesner Festschr. 112), have smooth walls. My specimen shows dentate tracheids. There is also evidence of transition from small to large pits (I. W. Bailey in Am. Nat. xliv. 292). Both large and small pits appear in my specimen of P. Merkusii.
Of other wood-characters, the presence or absence of tangential pits in the tracheids of the late wood establishes a distinction between Soft and Hard Pines. These pits, however, while always present in Soft Pines, are not always absent in Hard Pines. The single and multiple rows of resin-ducts in the wood of the first year may prove to be a reliable sectional distinction, but this character has not been sufficiently investigated to test its constancy. The wood-characters, therefore, however decisive they may be for establishing the phylogenetic relations of different genera, must be employed in the classification of the Pines with the same reservations that apply to external characters.
Ray-tracheids with dentate walls and ray-cells with large pits are peculiar to Pinus. Therefore the presence of these characters, alone or in combination, is sufficient evidence for the recognition of Pine-wood. But the combination of smooth tracheids with small pits (subsection Paracembra) Pinus shares with Picea, Larix and Pseudotsuga.
Among Hard Pines the size of the pits has a certain geographical significance. The large pits are found in all species of the Old World except P. halepensis and P. pinaster; the small pits in all species of the New World except P. resinosa and P. tropicalis. The Asiatic P. Merkusii with both large and small pits is not strictly an exception to this geographical distinction. The four exceptional species by this and by other characters unite the Hard Pines of the two hemispheres.