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. THE BARK. Bark is the outer part of the cortex that has perished, having been cut off from nourishment by the thin hard plates of the bark-scales. In the late and early bark-formation is found a general but by no means an exact distinction between Soft and Hard Pines. In the Soft Pines the cortex remains alive for many years, adjusting itself by growth to the increasing thickness of the wood. The trunks of young trees remain smooth and without rifts. In the Hard Pines the bark-formation begins early and the trunks of young trees are covered with a scaly or rifted bark. The smooth upper trunk of older trees is invariable in Soft Pines, but in Hard Pines there are several exceptions to early bark-formation. These exceptions are easily recognized in the field, and the character is of decisive specific importance (glabra, halepensis, etc.).