. A manual of the North American gymnosperms [microform] : exclusive of the cycadales but together with certain exotic species. Bois; Trees; Gymnosperms; Gymnospermes; Arbres; Wood. 68 ANATOMY OF THE GYMNOSPERMS i M from one another. Nor is it apparent at first sight in those cases of two-seriate pits where, as in Cupressoxylon Dawsoni from the Cretaceous, Larix americana. Sequoia, and various species of Pinus, the pits are always paired off in such a way that the axis of each pair is at right angles to the axis of the cell (fig. 8). Two explanations are here possible: (i) the spirals are in r
. A manual of the North American gymnosperms [microform] : exclusive of the cycadales but together with certain exotic species. Bois; Trees; Gymnosperms; Gymnospermes; Arbres; Wood. 68 ANATOMY OF THE GYMNOSPERMS i M from one another. Nor is it apparent at first sight in those cases of two-seriate pits where, as in Cupressoxylon Dawsoni from the Cretaceous, Larix americana. Sequoia, and various species of Pinus, the pits are always paired off in such a way that the axis of each pair is at right angles to the axis of the cell (fig. 8). Two explanations are here possible: (i) the spirals are in reality two-seriate, and are projected through the alternate members of the two rows of pits; or (2) the disposition of the pits repre- I sents an extreme phase in the flattening of the original spirals conformably to a higher type of development. This view, which seems the more reasonable, is in direct harmony with De Bary's law, while it receives additional support from the form and direction of the pit orifice. The orifice of the pit is variable, at differ- ent times being round, when the pits are also round and more or less distant; oval or oblong, when the pits assume corresponding forms; or, in the summer wood, lenticular or oblong. The transversely elliptical pits of Pinus stro- bus (fig. 9), the orifice of which is also trans- versely oblong, as also the similar pits of Pinus ""gens. Torde'rTd ''"''^"''^ ^^^^ ^^' ^^^''^ Substantial proof in pits on the radial Confirmation of the probable correctness of walls of the sum- this view. In the summer wood the pit orifice mer wood, x 280 , ... commonly assumes a position which appears to offer a direct contradiction to this conclusion. In Pinus strobus (fig. 10) the orifice is oblong and parallel with the tracheid axis. In Pinus pungens, as in many others of the same genus (fig. 14), the narrow orifice is extended above and below into a diagonal slit of great length, forming a narrow angle with the tracheid
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