68 



ANATOMY OF THE GYMNOSPERMS 



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- 

 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 

 cubensis (fig. 8), afford substantial proof in 

 confirmation of the probable correctness of 

 this view. In the summer wood the pit orifice 

 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 

 axis. At first sight this would seem to imply that these features 

 represent primitive spirals, the original direction of which has 

 not been greatly if at all modified, but one or two considerations 

 will assist us to a correct interpretation of this feature. In the 



FIG. 14. PINUS PUN- 

 GENS. Bordered 

 pits on the radial 

 walls of the sum- 

 mer wood. X 280 



