348 ANATOMY OF THE GYMNOSPERMS 



n-sin ducts are rather small, with 3 rows of cells in the middle portion. 

 No markings can be made out on the wood cells in this section" 

 (Knowlton). 



This species is very closely allied to the preceding, and should perhaps be 

 referred to it. The main points of difference are the following: smaller 

 resin ducts that are occasionally found in the summer wood ; smaller 

 wood cells throughout ; smaller and shorter compound medullary rays ; 

 ordinary rays always in a single series of 2-12 cells (average 6) instead 

 of 2-30 or more (average 12) (Knowlton). 



This species cannot be separated from the preceding on the basis of the 

 characters given, and it is undoubtedly the same, though recognized here 

 provisionally (D.P.P.). 



Trunks of small and medium size. Material silicified. 



Tertiary of the Yellowstone National Park at Specimen Ridge, near the head 

 of Crystal Creek (Knowlton). 



45. P. columbiana, Penh. 



Transverse. Growth rings variable though generally very broad in large 

 stems. Spring wood usually predominant, the transition to the summer 

 wood gradual but in the narrower rings more or less abrupt and some- 

 times conspicuously so ; the tracheids large, thick-walled and often con- 

 spicuously so, definitely rounded, often radially oval, chiefly uniform, 

 more or less equal, in regular radial rows. Summer wood conspicuous, 

 dense, and often rather thin. The structure as a whole is that of a rather 

 dense wood of medium hardness. Medullary rays prominent, not very 

 numerous, resinous, and distant upwards of 9 or more rarely 1 5 rows of 

 tracheids. Resin passages conspicuous, rather large, and scattering 

 throughout the growth ring, the parenchyma cells large, thin-walled, 

 and in 2 rows, or forming large and irregular tracts upwards of 6-9 

 tracheids wide, resinous; thyloses not obvious. 



Radial. Medullary rays resinous ; the tracheids rather numerous, marginal 

 and interspersed, not obviously predominant, very variable and often 

 as high as or higher than, long, sparingly dentate ; the parenchyma cells 

 all of one kind and rather thin-walled, straight, and equal to about 4 

 wood tracheids; the upper and lower walls strongly (?) pitted; the terminal 

 walls straight or diagonal and apparently not pitted; the lateral walls 

 with simple, round or lenticular pits of medium size, 2-4, chiefly 2, per 

 tracheid. Bordered pits on the tangential walls of the summer tracheids 

 small and not numerous ; those of the radial walls rather large, round, 

 or oval in i compact row, and generally numerous. 



Tangential. Fusiform rays rather numerous, short, the broad central tract 

 'with thin-walled parenchyma chiefly broken out, the unequal terminals 

 composed of broad, oval cells chiefly 'in i row. Ordinary rays low to 

 medium, i-seriate, not materially contracted by the interspersed tra- 

 cheids ; the parenchyma cells somewhat unequal and variable from 

 oblong to oval or broad and round. 



Calcified fragments of stems and branches, and also cones in the Tertiary 

 of Kettle River, near Midway, British Columbia. 



