240 ANATOMY OF THE GYMNOSPERMS 



" Tangential. The medullary rays are very abundant. They are always 

 simple and consist of a single layer, which ranges from i to 15 cells 

 in height, the average being about 7 or 8. The tracheids do not show 

 bordered pits on the tangential walls, a fact of considerable impor- 

 tance " (Knowlton). 



Remains silicified. 



From the Potomac Formation at Spring Hill, Virginia (Knowlton). 



14. * * C. arkansanum, Knowlton 



" Transverse. The annual ring is either entirely absent or so obscured by 

 the mass of crushed cells as to be indistinguishable. In a single, 

 exceptionally well-preserved spot the tracheids are seen to be arranged 

 in nearly uniform radial rows, there being generally about 3 or 4 rows 

 between 2 medullary rays. The rays are abundant and consist of a 

 single cell. 



''Radial. The tracheids are rather thick-walled and provided with a single 

 row of pits. The pits are rather distant, the outer circle having a diam- 

 eter of .oii-.oi45 mm., and the inner a diameter of .oo28-.oo48 mm. 

 The medullary rays are abundant, and usually only a single series thick, 

 although a few may be found with 2 series of cells in the center. 



" Tangential. The material is not sufficiently well preserved to determine 

 the medullary rays satisfactorily" (Knowlton). 



Remains silicified. 



From the Tertiary deposits (Orange sands) of Poinsett County, Arkansas 

 (Knowlton). 



15. * * C. Dawsoni, Penh. 



Transverse. Growth rings variable, chiefly medium to broad. Tracheids 

 of the spring wood large, thin-walled, conspicuously squarish, and 

 passing gradually into the usually thin but rather prominent summer 

 wood, which may occasionally become thicker and without definite 

 internal demarcation. Resin passages wholly wanting. Resin cells 

 numerous and conspicuous throughout the growth ring, often in more 

 or less prominent, tangential row. Medullary rays prominent, resin- 

 ous, i cell wide but rather broad, distant 2-9 rows of tracheids. 



Radial. Medullary rays devoid of tracheids. Ray cells resinous, contracted 

 at the ends, equal to 5-6 spring tracheids ; the upper and lower walls 

 thin and sparingly pitted ; the terminal walls straight, or sometimes 

 strongly curved, not pitted or locally thickened ; the lateral walls 

 with round or oval pits, 1-2 per tracheid in vertical series, or in 

 marginal cells and low rays, or over very broad tracheids, becoming 4 

 per tracheid. Bordered pits large, in I row, often more or less in pairs, 

 and so, over broad tracheids, becoming more or less 2-rowed. Resin 

 cells numerous, 35-40 /j. wide, 200 p. long. 



Tangential. Medullary rays of one kind only and i -seriate; the cells 

 rarely in pairs, large, thin-walled, oval or oblong, usually broad, and 

 often becoming transversely oval in all except the terminal cells. 

 Resin cells as in the radial section. 



