244 ANATOMY OF THE GYMNOSPERMS 



The individual cells have a length of .12-. 25 mm., and a diameter of 

 about .05 mm., slightly less, it will be observed, than the tracheids 

 among which they run. 



" Tangential. The medullary rays are always simple ; that is, they consist 

 of but a single row of cells, which varies from 2 to 49 cells in height. 

 The tracheids are provided on the tangential walls with a few scattered, 

 bordered pits. These have a diameter of .oi6-.o2i mm." (Knowlton). 



Remains silicified. Specimens represented by a trunk nearly 40 feet long 



and almost 2 feet in diameter. 

 Potomac Formation at Washington, D.C. (Knowlton). 



21. C. Calli, Knowlton 



" Transverse. The annual rings are very distinct, being marked by a 

 layer of fall wood 6-15 or more cells in thickness. These cells are 

 very thick-walled, the lumen being reduced to a mere line. The cells 

 of the spring wood are very large and begin abruptly at the ring, and 

 gradually diminish in size until they reach and pass into the fall wood. 

 The medullary rays as seen in this section are numerous and are sepa- 

 rated by 2-4 rows of tracheids. 



''Radial. In the spring and summer wood the tracheids are very broad 

 and provided with 2-3 rows of regularly and closely packed bor- 

 dered pits. These pits have an average diameter of .012 mm. and an 

 average inner diameter of .003 mm. The medullary rays are thin- 

 walled and in some cases, at least, provided with pits, of which there 

 are usually 3 in thickness of each tracheid. The resin tubes consist of 

 a chain of short, rectangular cells; they are moderately numerous. 



" Tangential. The medullary rays are arranged in a single series of super- 

 imposed cells, which varies from 2 to 25, the ordinary number being 

 6-15. The tracheids are not provided with pits on the tangential 

 walls" (Knowlton). 



Remains silicified. 



From the Tertiary clays of Greene County, Arkansas (Knowlton). 



11. * JUNIPERUS, Lixx. PLATES 40 AND 41 



Transverse. Growth rings generally narrow, often unconformable and 

 coalescent on the narrow side ; the summer wood usually thin but 

 dense. Resin passages wholly wanting. Resin cells rather numerous, 

 prominent, and chiefly in tangential bands, often giving rise to the 

 appearance of secondary growth rings. 



Radial. The numerous and often very resinous rays chiefly without tra- 

 cheids. Ray cells with thin and entire or sometimes coarsely pitted 

 terminal walls ; the lateral walls with bordered pits. Bordered pits 

 round or oval, chiefly in i row, generally numerous. Tracheids wholly 

 without spirals. 



Tangential. Fusiform rays wholly wanting. Ordinary rays I -seriate, some- 

 times 2-seriate in part, the cells oval, chiefly broad. 



