FOSSIL FLORA. 763 



PlTYOXYLON ALDERSONI 11. SJ3. 

 Pis. CVI, CXII, CXIII, OXVIII, figs. 3, 4; PI. CXIX, fig. 2. 



Diagnosis. — Trunks of large size, 3 to 5 feet in diameter; annual rings 

 very distinct, often 8 or 9 mm. broad, very sharply demarked; resin ducts 

 numerous, large, scattered, occurring in late summer and fall wood; wood 

 cells long, with a single irregular row of medium-sized pits; medullary rays 

 in a single series, or occasionally with divided cells; rays from 2 to 25 

 cells high, the average being about 10 or 12 cells. 



Transverse section. — The annual rings are very distinct, being plainly dis- 

 cernible to the naked eye. Some of the broadest rings are fully 9 mm. 

 wide, and none are less than 6 nun. The demarcation between fall and 

 spring wood is very pronounced (see fig. 4 of PI. C XVII I and 2 of PL 

 CXIX), the cells of fall being small, compressed, and thick-walled, while 

 those of the early spring wood are very large, and, of course, thin-walled. 



The cells of the spring and summer wood continue for a width of 

 5 mm., but little, if any, diminished in size. Then they become slightly 

 smaller and thicker-walled and pass gradually into the fall wood. 



The resin ducts are very large. They are not found in the summer 

 wood, but occur irregularly in the early fall and late fall wood. 



The medullary rays, as observed in this section, are straight and 

 separated by from 3 to 8 or 10 rows of wood cells. The individual cells 

 are apparently long. 



Radial section. — Notwithstanding the fact that the wood seems to be per- 

 fectly preserved, it does not reveal the structure well in this section. The 

 wood cells are seen to be sharp-pointed where they join. They are, of 

 course, broad in the spring- and summer wood, and very narrow and thick- 

 walled in the fall wood. It is very difficult to make out the pits, but in 

 exceptionally well preserved portions a few may be faintly seen. They 

 are scattered, but in a single series. They are so obscure that no satis- 

 factory measurements can be made. 



The medullary rays in this section are long, thick-walled, and without 

 markings, so far as can be made out. 



Tangential section. — This section is very plain. The medullary rays are 

 numerous and in a single series, although occasionally a ray may be observed 

 in which there are 2 series of cells for a short distance. In such cases the 



