76H GEOLOGY OF THE YELLOWSTONE NATIONAL PAl.'K. 



most are double, while occasionally there are 3 in a row or series, and 

 exceptionally as many as 4. The number in a square millimeter is only 

 from 4 to 6. The smallest of the single ducts range in diameter from 

 0.03 to O.OG mm. The largest observed are 0.21 mm. in long diameter 

 and 0.16 mm. in short diameter. The largest dou))le duct — that is, when 

 there are two together — is 0.31 mm. The largest of the sei'ies of 3 is 0.36 

 mm., and the largest of the few in series of 4 is 0.44 mm. The average 

 diameter of large and small ducts is probably about 0.12 mm. 



The medullary rays are 1 to 3 cells wide, and run irregularly among 

 the ducts. They are about 0.01 nun. broad. 



Radial section. — Thc wood ccUs are long, slender, and apparently sharp- 

 pointed. Thei'e is evidence also that some of these are divided uj) into 

 short cells by square divisions. 



The medullary rays form plates of short, rather tliin-walled cells. 

 They are from 0.02 to 0.O4 mm. in diameter and from O.Oo to 09 mm. in 

 length. They do not appear to be marked, yet there is some evidence that 

 there were minute pits; but the specimens are not well enough preserved 

 to be certain of this. 



The ducts shown in longitudinal section (PI. CXIX, fig. 3) are very 

 pronounced. The individual cells are from 10 to 0.20 inm., or some- 

 times more, in length. The walls fire covered ^vith small round pits, which 

 occasionallv pass into regular scalariform markings (see tigs. 4 and 5 of 

 PI. CXIX.) Each duct is surrounded by a mass of tissue from 2 to (i or 8 

 layei'S of cells thick, of which mention was made under the discussion of 

 the transverse section. The individual cells of this sheath are of about the 

 same size and appearance as the large cells of the medullary rays. 



Tangential section. — Tlic fiue pliotomicrograpliic reproductions of this section 

 (PI. CXVI) give a far better idea of the structure than any description can. 

 The medullary rays, it will be observed, are very numerous (about 3 to 

 each square millimeter). They are from 1 to, exceptionally, 4 layers of 

 cells broad and about 12 layers high, the extremes being 5 and 20. 



This plate shows admirably the ducts and related tissue. The one in 

 the center of the plate shows well the manner of division, although the 

 magnitication is hardly sufficient to show the pits or markings. 



This species is one of the handsomest with which I am familiar. It 

 has affinities with a number of described forms, as, for example, Launis 



