768 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



The ducts (PI. CXX, fig'. 4) are very numerous. They occupy at 

 least one-third of the area, exclusive of the rays. They are almost always 

 single, although often placed close together, especially in the beginning of 

 the spring wood. They are uniformly oblong in shape. In the spring- 

 wood the average size is 0.09 mm. in long and 0.06 mm. in short diameter. 

 In the fall wood they are from 0.03 to 0.06 mm. in long and 0.025 to 0.05 

 mm. in short diameter. 



The annual ring consists of a layer of slightly thicker wood cells, but 

 it is mainly distinguished by the abruptly larger ducts in the spring wood 

 (see fig. 4 of PI. CXX). 



The medullary rays are very abundant as seen in this section. They 

 are from 1 to 10 or 15 cells broad. Fully 30 per cent of the area is 

 covered by the medullary rays. The rays uniformly contain a black car- 

 bonaceous substance, these making them stand out in bold relief. 



Radial section. — The most prominent feature in this section (PI. CXX, fig. 

 5) is the medullary rays. They form high plates of usually short cells 

 with black carbonaceous contents. The ducts are also prominent, and 

 appear to be marked with scalariform thickenings; but as they are quite 

 obscure, this is not positive. 



Tangential section. — The structure of this section is very clearly revealed 

 under the microscope. The medullary rays are very numerous. They 

 range from 1 to 10 or 15 layers of cells broad and more than 100 high. 

 The cells are round, thin-walled, and usually or not at all compressed. 

 They take up, as already stated, fully 30 per cent of the space. In some 

 cases the rays are 0.5 mm. long and 0.35 mm. broad (cf. fig. 3 of PI. CXX). 



The wood cells are long and sharp-pointed. So far as can be made 

 out, there are few if any square divisions of the cells. 



The ducts, of course, show well in this section, but the markings, if 

 present, are now obscure. 



This species is quite closely related to the living Platanus occidentals 

 L., the common sycamore or plane tree. The living wood shows the indis- 

 tinct annual ring, the irregular wood cells, and numerous medullary rays 

 almost identical with the fossil wood. There are certain minor points of 

 difference, such as markings on the rays, lignification of the ducts, etc., but 

 they are certainly close enough to make their generic identity reasonably 

 sure. 



