1895 THE 3II0R0SC0rE. 137 



close structural resemblance of the barks, and the very 

 intimate relationship of the two species in habit as well 

 as in structure, suggest that the presence of starch in the 

 one and its absence in the other was only a seasonal dif- 

 ference. But this is a point which requires further in- 

 vestigation. The medullary rays in both barks are com- 

 posed of single rows of cells, and these are radially elon- 

 gated and of large size as compared with those of 

 adjacent tissues ; but those of Tsuga Mertensiana are, 

 on the average, larger, and the rays in this species, as 

 seen in a longitudinal-tangential section are composed, 

 on the average, of a larger number of cells. These dif- 

 ferences in the medullary rays are perhaps the most con- 

 stant ones between the two barks. 



Both barks contain abundance of crystals of oxalate of 

 calcium. These are mostly in the form of long prisms, 

 and are contained in rows of elongated cells of narrow 

 diameter, which traverse the bark in the direction of its 

 length. The crystals are frequently associated in the 

 containing cells with resinous and coloring matters. In 

 form and arrangement they do not differ in the two 

 barks, but appear to be rather more abundant in the Pa- 

 cific Coast species. 



Oleo-resin cells appear to be about equally abundant 

 in the two species. Those that do not also contain crys- 

 tals are isolated or in rows of two or three, and the cells 

 are shorter and broader than the crystal cells, though 

 they are not usually so large as the parenchyma cells 

 with which they are associated. They are scattered 

 without apparent order through the inner bark. Besides 

 the oleo-resin cells proper, just described, oleo-resin oc- 

 curs in many cells not especially devoted to secretions. 

 This is particularly true of the cells in the older portions 

 of the bark. 



DESCRIPTION OF FIGURES. 



Fig. 1. — Small portion of cross-section of bark of Tsuga Canadensis^ mag- 

 nified about 50 diameters, c, c, c, secondary cork formation ; a, dead phloem 



