[Vol. 2 



712 



ANNALS OF THE MISSOURI BOTANICAL GARDEN 



A more constant difference that serves to separate P. 

 Tsugae is the color of the context. In P. lucidus and P. 

 Curtisii the context is never pure white, but is usually sep- 

 arated into an upper light-colored and a lower brown layer. 

 This lower layer is more firm than the upper one and often 

 contains horny fibers. In P. Tsugae the context is uniform in 



texture and almost pure white 

 throughout, but often with a very 

 slight tinge of brown next the tubes. 

 Under the microscope this effect is 

 magnified. There are no brown 

 hyphae in the context of P. Tsugae, 

 while in the other two species 

 broAvn hyphae are very pronounced, 

 especially in the layer of con- 

 text next the tubes. A compari- 

 son of the size of the hyphae in the 

 three species is interesting but does 

 not always give conclusive evidence 

 as to the identity of the species. The 

 hyphae of P. Curtisii vary from 4 

 to 6 ft in diameter. Those of P. 

 lucidus are more variable. In some 

 cases they cannot be differentiated 

 from those of P. Curtisii in point of 



size, but in some specimens they attain a diameter of 10 y.. 

 Those of P. Tsugae often attain a diameter of 15 /*. The dif- 

 ference in the branching of the hyphae of these three species is 

 very striking and is shown in figs. 5 and 6, all drawn to the 

 same scale. Figure 5a represents the hyphae of P. Curtisii, 

 which are not extremely branched but can by no means be said 

 to be unbranched. Figure 5b shows the hyphae of P. lucidus, 

 and the branching does not differ materially from that of 

 P. Curtisii. In both species the large hyphae may extend 

 more than across the field of the high-power microscope and 

 not branch at all in that distance. This condition is never 

 found in the hyphae of P. Tsugae. There the hyphae are ex- 

 tremely branched, as shown in fig. 6. The large hyaline 



Fig. 5. a, hyphae of P. 

 Curtisii; b, hyphae of P. luc- 

 idus; c, width of hyphae of P. 

 Tsugae. 



