1916] Yates: Histology of Calijoyiiian Boletaccae 231 



PlLEUS 



The pileus in the Boletaceae may be divided into twd parts, whose 

 structure should be considered separately. The tubes should be con- 

 sidered apart from the rest of the pileus, since their histology is very 

 different from that of the context and rind. 



The pileus, exclusive of the tubes, may be divided into a surface 

 rind and the context. The rind varies greatly in its structure in the 

 different species. Sometimes this surface layer is formed merely of 

 densely interwoven hyphae differing from those of the context only 

 in their size and in their color. In such cases the hyphae of the rind 

 may be either larger or smaller in diameter; their color, however, is 

 always dark. Such a structure of the surface rind is often char- 

 acteristic of plants whose pileus is described as glabrous. In other 

 cases we find in addition to the layer described above a second layer 

 outside of this made up of long hairs or hyphae standing in a position 

 perpendicular to the surface. The hairs, according to the species, may 

 be branched or unbranched, septate or non-septate. Sometimes they 

 are embedded in a jelly, and in such cases the pileus is usually viscid or 

 slimy. In other plants the hairs are not embedded in a jelly and then 

 the pileus is tomentose, or scaly, depending on the arrangement of the 

 hairs. The hairs are usually colored some shade of brown. In some 

 species the hairs occur alone without any other differentiated layer 

 between them and the pileus. In this case the hairs pass over directly 

 into the context merely by a loss of coloring matter and by the hairs 

 becoming intertwined with each other. There may or may not be a 

 difference in size between the hairs and the hyphae which make up 

 the context. 



Context 



The context consists of hyphae which branch m\d anastomose fre- 

 quently and which are densely interwoven. These hyphae vary greatly 

 in diameter, which, however, is usually constant, within certain limits, 

 in a given species. Immediately above the tubes the hyphae are less 

 densely interwoven and fewer in number, enabling the tubes to be 

 readily separated from the pileus. 



Hymenium 

 The structure of the hymenium lining the interior of the tubes 

 varies but little in the different forms. The hj'phae of the trama, or 

 tis.sue between the tubes, maintain a position parallel to the long axis 



