260 University of California Puhlications in Botany [Vol. 6 



Histology 



Under the microscope the surface of the pileiis is seen to be cov- 

 ered with slender hairs 1 to 2/x in diameter and about 120/a in length. 

 These hairs are dark brown in color and septate (pi. 25, tig. 27). 



The context of the pileus is made up of hyaline, much branched 

 hyphae 8 to 12/* in diameter. 



The hymenium has a rather characteristic structure in Boletus 

 luteus. The basidia project almost 12/x, above the club-shaped para- 

 physes. There are also certain clusters of cystidia present. These 

 cystidia are 6 to 8/u, in diameter and project 25 to 30/t above the 

 paraphyses. The cystidia often occur in groups of 2 or 3 and in such 

 cases they often bear a striking resemblance to the "glands" found 

 in species of Rostkovites. 



The surface of the stipe also has a very characteristic structure in 

 Boletus luteus. On the outside is seen a honeycomb-like material 

 which is almost black in mass but of a brownish-yellow color in thin 

 section. This mass is formed of hyphae 2 to 3/x, in diameter. The 

 openings in the honeycomb are 5 or 6 sided and about 40/a in diameter, 

 with partitions 8/a in thickness between the openings. This layer is 

 some lOOfx in thickness but does not occur uniformly over the surface 

 of the stipe. It is these patches of black "honeycomb" which appear 

 to the naked eye as black dots or granules. 



Beneath the "honeycomb" is a rind which extends uniformly over 

 the surface but varies in thickness from 20 to 60/x. This rind is made 

 up of densely interwoven hyphae, 4 to 6/a in diameter, which arise as 

 branches from the hyphae of the context. The hyphae of the rind are 

 light yellow in color, while those of the context are hyaline, 4 to 10/u, 

 in diameter just beneath the rind and increasing to 20/x in diameter 

 near the center of the stipe. Near the periphery of the stipe the hyphae 

 lie in a position nearly parallel to each other and to the long axis of 

 the stipe, but toward the center there is an abundant branching and 

 anastomosing of the hyphae which gives rise to a pseudo-parenchy- 

 matous tissue (pi. 25, fig. 30). 



