The spore wall of five selected species of the Xerocomus group of Boletus was studied with a transmission electron microscope (TEM). The wall is composed of five layers in all the species studied: a very thin electron-dense outer layer 1 (ectosporium), a moderately electron-dense layer 2 (perisporium), a thick and amorphous electron-dense middle layer 3a (exosporium) that passes gradually into a thinner, granular or granular-fibrillar and moderately electron-dense layer *3b (episporium), and an almost electron-transparent layer 4 (endosporium). A smooth spore surface was found in Boletus pulverulentus and B. chrysenteron. A striate exosporium covered by the ectosporium and the perisporium was found in B. pruinatus, rough warts originating from a disrupted perisporium and ectosporium in B. parasiticus, and very fine warts composed of outgrowths of the ectosporium and part of the perisporium in B. subtomentosus. A species of an other group of the Boletales with conspicuous ornamentation (Strobilomyces strobilaceus) was examined as comparative material. The results of this TEM study are compared with SEM photographs of the spores of Xerocomus published by other authors, and methodologicalproblems with the examination of spore wall ornamentation are discussed. The data revealed confirm the high value of spore wall architecture and ornamentation in the taxonomy of this genus. The separation of B. parasiticus in a new genus Pseudoboletus Sutara is supported by its spore wall ornamentation that is unique in the Boletaceae.
Holec J. (1994): The ultrastructure of the spore wall and ornamentation in the Xerocomus group of Boletus. – Czech Mycology 47(3): 173–184.
