454 COMPARATIVE MORPHOLOGY OF FUNGI 



ous. Sometimes the pellicle is covered with cystidia-like cells, which 

 at the edge of the pileus, gradually change to cystidia; possibly they 

 function as hydathodes (Fayod, 1889; Istvaniffi, 1896; Knoll, 1912). 



The elevation of the hymenophore is generally caused by the sub- 

 hymenial hyphae, as those of the Polyporales, increasing their growth 

 over the original surface of the fructification. In the lowest forms, 

 e.g., in the first gymnocarpous type, these elevations, as in the Dictyola- 

 ceae, spread out centrifugally from the annular furrow to the margin 

 in the form of a few comparatively broad fleshy, or spongy folds. In the 

 hemiangiocarpous (second and third) types, the development is more 

 complicated and hence in part still controversial. In the (probably) 

 simpler forms, the tramal hyphae grow from above into the annular 

 furrow, and, under certain conditions, enter upon a temporary union 

 with the opposite side of the furrow, the partial veil. In the higher 

 forms, as Amanita, Amanitopsis and Psalliota campestris, the numerous 

 cavities into which the tramal plates grow (horizontal in cross-section 

 and running a distance around the fructification) are vertical and 

 radiate regularly. The tramal plates are formed from the ground 

 tissue. The lamellae in this highest type arise, not from renewed 

 growth of the ground tissue which covers the cavity, but from the 

 original ground tissue. Throughout the whole development they are 

 connected with the peripheral ground tissue of the partial veil and 

 become partially loosened from it only at maturity. Every individual 

 cavity is laid down singly and schizogenetically. 



In all these other angiocarpous forms, the lamellae are usually very 

 numerous and very narrow; their characteristic structure has given to 

 the order the name of gill fungi. The lamellae radiate and, in typical 

 cases, are separate throughout their whole length. Only in a side line, 

 which also differs in other respects (Paxillaceae, Boletaceae), are they 

 connected by partitions into daedaloid structures. In contrast to the 

 Polyporaceae, they proceed, in the higher forms at least, to form the 

 hymenium when they have completed their growth and attained their 

 final form. The hymenium forms over the whole fructification com- 

 paratively rapidly and simultaneously. Exceptionally, e.g., in Armillaria 

 mellea, basidia develop on the aerial mycelium, singly or in more compact 

 layers which form a sort of hymenium. 



The basidia correspond in form and development to the four-spored 

 basidia of the Polyporales. Exceptionally in the lower forms e.g., 

 Mycena we find still a third step in the division of the basidial nucleus, 

 without the spore number varying from four. Besides, in spite of the 

 normal quadrinucleate condition of the basidia, in Amanita bisporigera 

 two nuclei remain behind in the basidium (Lewis, 1906). 



As imperfect forms, oidia have been observed in numerous genera, 

 occasionally also conidia, gemmae or bulbils. 



