HELIOTROPISM OF ASCI IN DISCOMYCETES 319 



The formation of radially-projecting dissepiments and ridges on 

 the exterior of a pileiis in the Morchellaceae (Fig. 157) is comparable 

 with the formation of gills on the under side of the pileus in the 

 Agaricaceae ; for in both groups of fungi the same mechanical advan- 

 tage is obtained, namely, a considerable increase in the area of the 

 spore-bearing layer or hymenium without the hymenophore losing 

 its compactness as a whole. The gills of the Agaricaceae are often 

 closely packed : indeed, the 

 upper part of the interlamellar 

 space between two adjacent gills 

 in many agarics, e.g. Psalliota 

 campestris [videYig. 139 in Vol. II, 

 p. 390), is often reduced to 1 mm. 

 or less. On the other hand, in 

 most of the Morchellaceae, the 

 alveolar spaces are usually several 

 mm. in diameter. Correlated 

 with this difference in the spatial 

 arrangement of opposing hy menial 

 walls is the fundamental difference 

 in the structure and power of the 

 guns in the two groups of fungi. 

 The basidia of the Agaricaceae are 

 relatively small and feeble guns 

 with a range of only • 1-0-2 mm., 



and the basidia on one gill cannot therefore bespatter with spores 

 the opposing hymenium of another gill, even when the interlamellar 

 space is only 0- 5 mm. in width {cf. Fig. 139 in Vol. II, p. 390). In 

 contrast with such basidia, the asci of the Morchellaceae are rela- 

 tively large and powerful guns with a range of 1-3 cm., and they 

 therefore requh'e much more room for discharge than basidia. 

 The actual space provided for them is what is necessary to permit 

 of all the asci turning toward, and shooting their spores through, 

 the mouths of the alveoli.^ 



1 For other remarks on the manner in whieli the basidium lias influenced the 

 structure of hymenomycetous fruit-bodies and the ascus has influenced the structure 

 of discomycetous fruit-bodies vide these Researches, Vol. I, 1909, pp. 22-24 ; also 

 cf. Vol. II, 1924, pp. 67-68. 



Fig. 159. — Morchella crnssipes. Asci 

 sharply curved at tlieir ends. Mag- 

 nification, 820. Copied by A. H. R. 

 Bviller from Boudier's Icone-s Myco- 

 logicac, T. II, PI. 194. 



