272 RESEARCHES ON FUNGI 



firmly fixed in the hymenium against which, in the course of its de- 

 velopment, it has come to press. If one cuts out a cubical block 

 from such a gill-mass as is shown in Fig. Ill (p. 265), the gill-pieces 

 do not fall apart but remain attached to one another by the cystidia. 

 Owing to the strength of the cystidia crossing the interlamellar spaces 

 and their firm attachment at both ends, one almost invariably 

 breaks any two adjacent gills which one attempts to separate. It is 

 evident that the cystidia lock the gills together laterally and thereby 

 greatly increase the mechanical stability of the whole gill-system. 



In such a pileus as is shown in Fig. 110 (p. 265) the weakest 

 radial planes are coincident not with the interlamellar spaces crossed 

 by the cystidia but with the median planes of the gills. Of this 

 fact one may easily obtain experimental proof by breaking the 

 pileus from below upwards and observing the surface of fracture. 

 Let us now examine the fractiu-e-surface which resulted from 

 breaking the pileus of Fig. 110 into two halves and which is shown 

 in Fig. 111. About seven gills have been broken through on each 

 side of the stipe, and their remaining portions are locked together 

 by cystidia. The black streaks and patches alone are the hymenial 

 spore-producing surfaces of the gills, and they are black owing to 

 their being covered with black spores held upon the sterigmata. 

 The lighter grey areas, which are much more extensive than the 

 black, are tramal surfaces. Hence it is clear that, when the pileus 

 was split into two halves, the part of each gill which happened to 

 lie in the plane of fracture split for the most part down its median 

 plane so that its two halves became separated from one another, one 

 half remaining attached to one pileal hemisphere and the other half 

 to the other pileal hemisphere. 



When two adjacent gills are forced apart by mechanical pressure 

 suitably applied, most of the cystidia break away from one of the 

 gills at their apical ends and remain attached to the other gill by 

 their basal ends (Fig. 122, i, p. 287), while certain others break away 

 from one of the gills by their basal ends and remain attached to 

 the other gill by their apical ends (Fig. 122, j). Worthington 

 Smith 1 doubtless saw cystidia projecting from a gill with their basal 

 ends pointing outwards, for his illustration shows one upside down. 



^ W. G. Smith, " Cystidia in the Mushroom Tribe," GreviUea, vol. x, 1881, p. 78. 



