34 RESEARCHES ON FUNGI 



We have seen that in the Agaricinese the extent of the 

 hymenium has been increased by the production of radial wedge- 

 shaped gills with vertical median planes, so that the fruit-bodies 

 are characterised by an admirable compactness. However, certain 

 principles underlying the spacing of the gills in reference to one 

 another still require an elucidation. The gills are usually crowded 

 together on the underside of a pileus. Two adjacent gills, how- 

 ever, must be a certain distance apart in order to permit of the 

 liberation of the spores. It will subsequently be shown 1 that for 

 Psalliota campestris, &c, the spores are actually shot horizontally 

 for about 0-1 mm. into the interlamellar spaces before their paths 

 of movement become vertical. Two adjacent gills, where they are 

 closest to one another, i.e. near the pileus flesh, must therefore be 

 separated from one another by a distance which at least just 

 exceeds 0-1 mm. In the Mushroom the minimum space between 

 the gills was actually found to be about 0*2 mm. (Plate I., Fig. 4). 

 Probably nearly 50 per cent, of this should be regarded as a 

 margin of safety. When a mature pileus is tilted slightly, so that 

 the plane of the flesh is no longer horizontal, the gills, displaced 

 from their vertical planes, react to the stimulus of gravity by 

 growth in such a manner that they quickly come to take up 

 vertical positions once more. 2 This, however, entails a reduction 

 in the margin of safety, for the spaces between the gills become 

 narrowed. If the pileus is tilted beyond a certain amount, it neces- 

 sarily follows that, when the gills have adjusted themselves, the 

 margin of safety must have disappeared altogether. This must lead 

 to a diminution in the number of spores escaping from the pileus. 



In the Mushroom, judging from a study of gill-dimensions as 

 embodied in Plate I., Fig. 4, the margin of safety would not be 

 used up until the pileus had been tilted to an angle of about 30°. 

 In this instance, and probably quite generally for Agaricinere, pro- 

 vided only that the gills have taken up vertical planes, just as 

 many spores can be liberated from a slightly tilted as from an 



1 Vide infra, Chap. XL 



2 Cf. A. H. R. Buller, "The Reactions of the Fruit-bodies of Lentinus lepidms, 

 Fr., to External Stimuli," Ann. of Bot., vol. xix., 1905, p. 432. Also vide infra, 

 Chap. IV. 



