374 RESEARCHES ON FUNGI 



if we suppose, as a matter of theory, that the surface of the ground 

 is perfectly flat and that the wind is moving over its surface quite 

 uniformly at a rate of only 5 miles per hour, then the spores, before 

 coming to earth after leaving the base of the gills, would be carried 

 a distance of not less than 365 feet. But, in nature, on windy days, 

 it must often happen that a gust of wind, after sweeping beneath 

 a pileus, becomes deflected upwards and bears innumerable spores 

 to a great height into the atmosphere, in which case it may be 

 some hours before many of them settle, during which time they 

 would probably be carried for some scores of miles. 



The adjustments by which the hymenial surfaces on the gills 

 are placed in the optimum position for spore-liberation in the 

 Mushroom are no less than four in number, and may be summar- 

 ised as follows : (1) turning the pileus into an erect position by an 

 upward curvature of the stipe ; (2) raising the pileus several centi- 

 metres above the ground by growth in length of the stipe ; (3) 

 placing the gills with their long axes horizontal by an expansion of 

 the pileus ; and (4) setting the median planes of the gills in vertical 

 positions by the turning of the gills themselves about their lines of 

 attachment to the pileus-flesh. The expansion of the pileus is due 

 to causes inherent in the fruit-body itself, but the other movements 

 are controlled by the external stimulus of gravity : the upward 

 curvature of the stipe and its continued growth in a direction away 

 from the earth's centre are phenomena of negative geotropism, 

 whilst the movement of the gills about their lines of attachment 

 is a phenomenon of positive geotropism. 1 



The flesh of both stipe and pileus is firm and massive. The 

 fruit-body is thus endowed with considerable rigidity, so that it 

 remains immovable even when the wind is blowing. This rigidity 

 is necessary in order to keep the lamellae fixed in their nicely 

 adjusted positions. If the fruit-body were to sway about with every 

 breeze like grass stems, a vast number of spores would be prevented 

 from escaping from the sides of the gills. The spores are shot out 

 into the interlamellar spaces a horizontal distance of not more than 

 about O'l mm. 2 In still air, after arriving at this distance from 

 the gill, the spores make a sharp turn and then fall vertically 

 1 Vol. i, pp. 50-56. 2 Vol. i, p. 142. 



