PSALLIOTA CAMPESTRIS 373 



to increase very greatly, and yet very compactly and very 

 economically, the area on the under side of the pileus available for 

 the development of spores. In one fruit-body, which was 9-8 cm. 

 in diameter, the gills were found to have an area of a little more 

 than 20 04 times the area of the under surface of the pileus-flesh 

 to which they were attached. The specific increase of hy menial 

 surface due to the production of gills in this particular fruit-body 

 was therefore 20-04. 1 



The stipe is an organ which serves to raise the pileus, and 

 therefore also the hy menial layer upon the gills, to such a height 

 that a considerable space comes into existence between the under 

 side of the pileus and the surface of the ground. The presence of 

 this space enables the wind or air currents to pass freely below the 

 pileus and thus to carry away the falling spores. Since the spores, 

 after emerging from the interlamellar spaces, fall in still air at the 

 rate of about 1 mm. per second, 2 and since the free space beneath 

 the pileus is often 50 to 70 mm. in depth, it is clear that even a very 

 feeble lateral current of air is sufficient not only to prevent the 

 spores from settling directly beneath the pileus but also to carry 

 them for long distances before they come to earth. Everyone 

 knows from observation what long distances thistledown or the 

 hairy seeds of the Willow may travel on a windy day. Yet, in 

 still air, the rate of fall of the spores of the Mushroom is much 

 slower than that of any hairy fruits or seeds. 3 It is clear, there- 

 fore, that when the wind is at all strong the spores of the Mush- 

 room must often be carried many miles before they settle. If in a 

 particular fruit-body we suppose the free space between the base of 

 the gills and the ground to be 6 cm., and if we take the approximate 

 average velocity of fall of the spores to be 1*2 mm. per second, 

 then a simple calculation shows that the approximate length of 

 time required for the spores to fall from the base of the gills to the 

 ground in still air would be 50 seconds, i.e. nearly a minute. 4 Now, 



1 Cf. vol. i, pp. 27-31. The total area of all the gills in this fruit-body was 

 195-2 square inches, i.e. upwards of a square foot. 



2 Vol. i, p. 180. 



3 In still air, it was found that the fruits of the Thistle, Cnicus arvensis, fall 

 104 times as fast as the spores of a wild Mushroom. Vide Chap. I, p. 39. 



4 Vol. i, p. 216. 



