PSALLIOTA CAMPESTRIS 381 



the long ones. As the pileus grows in size and the spaces at the 

 pileus-periphery between the first-formed gills become wider, new 

 and shorter gills are interpolated between the older ones ; and this 

 process of gill-interpolation goes on continuously as spaces become 

 provided until all the gills have been formed. The very shortest 

 gills are always the last ones to appear. The determining factor 

 in the origin of a new gill seems to be the divergence beyond a cer- 

 tain degree of two older gills. As soon as an interlamellar space 

 has attained a certain width, a new gill-rudiment arises midway 

 between them. It is owing to this mode of development of the 

 gills in general that the wonderful system of lamellae on the under 

 side of a mature Mushroom comes to be established ; and, by 

 comprehending it, we can understand why it is that two gills never 

 touch one another and why the interlamellar spaces are never 

 reduced beyond a certain minimum and never exceed a certain 

 maximum. The law of gill-development secures that the under 

 side of the Mushroom shall be as crowded with lamellae as is 

 consistent with the efficient discharge of the spores. 



The Number of the Gills. The number of gills in any gill- 

 system varies from about 300 in small mushrooms to over 600 in 

 large ones. It is dependent to a great extent upon the diameter of 

 the pileus. This clearly follows from the consideration that as 

 two long gills pass from the stipe to the periphery they diverge 

 progressively more and more, and therefore leave room for the 

 interpolation of more and more shorter gills. 



A certain variety of the Common Mushroom was cultivated 

 artificially on a bed of horse manure. A small mushroom which 

 came up had a diameter in the fully expanded condition of 

 4-5 cm. It possessed in all about 320 gills : 120 long ones, 120 

 intermediate ones, and 80 very short ones. The spaces between 

 the long and intermediate gills numbered 240, but they were so 

 narrow that only one-third of them were divided by very short 

 gills. The gill-system in this particular mushroom, therefore, 

 consisted for the most part of gills of two lengths only. Four 

 other larger mushrooms, each of which was approximately 8 cm. 

 in diameter, possessed the following respective numbers of gills at 

 the pileus-periphery : 509, 586, 616, and 628, i.e. the gills of these 



