BASIDIA AND THE DISCHARGE OF SPORES 35 



shape and a smooth wall but also in its peculiar volume-to- 

 mass and surface-area-to-mass relations, a toy-balloon shows itself 

 particularly well fitted to be used as a large-size model of a 

 basidiospore. 



The rate of fall of a spore varies not only with its density but 

 also with its volume and with the amount of its surface exposed to 

 the air, so that, with density constant, the smaller a spore, the 

 slower is its terminal velocity. A variation of the rate of fall 

 with volume can readilv be demonstrated to an audience with a 



t> 



balloon. Thus, for instance, it was observed that the same balloon 



when falling 11 feet in still air took: (1) when uninflated, 1-5 



seconds ; (2) when partly inflated so that it was 10 inches long 



and 6 inches wide, 4 seconds ; and (3) when fully inflated so that 



it was 23 inches long and 6 inches wide, 5 seconds. In its three 



states, in succession, the balloon had about the same weight in air 



but there was a progressive increase in volume and surface area. 



The larger the volume of the balloon, the slower was its rate of fall. 



Any homogeneous elongated body falling in still air tends to 



orientate itself in such a way as to present the maximum amount of 



surface in the direction of the line of fall and thus to fall as slowly 



as possible. In accordance with this law a piece of paper 4 cm. 



long and ' 5 cm. wide, when falling, does not approach the earth 



with an end- wise movement but rather keeps its long axis directed 



transversely to its line of fall. For certain reasons which will not 



be discussed here, the piece of paper usually rotates rapidly about 



its long axis during its descent. An elongated cylindrical object, 



such as a piece of cotton fibre, hair, or spider's web, when falling 



(as may be seen in a beam of sunlight), also keeps its long axis 



parallel with the earth's surface but, unlike the flat piece of paper, 



does not rotate about its axis. Still smaller cylindrical bodies, 



such as the spores of Claviceps purpurea, doubtless fall like bits of 



cotton fibre ; but even oval spores, which are only two or three 



times as long as they are wide, also tend to fall with their long 



axes held horizontally. This I directly observed with a horizontal 



microscope for the spores of Polyporus squamosus, which are three 



times as long as they are wide. 1 Now this mode of falling of elongated 



1 Vol. i, 1909. pp. 185-189, Figs. 64-66. 



