98 RESEARCHES ON FUNGI 



5 mm. per second. Mushroom spores fall at a speed of about 1 mm. 

 per second. It is not surprising, therefore, that convection currents 

 carry the spores round in the beakers for a considerable time before 

 they settle down, and that the spores become spread fairly uniformly 

 in the air of any small closed chamber. In one experiment I placed 

 a piece of a Mushroom (Psalliota campestris) at the top of one end of 

 a box which was 107 cm. long, 7 mm. wide, and 13 cm. high, and 

 which was illuminated with a parallel beam of light sent through it 

 lengthwise. The spores were gradually scattered in the enclosed air. 

 Some were even carried to within a few centimetres of the end of the 

 box opposite to that in which the fungus had been placed. This 

 observation shows that very small convection currents are capable of 

 carrying the spores over a metre from a fruit-body in the lateral 

 direction. 



From observations which I have made upon the fall of spores in 

 glass chambers of various sizes, it seems that convection currents 

 are such that the spores in a sufficiently large chamber (large 

 beakers, &c.) tend to spread themselves uniformly within its con- 

 tained air, so that equal numbers of them come to occupy each 

 available unit of space. Richard Falck x observed the spore-deposits 

 made by fruit-bodies placed in chambers provided with vertical 

 series of small paper shelves, and he found that the shelves, even 

 when they had been placed one above the other at short intervals, 

 became equally covered with spore-dust. My own observations upon 

 falling spores, made by the beam-of-light method, have enabled me 

 to explain Falck's results in the following manner : Convection 

 currents are usually of such strength in the chambers that the 

 spores are moved about by them so[ that equal numbers come to 

 occupy each unit of space. As a result of this, there is the same 

 number of spores in the layer of air immediately over each shelf. 

 As the spores are falling by their own weight at the rate of about 

 1 mm. per second, 2 a certain number settle each second. Since the 

 conditions for the settling down of spores over each shelf are 



1 R. Falck, "Die Sporenverbreitung bei den Basidiomyceten," Beitrii<j> :nr 

 Biologie der Pflanzen, Bd. IX., 1904. 



2 The rate varies according to the species ; cf. the Table of velocities in 

 Chap. XV. 



