98 RESEARCHES OX FrXCil 



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 [PsaUiota campestris) at the top of one end of 

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

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

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

 Some Avere even carried to Avithin a fcAv centimetres of the end of the 

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

 observation shoAvs that very small convection currents are capable of 

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

 direction. 



From observations Avhich 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, &g.) tend to spread themselves uniformly Avithin its con- 

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

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

 made by fruit-bodies placed in chambers provided Avith vertical 

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

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

 became equally covered Avith 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 : CouA-ection 

 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 Aveight at the rate of about 

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

 conditions for the settling doAvn of spores oA'cr each shelf iire 



^ R. Falck, " Die Sporenverbi-eitung bei den Basidiomyceten," Jleifruge zia- 

 Bioloijic der Pflan::en, Bd. IX., 1904. 



^ The rate varies accordinif to the species ; rf. the Table of velocities in 

 Chap. XV. 



