THE BEAM-OF-LIGHT METHOD 99 



approximately the same, the shelves must eventually all become 

 uniformly covered with a spore-deposit. 



Falck has also called attention to the fact that, when the pilei of 

 certain fungi are suspended in a glass chamber, one sometimes 

 obtains curious and fantastic spore-deposits on paper placed at the 

 bottom of the vessel. These irregular spore-deposits are in my 

 opinion entirely due to the nature of the convection currents in the 

 glass vessels. Observations with the beam-of-light method have 

 taught me that, when the velocities of the convection currents are 

 high compared with the constant rate of fall of the spores due to 

 gravity, the spores become evenly distributed in each unit of space 

 in^the chamber, and that a uniform spore-deposit collects in con- 

 sequence upon the bottom of the chamber, shelves, &c. If, how- 

 ever, the spores, such as those of various species of Coprinus, are 

 large and heavy, and fall at the rate of several millimetres per 

 second, and if, in addition, the convection currents are not strong 

 compared with this rate of fall, then we have the conditions 

 for the formation of a localised and irregular spore-deposit at 

 the bottom of the chamber. In general it may be stated that 

 the appearance of any spore-deposit is decided partly by the 

 speed and nature of the movements of the air through which 

 the spores have fallen, and partly by the rate of fall of the spores 

 themselves. 



The pileus of a small Coprinus fruit-body which came up on 

 horse dung and was liberating its spores, was suspended at the top 

 of a closed glass chamber which was about 6 inches high, 4 inches 

 wide, and covered below with white paper. The chamber was placed 

 in front of the condensing lens of the lantern. It was observed that 

 a black spore-deposit was accumulating on the white paper along one 

 side of the base of the chamber. After about an hour it was assumed 

 that the chamber had taken on the room temperature. The arc- 

 light was suddenly turned on, and with the beam a stream of spores 

 could be seen leaving the gills, falling obliquely at a rate of several 

 millimetres per second, and settling where the spore-deposit had been 

 accumulating. Owing to the sides of the vessel becoming warmed 

 by heat accompanying the beam of light, new and marked convection 

 currents were soon formed. The result was that the stream of spores 



