THE BEAM-OF-LIGHT METHOD 95 



fascinating spectacle of millions of spores leaving the gills of a 

 Mushroom. 



It must not be supposed that the ordinary dust particles, which 

 are always present in the air in countless numbers, were mistaken 

 for spores. With a concentrated beam of light it is very easy to 

 see the dust particles. In my laboratory they are roughly of two 

 classes : the coarser ones are comparatively rare and consist of 

 fibrous matter, &c., whilst the finer ones are extraordinarily 

 numerous and doubtless of the most varied origin. The coarser 

 particles alone can be mistaken for spores, but a very little experience 

 is sufiicient to prevent confusion. These dust particles never 

 occur in the form of wreaths or curls, and only occasionally float 

 into the beam of light. Further, they are irregular in shape and 

 rarely affect the light in the same manner as a spore. The finer 

 particles are distinctly smaller than spores, never so regular in 

 size, and more numerous. They are also somewhat difificult to 

 see individually, for they do not glisten in the light nearly so 

 brightly as spores. It is evident, therefore, that spores and ordinary 

 dust particles can be distinguished in a beam of light with great 

 ease. 



After finding that the beam-of-light method could be applied 

 with so much success to a Mushroom, I made general use of it in 

 investigating the spore-fall of a large number of other species. It 

 has proved of great service in determining whether or not spore- 

 discharge was taking place in any particular fruit-body, in finding 

 out the length of the spore-fall period, and in studying the effect of 

 various external conditions upon spore-liberation. 



It may be stated quite generally that, w^henever spores are falling 

 from a fruit-body, they can be observed with the unaided eye in a 

 strong beam of light. So far as my experience goes, there are no 

 species of Hymenomycetes of which the spores are too small to be 

 seen in this macroscopic manner. Even a very slight discharge from 

 a fruit-body may be detected. Sufiicient evidence of its occurrence 

 is provided by a dozen spores streaming through a beam. 



It was found convenient in my own department to carry out 

 observations with the beam-of-light method in the following manner. 

 The lecture-theatre lantern, provided with an electric arc, was set 



