BASIDIA AND THE DISCHARGE OF SPORES 39 



for us to watch the smallest spores very carefully indeed if we 

 wished to make sure that they were actually falling. 



Every one has noted with delight the very slow rate of fall of 

 thistle-down when being driven before the wind. Yet thistle-down 

 falls very rapidly relatively to the spores of the Hymenomycetes. 

 This fact I have proved by direct comparison of the rates of fall 

 of thistle-down and spores in still air. 



Cnicus arvensis, which in Canada is called the Canada Thistle 

 although it was introduced from Europe, is as common a weed in 

 Manitoba as in England. I brought some capitula of this species, 

 which bore ripe but not yet fully expanded fruits, into the laboratory 

 at Winnipeg and allowed them to dry. Soon, each fruit opened 

 its parachute-like pappus and some of the fruits became detached 

 from their discs. I took four of these detached fruits and measured 

 their rates of fall in still air through a distance of 6 9 feet. Many 

 measurements were made and it was found that the fruits fell 

 6-9 feet in 11-12 seconds. The average rate of fall was 6-6 inches 

 or 167-6 mm. per second. 



The rates of fall of the spores of various species of Hymeno- 

 mycetes were measured, as described in Volume I, with a hori- 

 zontal microscope and a drum recorder. 1 The large spores of 

 Amanitopsis vaginata fall in moderately dry air at the rate of 

 4-29 mm. per second, the medium-sized spores of the Wild Mush- 

 room (Psalliota campestris) and of Polyporus squamosus at 1 -61 mm. 

 per second and 1-03 mm. per second respectively, and the small 

 spores of Collybia dryophila at 0-49 mm. per second. These rates 

 of fall along with the rate of fall of the fruits of Cnicus arvensis 

 may be set out in tabular form as follows : 



Rates of Fall in Still Air of Thistle Fruits and of the Spores 



of Hymenomycetes 



Cnicus arvensis . . . 167-6 mm. per second 



Amanitopsis vaginata . . 4-29 ,, ,, 



Psalliota campestris . . : .. 1-61 ,, ,, 



Polyporus squamosus . . 1-03 ,, ,, 



Collybia dryophila . . . 0-49 ,, ., 



1 Vol. i, 1909, pp. 166-177. 



67S39 



