558 FETCH : 



will be shown later, its actual fungicidal value, tested by its 

 power of killing the spores, is not great in this case. 



It is usually supposed that when plants are sprayed with a 

 fungicide, they are protected from the attacks of fungi, 

 because the fungicide kills any spores which may alight upon 

 the sprayed leaves. How this action occurs is a matter of 

 doubt. One view holds that the spores are actually killed by 

 contact with the fungicide ; this may be true of thin- walled 

 spores, but it is improbable with thick- walled spores, such 

 as Thielaviopsis , unless the period of contact extended over 

 several months. iVnother view is that the spores germinate, 

 and that the developing mycehum is killed by the fungicide. 

 This latter view would appear to be disproved by the experi- 

 ments with copper sulphate already quoted. It would seem 

 that the spores of Thielaviopsis would remain dormant in 

 contact with such a fungicide ; and would germinate when the 

 fungicide were washed away, provided that the exposure to 

 its action had not been too prolonged. 



In the experiments described below, the spores were 

 immersed in a solution of known strength, and wei'e transferred 

 at regular intervals to a nutrient solution, viz., sugar cane 

 extract. Obviously, by varying the concentration of the 

 fungicide, such experiments can be carried on indefinitely ; 

 but as little time was available it was necessary to restrict 

 them to those percentages which are commonly employed. 



The spores, except where otherwise stated, were taken from 

 cultures six days old. The mass of spores and mycelium was 

 transferred to a glass cylinder, 8 cm. high and 4 cm. diameter, 

 where it was torn into small pieces with needles, and pounded 

 with a glass rod to moisten the spores. The cylinder was then 

 placed in a basin, and the fungicide was poured into it and 

 allowed to overflow. Any floating spores were then rapidly 

 wiped off with a piece of filter paper. When the spores alone 

 were being tested, the hquid was poured into another similar 

 cyhnder after th.e fragments of mycehum had settled ; by this 

 procedure, the Hquid could be periodically stirred without 

 any danger of obtaining spores which had been protected by 

 the mj^cehum. When it was wished to take samples at short 

 intervals, the culture was divided into three parts, and the 



