RESPONSE AND ITS MEASUREMENT 445 



than they are against the growth or metabolism of mycelium (125, 147, 

 271, 341, 430a, 431). 



Fungistatic methods adapted for the assay of wood preservatives are 

 reviewed by Cartwright and Findlay (61). The type of assay chosen 

 has marked effects on the apparent efficacy of test compounds (110, 223, 

 348). Usually, loss in weight of treated wood blocks is the best cri- 

 terion of protection against decay; a somewhat more rapid assay based 

 on the static bending strength of a treated wood block has been intro- 

 duced (126). 



Special methods for fabric treatments are described by Siu (386). 

 Soil fumigants are usually of interest insofar as they kill organisms, but 

 a fungistatic method of bioassay has been devised (101). For the 

 practical testing of plant protectants, assays employing living inoculated 

 plants are well worked out (243, 245, 251, 280). 



The most progress has been made by the use of bioassays which are 

 based on inhibition of spore germination. This fundamental tech- 

 nique has been developed both practically and theoretically by McCal- 

 lan and by Horsfall, whose papers must be read for a thorough under- 

 standing of the method (172, 173, 174, 241, 242, 243, 252, 254, 258, 

 259, 436, 445). More or less standard procedures have been agreed 

 upon (71, 72). 



A standardized spore suspension of a test fungus — Monilinia fructi- 

 cola (=ScIerotinia fracticola) and Stemphylium sarcinaeforme (=Mac- 

 rosporium sarcinaeforme) are the most often used, but other sporulat- 

 ing fungi are suitable — is allowed to germinate in contact with the 

 toxicant, which may be either mixed with the spore suspension or 

 sprayed on the slide (33). If washed or vacuum-collected spores are 

 used, it is usually necessary to add nutrients, and these materials, 

 especially complex plant products such as orange juice, must be taken 

 into account in any consideration of the mode of action; orange juice, 

 possibly by reason of its content of citric acid, which can complex 

 metals, reduces both the fungicidal and the fungistatic effect of copper 

 on at least many fungi (91, 250, 287). 



In the spore germination assay, a simple plot of per cent inhibition 

 (or germination) against toxicant concentration or the logarithm of 

 toxicant concentration yields a sigmoid curve. This can be converted 

 to a straight line in the so-called logarithmic-probability system of 

 coordinates. The abscissa of this curve used in most studies is the 

 logarithm of the dose, the ordinate a calculated "probit" or "normal 

 equivalent deviation." Use of this coordinate system is based on the 

 general phenomenon that resistance in a population to a toxic agent 

 is distributed normally with respect to the logarithm of the dose (30, 



