Host and Parasite in certain Diseases of Plants. 435 



whence conidia again arise ; these are again sown on pears, and produce 

 a still more vigorous mycelium and crop of conidia, and so on. Calling 

 the first crop of conidia generation I, and the second crop generation 

 II, the third generation III, and so on, it was found that if the conidia 

 of generation I are sown on similar leaves of a S em/per vivum in a tiny 

 drop of sap they do not infect the leaf ; whereas those of generation II, 

 similarly sown, infect the leaf at once, and those of generation III 

 are still more virulent. 



Kissling,* who has paid special attention to this point, and has 

 carried the matter much further than other observers, measures the 

 infective power of the conidia by the size of the disease-spot they 

 produce in a given time. 



As I have shown, the penetrating germinal hypha causes the cells 

 in its neighbourhood to collapse and turn brown, because the excreted 

 ferment or poison destroys the cellulose, and makes the protoplasm 

 unable to retain the sap, which consequently suffuses and browns the 

 area concerned. Now it is obvious that the rapidity with which this 

 browning occurs may be taken as a rough measure of the progress 

 — and, therefore, of the destroying power — of the infecting hyphse, 

 other things being equal. Well, Kissling took the necessary precau- 

 tions, and set the conidia of succeeding generations I, II, and III to 

 work on the surface of various fruits and other parts of plants, of 

 course using the same substratum in any one series of experiments. 



3?IG. 13. Diagram constructed to show the relative progress of infections produced 

 by successive generations of conidia of Botrytis cinerea (see text). The three 

 different generations are denoted by differences in the characters of the curves. 

 The horizontal base line is divided into six equal parts representing days ; the 

 distances measured on the ordinates represent the diameters of the disease-spots 

 in millimetres, according to Kissling's experiments. 



* Loc. cit., pp. 22—29. 



