132 Journal of Agricultural Research voi. iv, no. 2 



Observations upon cultures for longer periods, two to four weeks, 

 indicated that the optimum temperatures for maximum growth are from 

 25° to 30°, with practically no difference between. The total growth 

 produced was less above and below these points. While it was still 

 proceeding normally but at a slower rate at the lower temperatures, at 35° 

 it was not only less but appeared to have reached its end. 



The discoloration of the medium which is very characteristic of cultures 

 of this parasite was faint or absent at and above 35° and quite intense 

 at and below 30° C. 



INVOLUTION FORMS 



High temperature is considered one of the factors influencing the 

 production of degeneration forms of bacteria,^ but this is apparently 

 not the case with the potato-scab organism. While the individual fila- 

 ments, which at lower temperatures are long and more or less curved, 

 appear very short and curled at 37° C, and especially so at 39° and 40°, 

 the writer does not consider them strictly involution forms. The gonidia, 

 also, can not be considered as involution forms, since morphologically the 

 same bodies occur normally upon scab spots on potato tubers and appar- 

 ently serve as fruiting organs. 



However, such abnormal growths may be produced by certain kinds 

 of culture media. The writer has observed some very interesting involu- 

 tion forms which constantly appear at all temperatures when the scab 

 organism is grown upon a synthetic agar that is much used in this labora- 

 tory for the cultivation of fungi.^ 



On this medium germination and growth proceeds normally at first, 

 but after two days, if incubated at 35° to 37°, which, as has already 

 been pointed out, are within the range of most favorable temperatures 

 for germination and early growth, the threads become distorted and 

 swollen at various places, both at the tips and in the middle. Some- 

 times even the gonidia themselves become abnormally enlarged at or 

 before germination. At the end of a month the entire growth will con- 

 sist of swollen, club-shaped, oval, or spherical segments of various sizes 

 (PI. XV, fig. 3 and 4). Not infrequently these abnormalities reach 4/i in 

 diameter. The consistency of the growth thus produced is soft and 

 somewhat slimy instead of being tough and hard, as is usually the case. 

 By leaving out one of the ingredients of the medium at a time it was 



iMigula, Walter. System der Bakterien . . . Bd. i, p. 52-53. Jena, 1897. 



- This synthetic agar was prepared according to the formula given by Darivin and Acton (Darwin, Francis, 

 and Acton, E. H. Practical Physiology of Plants, ed. 3, p. 68. Cambridge, 1909) and consists of — 



Gm. ' Gm. 



Dextrose 50 Magnesium sulphate 2. 5 



Peptone 20 Potassium monophosphate 2. 5 



Ammonimn nitrate 10 Calcium chlorid o. i 



Potassium nitrate S Distilled water i, 000 



