THE LARCH CANKER 47 



and a fev/ drops of water are allowed to lie between them. 

 A cover -slip is then very carefully removed from one of 

 the culture-chambers, and placed on the two rings, so that, 

 resting above the drops of water, the moisture on its under- 

 side does not dry up. The whole can be put under the 

 microscope and the spores removed by a sterilized needle 

 or glass filament and transferred to culture media in Petri 

 dishes or Cogit's flat glass bottles, the usual precautions 

 being taken against the introduction of foreign spores. 



A suitable medium was found to be either gelatine or 

 agar-agar containing malt extract (3 per cent.), meat 

 extract (0'3 per cent.), and citric acid {0-03 per cent.). 



There is at first some difference between the cultures 

 obtained from the large spores of the parasitic form and 

 the smaller spores of the saprophytic form. The germ 

 tubes of the latter are smaller, and growth is at first rather 

 slower, but after a day or two very little difference can be 

 found, and older streak cultures are indistinguishable. 



The mj^celium is characterized by forming a very close 

 felt-like mass, and it grows so evenly on all sides that 

 a culture from a single spore becomes a- circle (fig. 22) and 

 a streak culture has the form of an even band. When 

 growing on gelatine the mycelium puckers up the sub- 

 stratum into folds, but agar-agar remains flat. 



Temperature has a considerable modifying influence on 

 the growth of cultures, and the following table is instructive 

 in this respect. Twelve streak cultures were grown on agar- 

 agar in Petri dishes. The first four, A1-A4, were grown at 

 the temperature of the room (13°-14°C.), the next four, 

 As-As, in an incubator at 22°-23° C, and the last four, A9-A12, 

 in an incubator at 26°-27° C. 



It can be seen from the following table that a temperature 

 of 12°-13° C. is below the optimum for growth, and 26°-27° C. 

 is above the optimum. It is interesting to note that the 

 higher temperature encourages germination, which takes 

 place very slowly at the lower temperature, but for sub- 

 sequent growth the latter is more favourable. Also the 

 high temperature encourages excessive branching. 



