, 8 INTRODUCTION [ CH - 



natu and in artificial culture. In so far as a clear field is 

 tend to grow equally in all directions from the point where infection took 

 pace The same factor may account, as Stevens and Hall have suggested 

 for the alternate dense and sparse zones which characterize many fungal 

 colonies and are independent of changes in light and temperature. Energet, 

 .rowth results in the deposition of katabolic substances, and growth is corre- 

 spondingly reduced till a few scattered hyphae pass beyond the inhibiting 

 influence and give rise to a new ring of richly branched mycelium. In older 

 colonies the germination of fresh spores outside the zone of staling substanc 

 doubtless adds to this effect. 



The very definite character of the reaction is demonstrated by the ex- 

 periments of Graves, who used the germ-tubes of Rhizopus nigricans on con- 

 trasted agar media separated by a perforated mica plate. Hyphae developing 

 in agar made up with turnip juice grew towards the perforations which led 

 to fresh but otherwise identical agar on the other side of the mica plate. 



Hyphae under similar conditions turned from turnip juice, in which 

 growth had taken place, to plain agar, or indeed to any fresh substance 

 that was not in itself definitely repellant, even if, as in the case of plain agar, 

 its nutritive value was low. 



On the other hand, the hyphae turned away from the perforations in the 

 mica plate if these led to nutritive agar on which mycelium had already 

 developed and if the substratum on which the germ-tubes were growing was 

 by comparison fresh. This was the case even after the mycelium on the 

 staled agar had been taken away, the products of its metabolism which re- 

 mained in the agar having themselves a repellant effect. This effect was 

 removed by exposure to a temperature of iooC., showing that the staling 

 substances are altered or destroyed by heat. 



A somewhat different example of the inhibiting effect of these katabolic 

 staling substances was observed by Balls for the "sore-skin" fungus of cotton. 

 When the nutritive medium is limited in extent, growth comes to an end 

 more rapidly at high temperatures than at low. This is due not to the 

 earlier exhaustion of the available food supply, but to the more rapid ac- 

 cumulation of staling substances when growth is accelerated. Media in 

 which growth has come to an end prove capable of supporting further 

 growth if diluted with an equal quantity of water. By this means the staling 

 substances are sufficiently diluted to lose their inhibiting effect, while the 

 nutritive materials, even though diluted, are still sufficient to support growth. 



