Pethybridgk — Experiments vjith PJujtophthora. 15 



for nutrition, before the fungus has been able to invade tliem. Such plants, 

 of course, may become attacked with the blight in tlie ordinary way by 

 means of aerially borne spores later on. 



Shortly summarized, therefore, it may be said of diseased tubers that 

 tliey may — 



(1) die before planting-time ; 



(2) die if planted in the ground, without producing any overground 



stalks ; 



(3) produce small stalks above grouud wliicli soon die owing to direct 



infection with the fungus from the parent tuber; 



(4) produce healthy plants, whicli, provided thei'e be no opportunity 



during tlie season of becoming infected by aerially borne "spores," 

 remain free from tlie disease. 



With regard to the tubers whicli are killed before planting-time, these can 

 scarcely become a source of infection to the new crop. For, even if left 

 lying about aliove ground, tliey will probably be no longer capable of giving 

 rise to " spores " by the time tliat tlie now foliage has developed. Tlie case 

 of those tubers which are not actually dead at planting-time is, however, 

 somewhat different. Tliese, if too obviously diseased to be used as " seed," 

 may be left lying about, and may still be capable of producing " spores " 

 under suitable conditions when the new foliage has developed, and they 

 would therefore be a source of danger to the ensuing crop. 



Little danger is to be feared from any tubers which, being badly diseased, 

 are planted and produce nothing above ground, for the chances of any 

 " spores " whicli may have been formed on tlie tuber below ground finding 

 their way in a living condition to the surface are but small. On the other 

 hand, tubers which produce small diseased shoots above ground may be 

 higlily dangerous sources of infection, for on these shoots "spores" are 

 produced which can easily be carried to neiglibouring healthy plants. 



Tliere is, therefore, no question but that in a few cases the mycelium in a 

 diseased tuber may succeed in reacliing a shoot which has got above 

 ground ; but this takes place comparatively early, and such shoots are soon 

 killed off. This early killing off of young shoots by invasion of mycelium 

 from the tubers is, however, a very different thing from the sui3posed infection 

 of well-grown plants by such mycelium some two or three montiis later in the 

 season. If this actually occurs, the mycelium must be supposed to be growing 

 with almost inconceivable slowness, or to be lying in a dormant condition 

 during this period while these apparently healthy plants are developing. The 

 recently propounded theory of mycelial infection to be discussed in this paper 



