Pethybkidge — ExperimenU ivltli PJujtophthora. 17 



without affecting the tissues of the stalk and its branches ? It is impossible 

 to understand how tliis could occur without some indication of it being given 

 by the wilting or other abnormal behaviour of the stalks and foliage, such as 

 occurs in cases of Black Stalk Rot, Leaf Eoll, &c. ; but no such phenomena 

 have ever been observed in the case of attacks by Phytophthora, 



Anyone who is acquainted with the history of tlie numerous investigations 

 which have been carried out on this disease during the past fifty years 

 must know that experiments have proved tliat from diseased tubers perfectly 

 healthy plants may be produced, which, provided they are kept free from all 

 means of external infection by means of spores, remain unattacked tlirough- 

 out the season. Indeed, long ago the Prussian Government set the problem 

 as to whether healthy or diseased plants were produced from diseased tubers, 

 for solution to a body of scientific men on the staffs of its Agricultural 

 Academies and Experimental Stations, with the result that it was found that 

 diseased tubers produced healthy plants. Experiments carried out by 

 Professor Carroll and tlie late Professor Wright at tlie Albert Agricultural 

 Institution, Glasnevin, gave a similar result ; and my own experiments, which 

 will be dealt with presently, prove the same thing, so that there is no 

 necessity to labour this point. And it is not impossible or even difficult to 

 explain how tliis can be the case. Those who have studied and experimented 

 with diseased tubers know how comparatively ephemeral the growth of 

 Phytophthora on them is when they are kept in moist warm air, and how 

 comparatively soon the growth of Phytoplithora is over, and such tubers or 

 pieces of them become swamped with growl hs of other fungi and bacteria, 

 and become completely destroyed. When such a tuber is in tiie soil during 

 tlie period from March till June or July, there is even more chance of tliis 

 taking place. Meanwhile, liowever, tlie sprouts, especially if well started at 

 planting-time, have developed into healthy stalks independent of the parent 

 set. 



Sufficient has now, I think, been said to show that, on a priori grounds, the 

 theory of infection from dormant mycelium is a most improbable one. Hence 

 it becomes necessary to examine carefully the evidence on which it is based. 

 This, given in its chief upholder's own words, is as follows : — 



" Three potatoes showing rusty stains in the flesh, indicating the presence 

 of the mycelium of potato disease (FhytoiMhova infedans), were each cut into 

 two equal parts. Each half potato was planted separately in a plant-pot, 

 the soil and manure used being tiie same for all, and was sterilized by steam. 

 Three of the pots were placed in a house having a temperature ranging 

 between 70° and 80° F., and very often with moisture at saturation point. 

 Each pot was covered with a bell-jar. The remaining three pots were placed 



