16 Scientific Proceedings, Royal Dublin Society. 



involves the view that the mycelium becomes dormant for a period and then 

 resumes its activity. 



The chief advocate of the theory, which, on the face of it, as will be 

 presently shown, has little to recommend it, is Massee (7), whose most recently 

 publislied views may be summarized as follows : — Wlien a diseased tuber is 

 planted, the produce of such a tuber is always diseased ; yet under certain 

 conditions of weather (in bright, dry seasons) tlie stem and leaves of tlie 

 plant may remain perfectly free from tlie disease. On tlie other hand, during 

 a few cloudy, damp, sultry days in July the mycelium will take possession 

 of the stem and leaves, which will succumb within a few days. The 

 fact that simultaneous outbreaks of an epidemic may occur, extending over 

 wide areas, is considered to be best explicable in this way. 



According to this view, then, not only does the mycelium of Pliy tophthora 

 lie dormant in the tubers during the winter, but also during the long period 

 from planting-time — say, in March — until the time of the appearance of the 

 blight in June or July. Incidentally it may be remarked that it is diiBcult 

 to see how the produce of a diseased tuber can be diseased if at the same time 

 the mycelium is lying dormant in the tuber ; and the theory apparently states 

 tliat, unless a season of unfavourable weather sets in, the stem and leaves 

 remain free from the disease, and therefore in an exceptionally dry summer 

 the produce from a diseased tuber would not be diseased ! 



My own observations of the behaviour of diseased tubers during the 

 winter lead me to believe that even then the mycelium in them is not in a 

 true state of hibernation or dormancy. During this period certain changes 

 are going on ; the tuber itself is slowly sprouting, and the mycelium 

 invading fresh, healthy tissue at a rate depending largely upon the tem- 

 perature at which the tubers are kept. But even if it is admitted that 

 the mycelium in the tubers is dormant during the winter, it is almost 

 impossible to entertain the idea that, when the tuber is planted, it still 

 remains dormant. Surely planting the tuber in moist soil, which is also 

 often considerably warmer than the surrounding air, would, if anything, 

 encourage the mycelium to more active growth than before, rather than cause 

 it to remain in, or take on, a latent condition ! Another strong argument 

 against the acceptance of this theory is that, according to it, the attack of the 

 stalks must proceed from below upwards, whereas the exact contrary is what 

 is actually found to take place in the fields. On healthy plants the blight 

 always appears first as spots on the leaves or on the upper parts of the stalk 

 or its branches, and never in the form of decay at its base. Even if the 

 mycelium did lie dormant in the tubers, it must, to produce such spots, grow 

 up through the stalks to reach such places ; and how could it possibly do this 



