Pethybridge and Murphy — On Plujtophthora infestans. 573 



attack is followed by that of another, or of other organisms, when the 

 subsequent fate of the tuber will depend upon the nature of these secondary 

 organisms, and upon the conditions under which the tuber is kept. In 

 reality, therefore, the rot produced in a tuber by this fungus is essentially a 

 form of dry-rot. 



In practice it is not an easy matter to grow the fungus in a whole tuber 

 in such a way that one can be absolutely sure that the growth will remain 

 pure all the time. Our methods of procedure with whole tubers, and the 

 results, have been as follows : — Clean, healthy tubers, with unblemished skins, 

 were selected and carefully washed in running water, a soft brush being used. 

 They were steeped for a period in a dilute solution of either formalin or 

 mercuric chloride, and then dried. Inoculation was made with a small 

 portion of mycelium, generally bearing conidia, from a pure culture of the 

 fungus on an artificial medium, through a shallow stab into the skin of the 

 tubers. The tubers were then allowed to stand in a covered glass dish, 

 sometimes on a piece of moistened filter-paper, sometimes without this, at 

 room-temperature. 



As a rule, in from five to seven days, a somewhat dark, sunken area is 

 formed around the original point of inoculation ; aerial mycelium, bearing 

 conidia, may or may not develop at the inoculation-wound. This dark, sunken 

 area gradually extends ; and in the course of four weeks or so the greater part 

 of the skin of the whole tuber may have become similarly affected ; on the 

 other hand, in some cases, after the lapse of a similar period, the diseased 

 area may be much smaller. 



Inoculated tubers lose some of their water more quickly than control tubers, 

 treated similarly (i.e. stabbed, but with a sterile needle), do. This water tends 

 to condense and to collect on the lower surface of the tuber between it and 

 the bottom of the glass dish. When this is the case or when the inoculated 

 tuber has been placed on moistened filter-paper at the outset, the lower part 

 of the tuber becomes affected more rapidly than the upper. Experiences of 

 this kind and others which have been encountered lead us to believe that the 

 skin of a potato must not be looked upon as a mere physical membrane 

 impervious to water, for its properties, with regard to the passage of water 

 both inwards and outwards, ajDpear to be radically altered when the living 

 cells adjacent to it become killed. This idea is supported by the experiments 

 of Stoward,^ who found that certain chemical substances in solution pass 

 much more readily into a tuber through the skin over dead portions of tissue 



' Stoward, F. — "Xhe Effect of certain Chemical Substances on the Vitality of the Buds of Potato 

 Tubers, and their Disinfective Action on Potato Blight [Fhytophthora infestans).'''' Proc. Eoy. See. 

 Victoria, vol. xxiv (New Series), Pt. 2, 1912. 



SGIENT. PEOC, R.D.S., VOL. SIII., NO. XXXVI. 4 Q 



