Pethybridge— The. Verticillium Disease of the Potato. 79 



On certain media, viz., beef extract agar and gelatine and potato-stalk 

 extract gelatine, this blackening of the submerged mycelium does not occur 

 at all. Further, although the fungus readily develops this black submerged 

 mycelium (except on the media just mentioned) when cultivated soon after 

 its isolation from the potato, yet in the course of time, after prolonged 

 cultivation on these media, its power of producing black submerged mycelium 

 gradually diminishes until it becomes entirely lost. 



Most of the cultures were kept in the diffused light of the laboratory, but 

 a special comparative series was made to see whether light exercised any 

 influence on the behaviour of the fungus. Parallel cultures were for this 

 purpose (a) kept in total darkness, and (b) exposed directly to daylight in a 

 window facing north but protected from direct sunlight. No very striking 

 differences were observed in the cultures, but in the dark the aerial growth 

 was pure white, while in the light it took on a faint pink fringe, and the 

 blackening of the submerged mycelium which occurred both in the dark and 

 in the light began to be perceptible in the cultures kept in the dark slightly 

 sooner than in those exposed to light. On all the gelatine media used liquefac- 

 tion of the gelatine invariably occurred. This took place perhaps somewhat 

 slowly : but in slants in test-tubes the medium was usually completely 

 liquefied in cultures at room-temperature one week old. 



Many attempts were made to cultivate the fungus on living potato tubers 

 and on cut portions of living green potato-stalks, but without success. A 

 limited amount of development occurred in the cells injured by cutting or 

 making the inoculation. In the case of the cut stalks further growth took 

 place only when they had been standing for such a long time that they had 

 begun to die. In the case of the tubers or cut portions of them no further 

 growth took place, and a layer of cork was developed beneath the wounded 

 surface. Hence, although the fungus is a vascular parasite of the potato, it 

 is quite incapable of causing a rot of the living tubers or stalks. 1 



The morphological characters of the fungus were accurately described and 

 figured by Eeinke and Berthold, and its growth in pure culture has not 

 resulted in the necessity of altering or adding to the description given by 

 them. It is believed, however, that the species which these authors met with 

 living on the skin of the tuber and in the cortex of the stalks below ground 

 was not the same as that inhabiting the wood -vessels, but probably one of the 

 two already referred to, which develop dark-coloured chlamydospores on their 

 submerged mycelium. 



1 The opposite results obtained by Carpenter (Journ. Agric. Research V, No. 5, 1915, 

 p. 203) are, as he himself points out, doubtful, and were probably due to imperfect 

 experimental methods. 



SCIENT. PROC. B.D.S., VOL. XV., NO. VII, N 



6 



