Cockayne. — Spread of Phytophthora infestans. 317 



in the Auckland Province, but it has not been heard of since 

 till last year." For my part, I consider that the epidemic out- 

 break in Auckland in 1904 was in no way connected with the 

 sporadic ones which occurred previously. The cause can be 

 attributed to the fresh importation of Phytophthora mycelium 

 in imported potatoes. There are only two ways that Phytoph- 

 thora can possibly have been introduced here — either by resting 

 mycelium in diseased tubers, or by asexual spores or oospores ; 

 but as these latter have never been definitely discovered, its in- 

 troduction by means of them is most unlikely. The asexual 

 spores of P. infestans are naturally short-lived, and are in no 

 way provided with any adaptations to withstand the desiccation 

 that they would undergo in passing through the tropics. There- 

 fore it is almost certain that the introduction of this disease into 

 New Zealand was by means of dormant mycelium hidden away 

 in the tissues of affected tubers. This view is greatly strengthened 

 by the discovery, on numerous occasions during the past two 

 years, of varieties of imported potatoes in which the presence 

 of Phytophthora mycelium was clearly demonstrated. 



All potatoes which are now imported into New Zealand are 

 carefully examined by the Agricultural Department, and those 

 lines found affected with Phytophthora are at once destroyed. 



Action of Phytophthora on Potato-tubers. 



There are still many gaps in our knowledge of the full life- 

 history of the Irish potato- disease, and the exact pathological 

 processes that obtain in so-called diseased tubers are but im- 

 perfectly understood. That the disease is transmitted from 

 season to season by means of mycelium permeating the tissues 

 of the tubers themselves has been now abundantly proved. 

 Massee has given the name "hybernating mycelium" to that 

 portion of the vegetative body of a fungus which has the power 

 of remaining quiescent during the dormant period of the host's 

 existence, and which can return to normal development as soon 

 as the host commences to develop. It follows naturally that 

 those fungi which are able to develop hybernating mycelium 

 do not require the same amount of varied spore-formation as 

 those which are not so equipped. The formation of winter 

 spores would be decidedly superfluous, and such fungi can be 

 more specialised in the direction of producing summer and 

 generally short-lived spores, whose object is to rapidly infect 

 large masses of their hosts, provided the environment is 

 suitable. 



The finest example of hybernating mycelium is found, as 

 Freeman has shown, in the fungus affecting the various species 

 of Lolium, especially L. temulentum. In over 70 per cent, of 



