Murphy — Bionomics of the Conidia of PJujtophthora infestans. 461 



important aid iii prolonging life. While original nngerminated conidia 

 produced in air were not observed to be viable after a period of five to seven 

 days in water, germ tubes and secondary conidia are much more resistant. 



The low oxygen concentration in which the slow development and 

 continued vitality of the fungus were directly observed for comparatively 

 long periods in microscopical preparations is believed to offer an explanation 

 of the similar resistance exhibited in the soil. It is reasonable to suppose 

 that all the conditions reproduced artificially must be realized in the soil, as 

 well probably as many others. 



According to Paissell and Appleyard (21), there are two distinct 

 atmospheres in the soil — the " free air " and the " dissolved air." The former 

 does not differ in principle from atmospheric air, but it contains a larger 

 and fluctuating amount of carbon dioxide (about 0'25 per cent.), and is almost 

 saturated with water vapour. The dissolved air contains practically no 

 oxygen, and is made up of carbon dioxide and nitrogen in varying pro- 

 portions. It is shown that the oxygen requirements of the soil are always in 

 excess of the supply, and thus the two bodies of air, though existing in close 

 relationship to each other, retain their individual characteristics. Tempera- 

 ture, cultivation, manuring, and rainfall in particular, however, affect the 

 soil atmospheres. A heavy fall of rain, which is nearly saturated with 

 oxygen, partially renews the dissolved air. This, as evidenced by the subse- 

 -quent rise in the COj content, allows the development of aerobic organisms. 

 Among the latter the conidia of P. infestans must find in the fluctuations of 

 the dissolved atmosphere the conditions which enable them to survive for the 

 long periods noted in the experiments with contaminated soil, and to germinate 

 when conditions are made more favourable by the accession of water rich in 

 oxygen. 



There is another feature in the fluctuation in the amount of carbon 

 dioxide in the soil which may influence the course of the blight fungus. The 

 channel along which the parasite makes its way from the tubers to the leaves 

 in spring is not known with sufficient certainty. The stem of the potato has 

 been proved in a few cases to be penetrable by the mycelium, but the 

 possibility of the transference taking place through the soil has hardly been 

 studied. This is the more curious since the downward course of the parasite 

 in the autumn from the leaves to the tubers is admittedly through the soil. 

 It may be significant that the time at which the parasite finds its way down- 

 wards coincides with the minimum CO2 content of the " free "soil atmosphere, 

 while in the spring, when the course is upwards, the CO2 content is at its 

 maximum. The results of an experiment to test the capacity of the blight 

 fungus to penetrate the soil were so suggestive that further attention is 

 being devoted to the subject. 



