MuwPHY— i?«o«,o?«/c?s of the Conidia of Plujtophthora infestans. 449 



Notwithstanding these findings, based as they are on laboratory experi- 

 ments, there are indications that the life of conidia in air may be longer 

 under more natural conditions. When a vigorous growth of conidiophores is 

 left undisturbed for some time, it is noticeable that the fringe of the growth 

 contains conidia which are few in number and of poor germinative capacity. 

 Further in towards the substratum numerous and more vigorous conidia may 

 be found. It is probable that the air in the centre of such masses of mycelium 

 is more moist than in the fringe, which would account, to some extent, for 

 the result. At the same time the comparatively low vitality of conidia 

 when removed from the conidiophores and kept in a saturated atmosphere 

 would go to show that another factor besides moisture must be involved. It 

 is possible that large masses of mycelium contain an abnormal proportion of 

 carbon dioxide, arising from the liyphae themselves, the substratum, or 

 from bacteria and other contaminations, and that this has an influence on 

 the conidia. The bearing of a reduced oxygen supply on the germination 

 and vitality of the fungus is discussed more fully in later paragraphs. 



The view sometimes expressed (3, pp. 38 and 39 ; 4), however, that 

 that the conidia may live for some weeks in dry air under certain conditions, 

 while in moist soil their existence is shorter owing to germination supervening 

 after the first rain, is in opposition to our results. 



An experiment was twice repeated in which columns of sifted loam and 

 silver sand, each 10 cm. deep, were contained in two upright glass tubes, each 

 having a diameter of 2-8 em. The tubes were open at the top, and had an 

 arrangement to permit of drainage water escaping at the bottom. After the 

 materials in the tubes had settled, a small quantity of contaminated sand 

 was placed on the surface of the loam and sand. Water in measured 

 quantities was then allowed to drip at fixed rates on the contaminated soil 

 in the tubes. The liquid which came through at the bottom was caught on 

 filter papers, which were transferred at regular intervals to the cut surfaces 

 of potato tubers. The experiments were continued for six and five days 

 respectively. 



It was found in these experiments that the filter papers became sparingly 

 pathogenic to potato tubers after the passage of water equivalent to a " rain- 

 fall " of 40 to 80 cm. or more, and seventeen to forty-one hours after the 

 " rain " began. In the first experiment the sand alone allowed the infecting 

 material to pass through, this occurring after twenty-six and a half hours, 

 and none coming through the loam. In the second test (begun twelve days 

 after the commencement of the first, the same tubes and soils being used as 

 before, but with the addition of fresh contaminated sand) infecting material 

 passed through the loam filter only, and that after seventeen, twenty-four 



SCIENT. PBGC. R.D.S., VOL. XVI, NO. XXXIV. 3 O 



