MauPHY — Bionomics of the Conidia of Phytophthorainfestans. 457 



production. Corresponding results were secured with preparations kept at 

 22°-23° C. 



This experiment was repeated using motile protozoa along with conidia, 

 which makes a beautiful preparation. The protozoa after twenty-four hours 

 are confined to a narrow zone near the edge of the cover glass, in which they 

 swim about freely. In this area, and extending somewhat within it, good 

 germination is to be found, whilst still further in there may be no germina- 

 tion whatever. Motile bacteria exhibit the same zonation to some extent, the 

 outer ones only being in motion. Penicillium spores have also been used with 

 somewhat similar results. In this case the rapid development of P. infestans 

 allows some zoospores to be produced, but a great many become abortive. 



These results, it is believed, are to be explained by the fact that the 

 competing organisms use up the oxygen in the water and replace it with 

 carbon dioxide faster than the conidia can utilize it for the purpose of 

 , germinating. There may be a question of other excretion products having 

 an inhibitory effect, but judging from the variety of organisms used this is 

 hardly likely. The same effect has been produced by sealing round the edge 

 of the cover glass with melted paraffin. Furthermore, the conidia themselves, 

 if the slide is very thickly seeded, bring about the same result. When 

 germination is general around the outside of a clump, there is often none at 

 all in the centre. Conidia scattered among the hyphae of living mycelium of 

 P. infestans frequently behave similarly. 



Formation of secondary conidia.— Second&rj conidia, as originally described 

 by de Bary (2, 3, 4), are the immediate products of previously existing 

 conidia, being borne on the germ tubes of the latter. The name " secondary 

 conidia" is not a particularly good one, for conidia morphologically and 

 physiologically identical are produced by hyphae which fructify in water, 

 and identical but smaller bodies may be formed by zoospores, as will be 

 shown. The term is used in a somewhat loose sense in this paper for the 

 new conidia which sometimes follow after the germination both of conidia 

 and of zoospores. Such secondary conidia seem to owe the characteristic 

 appearance which is common to them and to conidia produced by submerged 

 hyphae to the fact that they are formed in water, on which account a word 

 like " hydro-conidia," which would include all three, might be preferable. 



The conditions under which germination is arrested but subsequently 

 takes place are favourable for the production of secondary conidia. These 

 have been very common on our slides. When germ tubes are formed in the 

 presence of bacteria, or when bacteria are introduced after their formation, 

 secondary conidia will always be met with within a zone from about 2 to 4 mm. 



