458 Scientific Proceedings^ Royal Dublin Society. 



wide inside the edge of the cover glass. In preparations sealed with paraffin 

 they are found only at the extreme edge. Their formation is constant, and is 

 the result of specific conditions. At a certain degree of oxygen concentration 

 which just permits germination, or when the oxygen supply is gradually cut 

 o£P after germination has taken place, secondary conidia are produced. 

 Bacteria are the most reliable agents for bringing about this state of affairs, 

 as may be seen by introducing them into one of two uniform preparations. 

 At a still lower oxygen concentration ungerminated conidia are eventually 

 (and sometimes rapidly) killed ; but there is evidence derived from sealed 

 preparations to show that germ tubes continue living for several weeks, and 

 even increase in length, with a still scantier oxygen supply. This is possibly 

 connected with the greater amount of exposed surface, which facilitates 

 respiration. For some reason which has not been established, secondary 

 conidia appear to be materially more resistant than are conidia formed in air 

 to the adverse influences they meet with in water in competition with other 

 organisms ; and they are, therefore, an important adaptation to certain aquatic 

 conditions. 



The formation of secondary conidia has been mentioned by most workers 

 since their discovery by de Bary (2, 3, 4), but certain new observations may 

 be recorded. The conidia which give rise to them rapidly lose all their 

 contents, which pass in their entirety into the smaller and less symiiietrical 

 secondary bodies. If the primary conidium has two germ tubes a secondary 

 conidium may be produced by each. The hypha joining the primary and 

 secondary conidia is of variable length, or may exceptionally be lacking, the 

 second conidium being an asymmetrical prolongation of the first. The 

 germ tubes generally grow in the direction of the greater oxygen concentra- 

 tion. Frequently the new conidia are found in the free water surrounding 

 the cover glass, being borne on very long tubes springing from conidia 

 lying further in. Iliese hyphae grow straight to the margin, perhaps after 

 a preliminary turn. The secondary conidia have thinner walls and more 

 rounded and prominent papillae than the original ones, and they always 

 contain one (or occasionally two) yellowish oil-drops. Even when lying in 

 the free water around the cover glass they may remain ungerminated but 

 viable for about three weeks. Frequently, however, they germinate sooner, 

 and they may produce zoospores if the conditions are favourable ; but germ 

 tubes are the general rule, frequently followed by a third conidium, as 

 de Bary and others have described. These have frequently been seen, and 

 on several occasions even what may be called a quaternary conidium, four 

 conidia in a chain, each somewhat smaller than the preceding one. It is 

 conceivable that repeated germination and conidium formation may result in 



