400 SUMMARY OF CURRENT EESEARCHES RELATING TO 



Cohn's solution witli drops of putrid flesh-liquid full of Bacterium 

 termo. The questions which he endeavoured to solve were : — 1. 

 Whether ordinary diffused light exercises any influence on the de- 

 velopment of bacteria in Cohu's solution ; 2, whether any influence is 

 produced by the direct rays of the sun ; 3, whether the direct rays of 

 the sun rapidly kill bacteria in a dry state. In accordance with 

 earlier observations, it was first ascertained that the bacteria were 

 killed by direct but not by diffused sunlight. Experiment was then 

 made as to the effect of temperature on tliese results ; and the con- 

 clusion arrived at was that at moderate and low temperatures the 

 direct rays of the sun not only do not kill bacteria, but even have no 

 prejudicial influence on their development. This will account for the 

 want of harmony in the results of previous experiments. 



As regards the effect of direct sunlight on dry bacteria, Jamieson 

 arrived at the conclusion that, under conditions very favourable for 

 rapid and complete drying up, as when the bacteria are freely exposed 

 to both sun and air, they may be destroyed in a comparatively short 

 time, which, however, is not less than from two to four days, even in 

 summer. Any direct influence of the rays of light as such on bacteria 

 must therefore be regarded as not established. 



Bacteria connected genetically with an Alga.* — Among the 

 algje which abound on the damp walls of greenhouses, one of the com- 

 monest is, according to H. Zukal, Drilosij^hon Julianns Ktz., belonging 

 to the Scytonemeae, and distinguished by its thick outer calcareous 

 sheath, which is partially or entirely wanting in places. The fila- 

 ments have a tendency to produce hormogonia of two different kinds. 

 In the formation of the most common kind, the filament within the 

 outer sheath splits up into small pieces, which ultimately escape, 

 and each carries on an independent existence. These develope into 

 filaments which again produce hormogonia ; but in this process the 

 original comparatively thick form of filament is never reproduced ; 

 each resulting filament is slenderer than the last ; till finally they 

 become invisible except to the highest powers. The second kind of 

 hormogonium has a fusiform shape, and thick brownish cell-wall, and 

 consists usually of from four to eight cells. These may remain 

 dormant for a long time after their escajDe from the sheath, and may 

 hence be termed resting hormogonia. On germinating, they reproduce 

 the ordinary thick filaments. 



The gradually thinner filaments resulting from the ordinary 

 hormogonia manifest a tendency for their constituent cells to sepa- 

 rate within the external sheath in a moniliform manner, which becomes 

 more pronounced in the succeeding generations, the sheath at the 

 same time gelatinizing, and the whole organism assuming a nostoc- 

 like character. The cells finally become differentiated into larger 

 and smaller, and in this state the filament is known as Nostoc parieti- 

 num Eabenh. The cells of the filament eventually separate, and 

 assume the character of an Ajyhanocapsa. Or in other cases Glceo- 

 cnpsa-Yike structures result from the nostoc-filaments, which coustitute 



* Oesterr. Bot. Zeitschr., xxxiii. (188.3) pp. 73-8 (1 pi.)- 



