310 NOTES AND MEMORANDA. 



wlien they are young than when they are old ; and it exhibits also 

 other variations. 



Apart from the alteration in sensitiveness during development, 

 whole batches show themselves to be directly sensible to relatively 

 higher or lesser intensities of light. This appears to depend on the 

 intensity of the light in the spot where they were produced. 



Heat exercises for the most part an influence on the photometric 

 sensitiveness of swarmspores. As the temperature rises they become 

 in general more sensitive to light, less so as it sinks. 



If there is not a free current of air through the batches, photo- 

 metric swarmspores are sensitive to higher intensities of light. 



Insufficient nutriment prevents the swarmspores from coming to 

 rest, without influencing their sensitiveness for light.* 



Floating Algae forming Scum on the Surface of Water.— The 

 cause of the sudden appearance of green, red, or brown scum on the 

 surface of water, when due to an algoid growth, must be either an 

 extraordinarily rapid multiplication of the alga, or a change in its spe- 

 cific gravity, in consequence of which it rises from the bottom to the 

 surface of water, such as occurs also in the terminal buds of flowering 

 water-plants, as HydrocJiaris, Sfratiotes, CeratophyUum, Myriophyllum, 

 Aldrovanda, Utricularia, &c. The organism which constitutes this 

 " Wasser-bliithe " is usually some green alga belonging to the Chroo- 

 coccacefe, Oscillatorieae, or Nostocaceas. Professor F. Cohn has for 

 the first time detected a Mivularia "j" as the cause of this appearance, on 

 a stream near Lauenburg, in Pomerania, the surface of which was 

 completely covered with a green scum, consisting of an innumerable 

 quantity of minute globes from ' 15 to 0-3 mm. in diameter, bearing 

 a superficial resemblance to Volvox. Under the Microscope they were 

 found to consist of Kivularia-filaments imbedded in jelly, formed of 

 ordinary cells and heterocysts. Cohn considers it a new species, to 

 which he gives the name Rivularia fiuitans. 



About the same time, C. Gobi, of St. Petersburg | observed a similar 

 appearance on the surface of the sea-water in the Gulf of Finland, con- 

 sisting also of minute green globes from 0*3 to • 45 mm. in dia- 

 meter or larger, enclosed in a very thin jelly, to which he gave the 

 name Rivularia pelagica. The two sj^ecies were subsequently deter- 

 mined by Professor Cohn to be indistinguishable from one another. 

 The marine form was seen only when the water was tolerably still, 

 disappearing completely when it became rough, and was accompanied 

 by large patches of another green alga, Aphanizomenon fios-aquce Elfs., 

 which had hitherto been observed only in fresh or brackish water. 



Luminous Bacteria in Meat. — An account has been published § 

 of some observations of M. Nuesch on " Luminous Bacteria on Fresh 

 Meat." A fact of the same kind was noted by the famous Fabricius 

 ab Aquapendente in 1592, who appears to have been the first to 

 observe it. M. Nuesch had some pork chops which were sufficiently 



* ' Der Naturforseher,' xi. (1878) 485. 



t ' Hedwigia,' xvil. (1878) 1. 



X Ibid., 33. 



i^ ' Bidl. Sc. Dc'p. dii Nord ' (1878) 184. 



