76 OF THE OSCILLATORIA. 



CHAPTER VIII. 



OF THE OSCILLATORS. 



Of the members of this family, the Lyngbyse and the 

 species of Leptothrix which float on the surface of the 

 water may all be prepared in precisely the same way as 

 was recommended for the Confervas in the previous chap- 

 ter. Those like Phormidium, which form filmy masses, 

 will be treated of presently. But the genus Oscillatoria 

 itself requires a peculiar mode of preparation. The 

 individuals embraced in this group dwell by preference on 

 mud at the bottom of shallow ponds and ditches ; though 

 frequently, on a sudden rise of the water, they break away 

 from the soil and float, appearing like a cloudy film 

 radiating from a centre. All have a disagreeable odour, 

 which clings to them long after they have been deposited 

 in the herbarium. They vary much in colour ; the 

 common tint is an seruginous green, darker or lighter in 

 different species. Others are steel-blue or even brown — as, 

 for instance, Oscillatoria Frohlichii in many of its habitats. 

 During the process of drying, they change colour, owing to 

 the influence of the oxygen in the atmosphere, their tints 

 gradually becoming brighter and more intense. This is 

 well seen in the last-named species, which is usually of a 

 deep brown during life, but which after death, and while 

 drying, assumes first a blackish-green tint, and then a steel 

 colour. 



The family takes its name from a property peculiar to 

 some of them, but not common to all, of spontaneously 

 oscillating or waving gently backwards and forwards, like 

 a very slow-moving pendulum. To whatever cause this 

 mysterious movement may be due, the fact itself is of 



