PROTOPHYT.E. 95 



frequently spread themselves as a green slime over the surface 

 of ponds and ditches. Their multiplication by duplicate sub- 

 division is so rapid that, in spite of their minuteness, extensive 

 areas may be quickly covered, in circumstances favourable to 

 their growth, by the products of one primordial cell. 



But the most remarkable passage in the life-history of the 

 Protococci is their alternation between a ( still ' and a f motile ' 

 condition. A ( still' cell, consisting of a colourless matter, 

 through which green or red-coloured granules are more or less 

 uniformly diffused, forms, by repeated self-divisions, 2, 4, 8, 16, 

 32 new cells or segments, which are of a very different nature 

 from their inert parent, as they are provided with one or two 

 cilia whose rhythmical contractions propel them rapidly through 

 the water. For this reason they were formerly supposed to be 

 animalcules, and made to figure in treatises on natural history as 

 Monades, Astasiae, Uvellse, and under a variety of other names, 

 each change of form resulting from the development of their 

 growth being supposed to be a different genus of animal. 



By the loss of their cilia, and the thickening of their envelope, 

 the f motile ' cells pass into the ( still ' form, and in this con- 

 dition they may be completely dried up, and remain in a state 

 of dormant vitality for many years. It is in this condition 

 that they are wafted about in atmospheric currents ; and being 

 brought down by the rain into pools and cisterns, they may 

 rapidly multiply and maintain themselves until the water is 

 dried up, or any other unfavourable circumstance occurs which 

 either kills them throughout or forces them to pass from the 

 active into the dormant condition. 



The cysts of the animalcules, precipitated conjointly with them 

 by the rain, find, through their means, an abundant nourishment; 

 and thus a little world of animals and plants appears, as if by 

 magic, in the new formed waters. When the ponds dry up, then 

 the encysted and apparently lifeless animalcules, and the c still' 

 cells of the Protococci, rise on the wind into the atmospheric 

 ocean, all ready for a new precipitation, and the peopling of 

 some future pool. 



The Oscillatorice, another tribe of microscopical plants, con- 

 sisting of continuous tubular filaments, formed by the elonga- 

 tion of their primordial cells, usually lying together in bundles 

 or in strata, and sometimes invested by gelatinous sheaths, are 



