MICROSCOPIC FORMS OF VEGETABLE LIFE. 



and at the same time propelled forward; and as the oscillation continues 

 after the filament has left its nest, the little swimmer gradually moves 

 along, till it not only reaches the edge of the vessel, but often as if in 

 the attempt to escape confinement continues its voyage up the sides, 

 till it is stopped by dryness. Thus in a very short time a small piece of 

 Oscillatoria will spread itself over a large vessel of water." This rhyth- 

 mical movement, impelling the filaments in an undeviating onward 

 course, is greatly influenced by temperature and light, being much mere 

 active in warmth and sunshine than in cold and shade; and it is checked 

 by any strong chemical agents. The true Generation of Oscillator iacem 

 is as yet completely unknown. 



247. Nearly allied to the preceding is the little tribe of Nostocliacece; 

 which consists of distinctly-beaded filaments, lying in firmly-gelatinous 

 fronds of definite outline (Fig. 147). The filaments are usually simple,. 



Structure of Oscillatoria contexta, 

 A, portion of a filament, showing 

 the striations on the cellulose-coat, a, 

 a, where the endochrorne is wanting; 

 B, portion of filament treated with 

 weak syrup, showing a disposition to 

 a regular breaking- up of the endo- 

 chroine into masses; c, portion of fila- 

 ments treated with strong solution of 

 chloride of calcium, showing a more 

 advanced stage of the same separa- 

 tion. 



Portion of gelatinous frond of 

 Nostoc. 



though sometimes branched; and are almost always curved or twisted r 

 often taking a spiral direction. The masses of jelly in which they are- 

 imbedded are sometimes globular or nearly so, and sometimes extend in 

 more or less regular branches; they frequently attain a very considerable 

 size; and as they occasionally present themselves quite suddenly (espe- 

 cially in the latter part of autumn, on damp gar den- walks), they have 

 received the name of 'fallen stars.' They are not always so suddenly 

 produced, however, as they appear to be; for they shrink up into mere 

 films in dry weather, and expand again with the first shower. 1 Nostocs 

 multiply, like the Oscillatoriaceae, by the subdivision of their filaments, 

 portions of which escape from the gelatinous mass wherein they were im- 

 bedded, and move slowly through the water in the direction of their 

 length: after a time they cease to move, and a new gelatinous envelope 

 is formed around each piece, which then begins not only to increase in 

 length by the transverse subdivision of its segments, but also to double 



1 See Hicks in " Quart. Journ. of Microsc. Science.," N.S., Vol. i. (1861), p. 90. 



