474 



LIFE-HISTORIES 



chain somewhat larger cells are formed, each with a distinct 

 wall. Certain of these cells, called heterocysts ' have most of 

 their protoplasm replaced by a pale or colorless cell-sap and 



Fig. 306. — Fallen Stars {A'ostuc spp., Fallen Star Fanul\', Xostucacecp). 

 A-F, Nostoc paludosiim. A, chain-colony with resting spores near 

 the middle and heterocysts at the ends, 'p. B, resting spores. C- 

 F, germination of resting spore, and stages in the formation of a new 

 chain-colony. G-J, N^ostoc sphwricuin. G, gelatinous balls as they 

 appear in moist earth, 3 natural size. H, part of a slice cut through 

 the ball, ^a. / early stages in a growing colony, -';". (.Janczcwski, 

 Thurot.) — Protoplasts bluish green, enveloped in a gelatinous mass 

 which they produce. Found mostly in water or on moist earth in 

 various parts of the world. 



become incapable of further development. At points where 



the heterocysts thus limit growth the chain eventually 



^ Het'er-o-cyst < Gr. hcieros, different; kyslis, a bag. 



