CLASSIFICATION OF PLANTS. 



17 



damp ground, but more commonly attached to plants, stones, 

 etc., in water. The masses vary in color from light brown 

 to deep blackish green, and in size from that of a pin head to 

 several centimetres in diameter. 



In the higher forms special cells called heterocysts are 

 found. They are colorless, or light yellowish, regularly dis- 

 posed ; but their function is not known. Besides these, cer- 

 tain cells become thick-walled, and form resting cells (spores) 



FIG. 7. Forms of CyanophycesR. A, Nostoc. B, Glueotrichia, x 1. C, in- 

 dividual of Glwotrichia. D, Chroococcus. E, Nostoc. F, Oscillaria. G, 

 H, Tolypothrix. All x 300. y, heterocyst. sp. spore. 



the propagation of the plant (Fig. 7, G. sp). In species 

 where the sheath of the filament is well marked (Fig. 7, H), 

 groups of cells slip out of the sheath, and develop a new one, 

 thus giving rise to a new plant. 



The bacteria (Schizomycetes), although among the commonest 

 of organisms, owing to their excessive minuteness, are difficult 

 to study, especially for the beginner. They resemble, in their 

 general structure and methods of reproduction, the blue-green 

 slimes, but a^e, with very few exceptions, destitute of chloro- 

 phyll, although often possessing bright pigments, blue, vio- 



*&L red, etc. It is one of these that sometimes forms blood- 

 rm. spots in flour paste or bits* of bread that have been kept 

 very moist and warm. They are universally present where 



decomposition is going on, and are themselves the principal 



