CHAPTER I. 



SINGLE-CELLED PLANTS AND COLONIES. 



In the lakes and pools, in ditches and slow streams, on 

 the surface of damp rocks and wood, may be found many 

 sorts of microscopic plants, whose entire body is merely a 

 single cell. 



Blue-green alg^. 



11. Fission-algse. — The simplest forms of these, the fission- 

 algce, have the protoplasm only slightly differentiated. The 

 central part becomes the nucleus, while the whole of the 

 remaining protoplasm is colored by the chlorophyll and a 

 blue coloring matter caWed p/iycocvanin, so that in mass these 

 algpe look bluish-green or even blackish. For this reason 

 they are called blue-green alga; to distinguish them from those 

 in which only the yellow-green of chlorophyll is present. 



12. Gelatinous colonies. — The cell-wall may be a thin 

 sheet of cellulose, but commonly it is 

 composed of se\"eral layers, of which 

 the outer are changeti into mucilage. 

 This swells into a transparent jelly 

 when wet, either becoming homo- 

 geneous or showing distinct stratifica- 

 tion. When a number of such forms 

 grow in comi)any (fig. 12), this 



F,G. T.-A bk,e-.rec.n .i,a i^^^Y'^'^e material blends into a single 

 twS'is"''"'- .n^d'toio;;"'; "^^^^^ in which the protoi.lasm of the 

 S^fied'3o:"!a"v~l?"r associated plants seems to be em- 

 Sachs. bedded. 



13. Gelatinous filament-colonies. — In other cases, instead 

 of being associated only by the adhesion of the mucilaginous 

 portion of the cell-wall, the cells, still ]ira(tically inde- 

 pendent the one of the other, remain connected by the 



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