184 PRACTICAL BOTANY 



168. Nostoc : reproduction. If a single Nostoc cell is sepa- 

 rated within the mass from the chain in which it has grown, 

 it soon divides in essentially the same way as does the Crloeo- 

 capsa plant; but in the case of Nostoc, the new cells thus 

 formed are likely to remain together, and will redivide in the 

 same direction as did the cell at first, and thus form a new 

 chain. Nostoc is not, however, usually reproduced by one cell's 

 becoming free and behaving as just described. Divisions of 

 the cells do occur as stated, sometimes all or nearly all of the 

 cells of a chain dividing at the same time. This, if contin- 

 ued, would soon produce a plant of great length, a result 

 which does sometimes occur. Usually, however, in one or 

 more cells of the chain the protoplasm dies and the cell wall 

 greatly enlarges, thus producing a heterocyst. This hetero- 

 cyst apparently weakens the connection between the adjoin- 

 ing cells, and the chain separates at this point, the heterocyst 

 usually adhering to one of the new chains (Fig. 153, 7?). The 

 presence of two or more heterocysts may result in breaking 

 the chain of cells into three or more new Nostoc chains at the 

 same time. This kind of reproduction resembles that which 

 was seen in Grloeocapsa, in as much as the cells divide to repro- 

 duce the plant. It differs in the fact that new plant chains are 

 formed by an additional reproductive structure, the heterocyst. 



169. Oscillatoria : structure and nutrition. In the same kind 

 of regions in which Crloeocapsa and Nostoc are found, but more 

 abundant and more widely distributed, is the plant Oscillatoria. 

 Oscillatoria filaments grow together, sometimes forming mats 

 of a dark green, dirty-looking growth. At times when a few 

 plants are seen* growing free from others, they present a beau- 

 tiful clear green growth in which very little bluish coloring 

 matter can be detected. The mats, though often slimy, are 

 not covered by the masses of gelatinous substance that are 

 seen in the two preceding forms. 



Some of the Oscillatoria plants are so large that if placed in 

 a dish of water they may be studied in a general way without 

 magnification. They are thread-like plants, the length of each 



