218 



ESSENTIALS OF BOTANY 



the usual one in higher plants. In the latter every cell 

 commonly possesses a distinct nucleus, and this generally 

 goes through complicated changes of form during the pro- 

 cess of nuclear division (see Bergen and 

 Davis' Principles of Botany, Sect. 199). 

 277. Summary of the Blue-Green 

 Algae. — Many species of the blue-green 

 algae occur in fresh water, especially in 

 warm waters which are impure from 

 decayiag organic matter, and in hot 

 springs, some are found in sea water, a 

 good many flourish on damp earth, and 

 a few inhabit cavities in other plants. 



The blue-green algae are plants of 

 very low organization. Some of them 

 consist of single cells, living in a soli- 

 tary way, others have the individual 

 cells held together in masses by a 

 gelatinous substance, and still others, 

 like Oscillatoria, consist of thread-like 

 rows of cells. 



Blue-green algae differ from other algse in the nature of 

 their coloring matter,^ in the absence of well-defined nuclei 

 in the cells, and in the simple mode of reproduction, by 

 cell-division. From the readiness with which their cells 

 split into new ones they are often called fissioyi algae. 



Fig. 152. Oscillatoria. 



A lew cells are showTi 

 treated with reagents 

 to show the protoplas- 

 mic cell-conteuts more 

 clearly. At b the cell- 

 wall is sending a par- 

 tition inward, and the 

 protoplasmic contents 

 beginning to divide. 

 At c the division (fis- 

 sion) has proceeded 

 still further. (Much 

 magnified.) 



1 Tlieir color is never exactly that of chlorophyll, hut ranges from some- 

 thing not much unlike chlorophyll to hlue-green, orange-yellow, brown, red, 

 or violet. 



