20 PLANT STKUCTIJKES 



it will bulge outward. In all active living cells there is 

 this pressure upon the wall from within. 



Each cell of the Oscillatoria filament has the power of 

 dividing, thus formiug new cells and elongating the fila- 

 ment. A filament may break up into fragments of varying 

 lengths, and each fragment by cell division organizes a new 

 filament. Here again reproduction is by means of vegeta- 

 tive multiplication. 



J 21. Conclusions.— Taking Glmocapsa, Nostoc, and Oscil- 

 Uatoria as representatives of the group Cyanophyceae, or 

 " green slimes," we may come to some conclusions concern- 

 ing the group in general. The plant body is very simple, 

 consisting of single cells, or chains and filaments of cells. 

 Although in Nostoc and Oscillatoria the cells are organized 

 into chains and filaments, each cell seems to be able to live 

 and act independently, and the chain and filament seem to 

 be little more than colonies of individual cells. In this 

 sense, all of these plants may be regarded as one-celled. 



Differentiation is exhibited in the appearance of hetero- 

 cysts in Nostoc, peculiar cells which seem to be connected 

 in some way with the breaking up of filamentous colonies, 

 although the Oscillatoria filament breaks up without them. 



The power of motion is also well exhibited by the group, 

 the free filaments of Oscillatoria moving almost continu- 

 ally, and the imbedded chains of Nostoc at times moving to 

 escape from the restraining mucilage. 



The whole group also shows a strong tendency in the 

 cell-wall material to become converted into mucilage and 

 much swollen, a tendency which reaches an extreme expres- 

 sion in such forms as Nostoc and Glmocapsa. 



Another distinguishing mark is that reproduction is 

 exclusively by means of vegetative multiplication, through 

 ordinary cell division or fission, which takes place very 

 freely. Individual cells are organized with heavy resistant 

 walls to enable them to endure the winter or other unfavor- 

 able conditions, and to start a new series of individuals 



