2 74 Phillips on a Comparative Study^of the 



chromatin, reserve products, assimilation products, etc. 

 The peripheral zone is looked upon as a chromatophore by 

 some, while others consider it as the general cytoplasm, 

 throughout which the coloring matter is diffused. The wall 

 is composed of a substance unnamed as yet, but supposed to 

 be a modification of cellulose, probably quite close to fungus- 

 cellulose. It was in the hope of adding something to our 

 knowledge of these organisms, which might aid in the solu- 

 tion of these cytological problems that the present work was 

 undertaken. 



Material Used. 



The choice of organisms for this investigation was gov- 

 erned by several considerations : ( i ) The availability of 

 material. The greenhouses and Botanical Garden of the 

 University of Pennsylvania have furnished excellent places 

 for the collection and culture of these organisms, though 

 the collections were by no means confined to these localities. 

 (2) Plants were selected primarily with a view to the large 

 size of their cells. Those forms in which the cell contents 

 were especially masked for any reason, or where the cells 

 were very small, were discarded, and the larger and clearer 

 organisms used in preference. (3) Several different organ- 

 isms were employed in order to avoid the mistaking of arti- 

 facts for natural conditions, and to eliminate the "individual 

 factor" as much as possible. 



Though the work here reported deals mainly with the 

 species of Oscillaria, Nostoc and Cylindrospermum investi- 

 gated, the results with the other organisms were essentially 

 the same, and to speak of each organism in detail would be 

 but reiteration. Therefore the morphological structures 

 have been made the basis of report, they being so constant, 

 varying only with the forms of the cells of the different 

 plants, that they may be considered as belonging to all of 

 the forms except where otherwise noted. The plants studied 

 were as follows : Nostoc (three species), Nostoc in Collcnia, 



