UNIVERSITY 



OF 



Vol. 2] Gardner. — Cytological Studies in Cyanophyceae. 265 



Only a slight modification of the first type is necessary to pro- 

 duce the third or Primitive Mitosis type, which has been found 

 in a single species only (Figs. 40-44). In this type the chroma- 

 tin unites into a single contorted thread quite irregular and in- 

 definite in shape. This thread breaks into a definite number of 

 pieces preparatory to cell division. 



The foregoing description of types of nuclei applies to the 

 resting condition. 



It seems appropriate here to say a few words about resting 

 nuclei, since Kohl and Olive do not consider that the nucleus ever 

 assumes that condition. I am not sure that I understand their 

 point of view on this question, but if their meaning is a condition 

 of the nucleus in which the chromatin is distributed through the 

 nucleus in the form of fine granules when not dividing as is gen- 

 erally the case in higher organisms, then I can agree that the 

 nucleus of the Cyanophyceae never reaches a resting condition. 

 But if a resting condition in the ordinary sense is meant, in which 

 there is no visible sign of any nuclear division or cell-wall for- 

 mation, then the nucleus of the Cyanophyceae is as often in a 

 resting condition as the nucleus of any other organism. Olive 

 speaks of centers of activity of cell-division occurring with rhyth- 

 mic regularity in the trichomes of filamentous forms. This would 

 indicate that the cells between these centers of activity must be 

 in a resting condition. Under ordinary circumstances almost 

 any species shows from 30 per cent, to 40 per cent, of the cells in 

 which there is absolutely no sign of cell-division or change in the 

 shape of the nucleus to indicate preparation for division, and I 

 have had cultures dry up slowly in which at least 90 per cent, of 

 the cells were in a quiescent state. When cells are dividing rap- 

 idly the chromatin will be found in many cells to extend from 

 cross-wall to cross-wall, while in the same collection, and even in 

 the same plant, one may find the chromatin in other cells concen- 

 trated near the middle of the cell with a layer of protoplasm be- 

 tween it and the cross-wall. Many of the heterocysted group 

 produce spores which rest for a time, often for months. The 

 homocysted group produces hormogonia which act in the same 

 manner. These are simply small groups of cells, from two to 

 several, which survive when a filament breaks up from any cause. 



