CHLOROPHYCEAE 213 



In many, probably in most of the green algae, nuclear and cell di- 

 vision takes place at night. This is definitely known to be the case in 

 Spirogyra, Zygnema, Closterium, Ulothrix, and others. Mitosis is most 

 abundant about midnight, or an hour before midnight, and continues 

 up to three or four o'clock in the morning. The most extensive work 

 on the time of day at which nuclear division occurs is a paper by G. 

 Karsten, "Ueber embryonales Wachstum und seine Tagesperiode," 

 Zeitschrift filr Botanik, 7:1-34, 1915. Although the paper is in Ger- 

 man, the numerous tables can be understood by those who are un- 

 familiar with the language. The paper contains a bibliography of the 

 subject. 



Chromo-acetic acid, with or without osmic acid, is a good killing 

 and fixing agent for the entire group. We prefer the following formula : 

 Chromic acid, 1 g.; glacial acetic acid, 3 c.c; 1 per cent osmic acid, 5 

 c.c. ; water, 90 c.c. If material is to be imbedded, it is better to increase 

 the amount of osmic acid up to 7 c.c, since the staining of thin sections 

 is likely to be more brilliant. 



With any fixing agent, it is worth while to place a few filaments in 

 the mixture and watch the effect under the microscope. If plasmolysis 

 occurs with the chromo-acetic mixture, weaken the chromic or 

 strengthen the acetic until the suitable proportions are determined. In 

 the previous edition, the usual method was to weaken the chromic 

 acid. While this avoided any shrinking of the cell contents, the fixing 

 was not very thorough, and material often suffered during staining or 

 other subsequent processes. An extensive series of experunents, espe- 

 cially with coenocytic forms which are notoriously difficult to prepare, 

 proved that it is better not to let the chromic acid drop below 0.7 g. to 

 100 c.c. of water. Generally 3 c.c. of acetic will be enough to avoid any 

 shrinking. If there is still a tendency to shrink with 4 c.c. of acetic, 

 weaken the chromic down to 0.5 and let the solution act for 48 hours. 

 One function of the osmic acid is to make the killing almost instan- 

 taneous. If you put the little crustacean, Cyclops, into a 1 per cent 

 chromo-acetic solution, it will keep up its characteristic movements 

 for several minutes; but if you add one drop of 1 per cent osmic acid 

 to 25 c.c. of the chromo-acetic solution, the movements stop in a few 

 seconds. Besides, the osmic acid acts as a mordant for some stains, 

 especially haematoxylin. About 24 hours in any of the chromic series 

 and a 24 hours' washing in water will be sufficient for members of this 

 group. Only a few of the most commonly studied will be mentioned. 



With marine forms use sea water in making up the fixing agents and 



