176 
METHODS IN PLANT HISTOLOGY 
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 fiir Botanik , 7:1-34, 1915. Although the 
paper is in German, the numerous tables can be understood by 
those who are unfamiliar 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 for¬ 
mula : Chromic acid, 1 g.; glacial acetic acid, 3 c.c; 1 per cent osmic 
acid, 1 c.c. . 
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 experiments, especially with coenocytic forms which are 
notoriously difficult to prepare, proved that it is better to keep the 
chromic acid up to 1 per cent and strengthen the acetic acid, if neces¬ 
sary. The function of the osmic acid is to make the killing almost 
instantaneous. This high percentage of acetic acid, excellent for 
algae and fungi and many other forms, is not so good for many tissues 
of higher plants, because the proportion of acetic acid is too great. 
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 in washing, but use fresh water in making up alcohols and for the 
10 per cent glycerin. 
Volvox.— Volvox is found in ponds and ditches, and even in 
shallow puddles. The most favorable place to look for it is in the 
deeper ponds, lagoons, and ditches which receive an abundance of 
rain water. It has been claimed that where you find Lemna, you 
are likely to find Volvox; and it is true that such water is favorable, 
