552 Davis . — Cytological Studies on Oenothera . /. 
penetration from the tips is so slow that fixation is generally imperfect. 
A fixing fluid of the following formula was chiefly employed : i per cent, 
chromic acid 25 cc., 10 per cent, glacial acetic acid 20 cc., 1 per cent, osmic 
acid 10 cc., water 45 cc. The material was left in this solution 2-4 hours 
and then transferred for about 20 hours to a similar solution without osmic 
acid. The good effects of the osmic acid were in this way obtained 
without excessive blackening, and this may be readily removed with 
a very short immersion of the sections in a dilute alcoholic (70 per cent.) 
solution of hydrogen peroxide. A stronger chrom-acetic fixing fluid with 
osmic acid omitted also gave good results : 1 per cent, chromic acid 40 cc., 
10 per cent, glacial acetic acid 40 cc., water 20 cc. Iron-alum haematoxylin 
proved to be the most satisfactory stain for the nuclei, which are relatively 
small. 
The Vegetative Mitoses in the Anther. 
It is important, before considering the events of synapsis and the 
two mitoses (heterotypic and homotypic) in the pollen mother-cell, to 
describe the characteristics of the vegetative divisions of the sporophyte. 
These may be studied in the root tips, as Miss Lutz has already done for 
a number of forms of Oenothera. There are, however, obvious advantages, 
because of the close relationships of the nuclei concerned, in selecting 
sporophytic mitoses close to or immediately preceding the reduction 
divisions of pollen formation. 
The archesporium in the anthers of Oenothera is differentiated at 
a very early period as a single row of cells running lengthwise in the middle 
region of each of the four portions that become pollen chambers. As the 
anther grows there are cell-divisions in the archesporium both crosswise and 
lengthwise, and thus the single row of archesporial cells becomes trans- 
formed into a somewhat irregular double row throughout most of its length. 
There are also numerous nuclear divisions in the cells surrounding the 
archesporium (which cells form in the mature anther a well-defined tapetal 
tissue) and in other regions of the anther as well. Consequently the anther, as 
well as other parts of the developing flower, presents excellent subjects for 
the study of vegetative sporophytic mitoses. 
This section of the paper will be concerned chiefly with the final 
mitoses in the archesporium which convert the primary single row of cells 
into the double row that become the pollen mother-cells. These mitoses 
appear in no way different from those in the developing tapetum and 
in other parts of the anther, but are more easily studied because the arche- 
sporial cells are somewhat larger. 
The resting nuclei in the archesporium and throughout meriste-. 
matic regions of the developing flower are similar in structure. Each 
contains, as a rule, a single large nucleolus, usually accompanied by 
