640 Davis.—Cy to logical Studies on Oenothera. II. 
are chromatin centres or prochromosomes, but their form is exceedingly 
variable, and one cannot make constant counts. 
The study of the nuclei in older pollen grains and of the mitosis which 
differentiates the generative from the tube nucleus presents certain technical 
difficulties that make the investigation of this phase of the gametophyte 
somewhat difficult, and the writer has not yet taken it up in detail. 
The history of the homotypic mitosis in Oenothera biennis is identical 
in all essentials with that of gran diflora, Lamarckiana , and rubrinervis. 
The mitosis is clearly an equation division distributing the halves of chromo¬ 
somes whose premature division takes place during the anaphase of the 
heterotypic mitosis, the halves remaining associated as seven pairs throughout 
the period of interkinesis. 
The Reduction Divisions in the Ovule. 
In some respects the ovule furnishes better material than the anther for 
studies on the reduction divisions, since when the outer walls of the ovaries 
are carefully cut away it is frequently possible to obtain a better and more 
uniform fixation than in the case of the anther. It is, however, at times far 
more difficult to seriate stages. The study of Geerts (' 09 ) on the ovule of 
Oe7iothera Lamarckiana gives an opportunity for some comparison of that 
form with biennis. 
The reduction divisions in the ovule of Oenothera occur just previous to 
the differentiation of the embryo sac which arises from one of a group 
of four cells homologous with a pollen tetrad. The embryo sac therefore 
develops from the homologue of a megaspore and not from the homologue 
of a megaspore mother-cell as in many types (e. g. Liliiim , &c.). The four 
cells of the group (tetrad) are arranged in a line and arise, through a hetero¬ 
typic mitosis followed by two homotypic divisions, from a single large 
megaspore mother-cell which is differentiated in the nucellus of the very 
young ovule at the time of the development of the integuments. Geerts (’ 09 ) 
reports for Lamarckiana that the embryo sac always develops from the 
upper cell of the row of four (tetrad), the other three cells breaking down. 
This is not true for the present form of biennis , in which the embryo sac 
appears to develop rather more frequently from the lowest cell of the row 
(i. e. farthest away from the micropylar end), although often arising from the 
upper cell. 
Geerts (’ 09 , PL XXII) has pointed out in tabular form that the 
reduction divisions in the ovule of Lamarckiana take place at a stage 
of the flower’s development long after the homologous mitoses in the pollen 
mother-cell, and the same history is true of biennis. Indeed, the pollen 
tetrads are fully formed before the rudiments of the ovules have begun 
to develop their integuments. 
