1908] GATES— REDUCTION IN OENOTHERA 7 



break apart at the corners where they meet, and before diakinesis is 

 reached they are completely rounded off and independent, or they 

 frequently remain connected with other mother cells only at the ends. 

 In the meantime the cavity of the loculus grows rapidly, so that the 

 mother cells, in normal development, usually lie loose in the cavity. 

 The events of synapsis and reduction usually go forward simul- 

 taneously throughout a flower, with comparatively little variation in 

 the different parts of the same loculus or in the different anthers of a 

 flower. In one flower, however, wide variation was found in different 

 anthers, but comparative constancy in the loculus. One anther of 

 this flower was in synapsis, another in diakinesis, another in meta- 

 phase of the heterotypic mitosis, and in still another some of the 

 mother cells had completed the second mitosis. No abnormalities 

 in the cytological condition of this flower were observed. 



SYNAPSIS 



After the stage described in fig. 4, the nucleus increases greatly in 

 size, but without an appreciable increase in the size of the cell. The 

 single row of sporogenous cells divides, so that there are usually two 

 rows of pollen mother cells. Occasionally three or more mother 

 cells are found in the cross-section of a loculus. In general there are 

 fewer divisions than in the other forms, and this is at least one of the 

 reasons why the mother cells are on the average larger than, for 

 example, in O. Lamar ckiana. 



The resting nucleus of the pollen mother cell increases in size and 

 begins to show signs of approaching synapsis. Figs. 12, 13, 14 

 show stages in the beginning of this process. A number of these 

 stages were found — although they are uncommon — in the same 

 sections with regular synapsis stages. In some cases they occurred 

 side by side with mother cells in which the synaptic knot had already 

 been formed. A complete series of stages may be found in the same 

 section, from the beginning of contraction to the formation of a close 

 synaptic ball. The cytoplasm in these cells shows no contraction 

 whatever, but is perfectly fixed. For this and other reasons there 

 can be no doubt that this is a real contraction stage, leading to synap- 

 sis, and not a result of imperfect fixation, as one might judge on first 

 examination. 



