Beer, Development of tlie pollen grain aud antlier of some Onagraceae. 303 



fragmentation. When the special-mother-cells have been establi- 

 shed mitotic divisions are rarely met witli whilst fragmenting 

 niiclei occur on every side. Most of the tapetal niiclei now 

 contain a single large nucleolus and a very deeply staining 

 nuclear wall, besides tliis only a very little finely granulär 

 cliromatic material can be seen lying near or upon the nuclear 

 wall (see n Fig. ^-t). 



In anthers in which the first pollen-wall is just making its 

 appearance. I have several times seen the tapetal nuclei in the 

 prophases of mitosis butlhav^e never, at this period, succeeded 

 in finding the later stages of division and I believe that mitosis 

 is no longer completed by the nuclei. Nuclei which have every 

 appearance of undergoing fragmentation are, however, very 

 abundant both at this and at later stages of development 

 (Fig. 32, 33, 34, 40, 42) Strasburger^) and later writers, in 

 describing the tapetum of other plants, have found mitosis to 

 be the only mode of nuclear division and they believe the con- 

 stricted nuclei which occur in the cells to represent fusion and 

 not fragmentation of the nuclei. 



In Oeiiofhera it is impossible to imagine that karyokinesis 

 can be the only mode of nuclear multiplication. 



In the first place mitotic divisions are never very frequent 

 and it is difficult to account for the presence of six or seven 

 nuclei in a young tapetal cell through tlieir agency alone. 



Moreover, mitotic figures cannot be found in the tapetum 

 of Oenofhera after the appearance of the first pollen wall so that 

 if this is the only mode of division and the constricted nuclei, 

 which are common both at this and at subsequent stages, really 

 represent fusions it is impossible to see whence the constant 

 supply of nuclei comes for these repeated fusions and which 

 leaves the older tapetal cell with two or three nuclei to the last. 

 The way in which these constricted nuclei often hang together 

 by a narrow neck also favours the view that they are separating 

 from one another and are not uniting (see especially Fig. 34). 



The great disparity which after exists in the sizes of the 

 nuclei of a cell is also what one would expect with direct rather 

 than with indirect division (compare sizes of the two nuclei in 

 Fig. 37). 



For all of these reasons I consider that most of these con- 

 stricted nuclei represent a fragmentation and not a fusion of nuclei. 



Every constricted nucleus does not, however, necessarily 

 imply nuclear multiplication. 



There is no doubt that the tapetal nuclei alter their shape 

 and often become very irregulär in outline without this leading 

 to a division of the nucleus or representing a fusion (see Fig. 36, 37). 

 These changes in shape are evidently signs of the occurrence of 

 an active mctabolism in the cell and may be compared to the 

 similar phenomena which have been described in the secreting 

 cells of may animals. 



1) Strasburger, E. „Teilungsvorgang d. Zellkerne etc." (Arch. f. Mikro. 

 Anat. Bd. 21. 1882. pp. 574—575.) 



