42 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



heterozygous cytoplasm which makes it more susceptible to pressures 

 and other similar treatments. 



Mention may also be made here of fusions of entire cells of the 

 sporogenous tissue. Matsura (1935) reported that in Phacellan- 

 thus the separating walls between adjacent microspore mother cells 

 sometimes dissolve and fuse in pairs to form giant cells, which may 

 either give rise to polyploid gametes or degenerate without com- 

 pleting the meiotic divisions. In two haploid plants of Phleum 

 pratense, Levan (1941) observed the fusion of as many as 30 micro- 

 spore mother cells, giving rise to large plasmodia or "syncytes." 

 A similar behavior has also been reported by Stern (1946) in sugar 

 suspensions of the microspore mother cells of Trillium erectum. 

 Here the extent of the fusions appeared to be unlimited, although the 

 maximum number of nuclei actually observed in a cell was 32. 



Cytokinesis. The divisions of the microspore mother cells may 

 be of the successive or the simultaneous type. 8 In the former a 

 cell plate is laid down immediately after the first meiotic division 

 and another in each of the two daughter cells after the second meio- 

 tic division. In the simultaneous type, on the other hand, no wall 

 is laid down after the first division and the mother cell becomes 

 separated all at once into four parts after both the meiotic divisions 

 are over. 



The investigations of C. H. Farr (1916) and others have shown 

 that there is also another difference in the mechanism of cytokine- 

 sis. In the successive type the cell plate is laid down in the center 

 and then extends centrifugally on both sides, dividing the cell into 

 two equal halves. In the simultaneous type, on the other hand, the 

 division usually occurs by centripetally advancing constriction fur- 

 rows, which meet in the center and divide the mother cell into four 

 parts. 



Farr (1916) studied Nicotiana tdbacum in special detail. At first 

 there is an enlargement of the nucleus of the microspore mother 

 cell, accompanied by a thickening of the mother cell wall. No cell 

 plate is laid down after Meiosis I, and the spindle fibers of this divi- 

 sion disappear during the metaphases of Meiosis II. After the four 

 daughter nuclei have become organized, they assume a tetrahedral 

 arrangement and a spindle is re-formed between every two nuclei, 



8 For an account of the nuclear changes in meiosis, see Sharp (1943) and other 

 vvorks on cytology 



