CHAPTEE VI 



THE MALE GAMETOPHYTE 



THE reduced number of chromosomes appears at the first 

 mitosis in the pollen mother-cell, which is therefore the first 

 gametophytic cell (Fig. 53). In every case, so far as known, 

 two divisions occur in rapid succession, giving rise to four 

 microspores. Strasburger 8 has called attention to the two 

 modes of division. In one case, most frequent among Mono- 

 cotyledons, a wall follows the first nuclear division, dividing 

 the mother-cell into two hemispherical cells ; the second nuclear 

 division is also followed immediately by the formation of a 

 wall, making two equal cells from each of the hemispheres (Fig. 

 54). In the other case, more characteristic of the Dicotyledons, 

 the two nuclear divisions occur before any walls are formed, 

 all the walls being then formed simultaneously and in such a 

 way that each of the four cells has the form of a triangular 

 pyramid with a spherical base that is, each cell is the quadrant 

 of a sphere (Figs. 55, 56). The former method has been called 

 successive, the latter simultaneous division. The two modes 

 are not sharply characteristic of the two great groups of Angio- 

 sperms, but the successive method is dominant among Mono- 

 cotyledons and the simultaneous among Dicotyledons. In any 

 event the result is a tetrad^ a group of four cells each of which 

 is a microspore. In successive division there is a bilateral ar- 

 rangement of the microspores, and in simultaneous division the 

 arrangement is tetrahedral ; but both arrangements sometimes 

 occur in the same sporangium. 



The arrangement of the tetrad is not always restricted to 

 these two methods (Fig. 57). Wille 15 has described varying 

 arrangements of microspores in the tetrads of species of Juncus 

 and Orchis mascula] and in TypJia Schaffner 34 not only found 



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