Fig. 10. — Silphium integri folium. Longitudinal sections of microsporangia ; x 520. 

 A, single row of arehesporial cells ; in two cells division into primary sporogenoua 

 and primary parietal cells has already taken place, i?, sporogenoua and tapetal cells 

 sharply differentiated. C, later stage showing spore mother-cells in synapsis. I), 

 a tetrad ("only three microspores shown) formed within the spore mother-cell. — After 

 Mekrell. 2 ^ 



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