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A TEXTBOOK OF THEORETICAL BOTANY 



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Outer integument 

 Inner integument 



Fig. 778. Wehvitschia mirabilis. 

 Longitudinal section of an 

 ovule showing the nucellus 

 with embryo sac. 



The Male Gametophyte. 



The pollen grains of Wehvitschia are produced in a synangium formed of 

 three fused sporangia at the top of each stamen. The wall of the adult 

 sporangium is single layered, as in many Pteridophyta, and opens by a vertical 

 slit. 



The maturing microspore shows three nuclei. One of these is surrounded 

 by dense cytoplasm and is apparently the generative cell, one of the other 

 two aborts, so that the mature grain has two nuclei only, as in the Angiosperm 

 pollen grain before germination. The abortive nucleus may be a remnant 

 of a prothallial cell. The persistent nucleus becomes the tube nucleus at 

 fertilization. Pollination is probably effected by insects. 



The Female Gametophyte. 



The female gametophyte is unique. Only one megaspore mother cell is 

 formed, low down in the nucellus. This seems to form four megaspores, 

 the lowest of which becomes the embryo sac. In it, as it enlarges, nuclear 



