THE SPERM A TOP 11 Y TA 



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Fig. 217. — Development of the female gametophyte and .seed in a gyninospcrm 

 (Pine), Longitudinal sections through the base of the cone-scale. A, young 

 ovule, consisting of an integument (1) and a megasporangium or nucellus (2), 

 within which is a row of four megaspores which have developed from a megaspore 

 mother-cell. B, three of the megaspores have aborted, the fourth is enlarging. 

 C, the megaspore has germinated into a young gametophyte or em))ryo-sac (3), 

 which now consists of a layer of free nuclei surrounding a large vacuole. D, the 

 young sac has developed into the mature female gametopliyte (3), the bulk of 

 which consists of endosperm. At one end are two archegonia (4) within each of 

 which is an egg-cell. Pollen grains have entered the micropyle and alighted on 

 the tip of the nucellus. E, fertilization. Two pollen grains have germinated, 

 sent their tubes down through the nucellus, and discharged their contents into the 

 two archegonia, in each of which one of the male nuclei is fusing with the egg 

 nucleus (5). F, from the fertilized egg have grown two young'embryos (6), one 

 larger than the other, which have been pushed down into the middle of the endo- 

 sperm. G, mature seed. The integument of the ovule has developed into the 

 seed coat, the micropyle has closed, the endosperm has become greatly enlarged, 

 the nucellus has almost disappeared and the embryo has grown to its full size. 

 The smaller embryo in F failed to develop. This seed will now detach it.self 

 from the cone-scale and under favorable conditions will produce a new plant. 



