KEPEODUCTION 433 



which is the result of the fusion of the polar nuclei, and is 

 called the definitive nucleus of the embryo-sac. Each nucleus 

 is surrounded by protoplasm, the egg apparatus in parti- 

 cular showing three well-defined naked or primordial cells. 

 The antipodal cells become clothed with cell-walls. There 

 is a certain amount of protoplasm existing in the spore, 

 lying around the wall and forming bridles across it, con- 

 necting the peripheral substance with that in the centre in 

 which the definitive nucleus is resting. 



There are no apparent archegonia : the oosphere is one 

 of the three cells of the egg apparatus, the other two being 

 known as the synergidce. The oosphere is a product of 

 the last division of the original upper nucleus, the other 

 half being the polar nucleus which takes part in the fusion 

 described. 



As in the Spermophytes the spore always remains enclosed 

 in the ovule or sporangium, and its prothallium with the 

 female organs is enclosed in it, the method of fertilisation 

 of the oosphere by a free-swimming antherozoid is impractic- 

 able. The problem of bringing the sexual cells together is 

 met by causing the germination of the microspore to take 

 place on some part of the tissue near the megaspore and by 

 its prothallium taking the form of a tube which grows down 

 through the tissue of the parts surrounding the megaspore to 

 the megaspore itself. This tubular prothallium, known as the 

 pollen tube, bears usually two male gametes, which are thus 

 brought into the neighbourhood of the archegonia or the 

 egg apparatus respectively. In a few species of the Gymno- 

 sperms the male gametes are ciliated antherozoids, but 

 usually they are two conspicuous nuclear masses asso- 

 ciated with a little cytoplasm. 



In the Gymnosperms fertilisation is brought about by 

 the entry of a male gamete into an archegonium. In the 

 Angiosperms one of the generative nuclei fuses with the 

 oosphere. In many families the other one has been seen to 

 fuse with the definitive nucleus. 



After the fertilisation of the oosphere in both cases an 



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