442 VEGETABLE PHYSIOLOGY 



termed the antipodal cells ; and the nucleus in the centre 

 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 in 

 almost all cases by its prothallium taking the form of a tube, 

 which grows down through the tissue of the parts sur- 

 rounding 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 neigh- 

 bourhood of the archegonia or the egg apparatus respectively. 

 In a few species of the Gymnosperms the male gametes are 

 ciliated antherozoids, but usually they are two conspicuous 

 nuclear masses associated with a little cytoplasm. 



In the Gymnosperms fertilisation is brought about by 

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

 species with ciliated gametes these are not transferred 

 through pollen tubes. The pollen grain or microspore 

 penetrates into the microphyll of the ovule and is drawn 



