449, 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. Hach 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 synergide. 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 
