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SPERMATOPHYTA 
425 
Male gametophyte and fertilization. The microspore (Fig.482), 
before being shed, germinates, and a male gametophyte, or pro- 
thallus, is formed within the spore wall (Fig. 482). This consists 
of four cells: a tube cell, a generative cell, and two evanescent 
prothallial cells which begin to disorganize as soon as they are 
formed, so that only remnants of them appear in the pollen grain 
_at the time of shedding. After 
reaching the micropyle the 
pollen grain sends out a pol- 
len tube (Fig. 483) which 
grows through the nucellus 
and enters the archegonium. 
During the growth of the pol- 
len tube the tube nucleus mi- 
grates into the tube, and the 
generative cell divides to form 
two cells: a stalk cell (toward 
the degenerating prothallal 
cells) and a body cell (Fig. 
483). Later the body cell 
becomes free and passes into 
the tube, where its nucleus 
divides to form two male nu- 
clei. Fertilization results from 
the fusion of one of the male 
nuclei with an egg nucleus. 
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Fig. 480. Apex of female prothal- 
lus of pine, showing two archegonia. 
(x 75) 
The most striking difference between the sexual method of repro- 
duction in the conifers and that in the cycads and Ginkgo is that 
in the conifers there are no ciliated spermatozoids. The presence 
of ciliated spermatozoids is a character, derived from aquatic an- 
cestors; which has persisted throughout the division Pteridophyta 
and among the Gymnospermae in the Cycadales and Ginkgo, but 
has disappeared in the Coniferales and all the higher orders of 
the Spermatophyta. 
Seed. The fertilized egg germinates and produces an embryo 
which remains embedded in the prothallus until the germination 
