356 PRACTICAL COURSE IN BOTANY 
In the gymnosperms, — pines, yews, cycads, etc., —which 
represent the most ancient and primitive type of existing 
seed-bearing plants, 
the similarity of these 
processes to those of 
certain of the pterido- 
phytes is very striking, 
and it was through 
the study of these that 
the sequences of the 
process were traced in 
the much more obscure 
form in which they 
occur among the angi- 
osperms. From the 
endosperm in the seeds 
of gymnosperms arche- 
gonia were found to be 
developed (Fig. 510) in 
much the same way as 
Fie. 510. — Diagrammatic section through the in Selaginella, from the 
ovule of a gymnosperm belonging to the spruce ‘ 
family: 7, integument cuvering the ovule; v, endo- Pro thallium ; thu Ss 
sperm (corresponding to female gametophyte), : = 
which fills the embryo sac, containing two arche- 8 howin 8 the en d 0 
gonia, a; 0, egg cell; p, pollen grains; ¢, pollen sperm to be a modified 
tubes entering the neck, c, of the archegonia. 
and greatly reduced 
gametophyte. In some cases, it has even been found to 
protrude a little way out of the embryo sac and to take on 
a slightly greenish tinge — another rem‘niscence of its origin. 
Fertilization, too, takes place in precisely the same manner 
as in the pteridophytes, except that in all but the ginkgo 
and the cycads, the fertilizing cells in the pollen grains are 
non-motile, and find their way to the ovule by growing down 
into the embryo sac with the pollen tube, instead of swimming 
to it — an adaptation probably brought‘ about in response 
to changed conditions during the course of evolution from 
aquatic to terrestrial life. 
