292 EVOLUTION OF PLANTS 
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there is a gradual reduction of the gametophyte. This 
in the lower forms is long lived and much like the 
simpler liverworts in its structure, and bears both arche- 
gonia and antheridia. Other forms develop male and 
female gametophytes from similar spores, and, finally, 
heterospory has arisen in several groups of Pterido- 
phytes. In these, two sorts of spores are produced 
which on germination give rise respectively to exceed- 
ingly reduced male or female plants. Heterospory is 
found in several groups of living ferns, and in one 
genus, Selaginella, among the club-mosses. It is evi- 
dent from a study of fossil Pteridophytes that it was 
also developed in the Equisetinew. In Selaginella the 
germination of the spores begins within the sporangium, 
which sometimes falls away with the contained spores. 
The permanent retention of. the spores within the 
sporangium until the germination is complete, and the 
thickening of the sporangium-wall as a protection to 
the included gametophyte and embryo, the whole finally 
becoming detached from the sporophyte, is the origin 
of the seed of the higher plants, which is therefore only 
a further development of the macrosporangium of the 
heterosporous Pteridophytes. 
In the seed plants, or flowering plants, the reduction 
of the gametophyte reaches its extreme, but there is no 
absolute break between Pteridophytes and Spermato- 
phytes. The retention of the germinating macrospore 
within the sporangium has necessitated a different 
method of fertilization, hence the development of the 
pollen-tube. The lower Spermatophytes, especially the 
Cycads, while developing a pollen-tube from the ger- 
minating microspore, nevertheless produce spermato- 
