This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol1", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
Plants containing woody and vascular tissues in the stem and producing spores asexually, which, on germination, develop small mostly flat green structures called prothallia (gametophyte). On these are borne the sexual reproductive organs, the female known as archegones, the male as antherids. From the fertilization of the oosphere in the archegone by spermatozoids produced in the antherids, the asexual phase (sporophyte) of the plants is developed; this phase is represented by an ordinary fern, lycopod or horsetail.
This subkingdom comprises about 6,000 living species, of which more than three-fourths are confined to tropical regions. The number of extinct species known probably exceeds those living. They appeared on the earth in the early part of the Palaeozoic Era, reached great abundance in Carboniferous Time, but have since been mainly replaced by plants of higher organization, so that at present they form only a small proportion of the total flora. The time of year noted under the species indicates the season at which the spores are mature.