The Pteridophyta illustrate the cytological cycle with unusual clearness, since the two somatic phases (viz., the haploid prothal lus and the diploid fern plant) are so unlike, and so markedly independent the one of the other during their adult existence. It seems probable that in the course of evolution some simple cells, of which the lowest and largest is the ovum or egg, sunk in the tissue of the parent (fig. 4). When ripe the archegonium also ruptures on access to external water, the distal cells of the wall parting so that an open channel filled with mucilage leads down to the rounded egg. Fertilization (syngamy) consists in the fusion of the spermatozoid with the egg to which it is attracted by soluble substance diffusing from it into the water that bathes the open archegonium. The result of that fusion is the zygote, which is the starting point for the development of a new fern plant. This new individual appears first as a spherical mass of delicate cells, nursed in the cavity of the archegonium; but it soon bursts out with its first leaf and root, and its apical bud ready to grow into a new fern, while the prothallus rots form of alternation present in an algal ancestry has been reg ularized and standardized in the Archegoniatae in accordance with a passage from the relative uniformity of aquatic life to the more varied vicissitudes of life on land. In the Bryophyta the gametophyte was more adaptive, and became the dominant generation ; on the other hand, in the Pteridophytes, as also in all the higher land-plants, the sporophyte became specialized as the dominant land-living organism. But the Archegoniatae them selves were never wholly emancipated from dependence on ex ternal liquid water. They show their amphibious character by their zoidiogamic fertilization ; and this confirms their position as primitive land-plants.
For the most part the Pteridophyta are like the Bryophyta in possessing only one type of spore (homosporous) ; but they pro duce these in enormous numbers. A common shield fern may ripen over so millions of them in a season. This is a primitive mode of propagation characteristic of early vegetation.
On the other hand, some few Pteridophyte-types such as Selaginella and Isoetes, and those curious little fern-derivatives styled collectively the Hydropterideae, possess sexually-differ entiated spores, and are described as heterosporous; numerous smaller spores (rnicrospores) bear each a rudimentary male prothallus, while a few larger spores (megaspores), or only a single one, produce each a massive female prothallus. In this they show a state of specialized advance along lines that have led to the final success of the flowering plants. The Pteridophytes as a whole present the paradox of a great division of the vegetable kingdom that has achieved success by force of numbers, rather than by the more refined methods of physiological adjustment and of propagation even in the higher plants.
These paragraphs, of necessity rather technical, will suffice to introduce the Pteridophyta, Vascular Cryptogams, or fern-allies as they are sometimes called. They are represented by six natural groups or classes of organisms, of which four include forms both living and fossil, but two are known only as extinct and very early fossils. Such facts at once confirm their position as repre senting a primitive vegetation. They may be arranged in rough sequence according to their complexity of form and structure ; but this must not be understood as conveying any definite view as to affinity.