Science may be described as the art of systematic over-simplification.

Karl Popper (b. 1902), Anglo-Austrian philosopher. Quoted in: Observer (London, 1 Aug. 1982).

1. THE SEEDLESS VASCULAR PLANTS

The seedless vascular plants are often called the ferns and fern allies; these are all vascular plants, meaning that they possess true vascular tissue. They are intermediate in morphological complexity between the bryophytes and the seed plants. All are characterized by alternation of heteromorphic generations, in which the sporophyte generation is the dominant phase and the gametophyte generation is much reduced. This is just part of an evolutionary trend toward a more and more reduced gametophyte generation.

Three extinct Divisions known from the fossil record. Primitive vascular plants known only from fossil record. Four extant divisions

Psilophyta - 2 genera Psilotum TX, FL, LA, HI, Puerto Rico, Australia. Unique among the vascular plants in that it lacks roots and leaves. Tmesipterus - epiphyte on other plants.

Lycophyta -- 10 -15 genera 1000 species. Include the Lycopods - club mosses (18742). Selaginella - resurrection plant - heterosporous - 700 species. Isoetes -(quillwort) also heterosporous - lacks stomata CO2 from sediment. Found high elevations.

Sphenophyta - horsetails Equisitum - one genus- 15 species- may be the oldest genus still living for seedless plants.(20615, 17762, 30225, 18353, 18120, 20614)

Pterophyta - ferns. 11,000 species. largest group of plants outside of Angiosperms. (17307, 17308, 17309, 17310, 17469, 17759, 17861, 17470, 18144, 17760, 17758, 17471).

The first three divisions Psilotophyta (the whisk ferns), Lycophyta (the club mosses), and Sphenophyta (the horsetails) are collectively called the "fern allies"; there are few extant species, although all three groups are well-represented in the fossil record. The ferns (Pterophyta) differ from the fern allies in that they possess relatively large, complex leaves (megaphylls), as compared with the reduced leaves (microphylls) of the fern allies.

2. Pterophyta: the ferns

Structurally, the ferns can be subdivided into two groups based on the development and structure of the sporangium: eusporangiate or leptosporangiate. The difference is illustrated in Figure 16-28, Page 343. Largest order -Filicales. There are some special terms you should know for ferns: fronds, pinnae, rachis, fiddleheads, sori, indusia, prothallus. Review the fern life cycle - Page 381 (Figure 23-4). Note requirement for water for sperm.

3 Each of the four divisions of seedless vascular plants solved some of the problems of life on land, but none solved them all. For a while they did dominate the earth but were replaced by the Spermatophyta-- Why? The standard reason is of course that the Spermatophyta do things better, but its not quite that simple. Obviously, there still are primitive plants. What would be another reason for the shift in dominance?

A major climate change: it is becoming clear that all major evolutionary events have occurred simultaneously as a consequence of major changes, such as asteroid impacts. For this particular change (from seedless to seed plants), in general, it became cooler and drier. Not only did the seed plants have competitive advantage of height and growth, and better leaves but also better ability to withstand drought and cold. And possibly, better hormonal control of phenology and development.