Spore-bearing to seed-bearing
Notes for Bio 4420 (Plant taxonomy) at Utah State University
Key words: Spore bearing plants; Progymnosperms; Seed bearing plants; Seeds; Pollen grains
References: Gymnosperm evolution figure  based on Stewart 1983, p. 313 and 348.
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seedhorz.GIF (11149 bytes)  A SEED!
SPORE BEARING PLANTS
The groups that we have discussed so far (Lycopodiophyta, Equisetophyta, and Polypodiophyta), all of which are spore-bearing plant groups, had three severe restrictions:
  • The gametophytes were independent of the sporophyte
  • The male gametes had to swim through water to reach the female gamete
  • The internal stem structure was not particularly strong or efficient

Why were these restrictions? Gametophytes that are independent, free-living organisms have to become established rapidly. They must, therefore, fall in places where they can germinate and produce gametes. For contemporary members of these three groups this means in mesic areas. Some species also require that the right fungus be present for establishing a mycorrhizal association. If these conditions are not met, the gametophyte dies.  In other words, gametophytes (at least as we known them) tend to be fussy.  

Requiring water for fertilization means that there must be water around, but it also means that the sperm and egg must be at more or less the same elevation or that the sperm be upstream from the egg. An egg at the top of a tall plant is not likely to be fertilized. Incidentally, the amount of water needed is not large - the film remaining after a rainfall will do it, but such a film will also dry rapidly in a dry atmosphere.

The internal stem structure has to be both strong and effective in moving solutions around if a plant is to be able to grow tall and withstand both strong winds and drought. A few extinct members of the Equisetophyta grew to approximately 20m (65 ft) and extinct members of the Lycopodiophyta grew even taller, to at least 40m (130+ ft). These are impressive heights, but Calamites, the tall Equisetophyte, had a hollow stem and, compared with modern trees, very few fibers. Lepidodendron, the tallest Lycopodophyte, had an obconic stem, but the base was surrounded by cortical tissue that became thinner as the stem proper became thicker. As in Calamites, the kinds of cell present suggest that Lepidodendron did not have a particularly strong trunk. It was also determinate (once it started branching at the top, it could not grow taller).

The stem structure of Calamites and Lepidodendron was very adequate for life in the warm, equable climates of the Carboniferous period (280-345 MYBP), during which there were extensive lowland swamps. Moreover, the same swamps presumably provided good conditions for gametophyte development. Everything was just great, so far as these plants were concerned, but the climate changed.

PROGYMNOSPERMS
The next geological period, the Permian (195-280 MYBP) was a time of mountain building and the development of extensive arid and semiarid areas. This favored a small group of plants that we know of first from the mid-Devonian (370 MYBP), the Progymnosperms. These plants had a stem structure very similar to modern gymnosperms. Some fossils even show what look like annual growth rings. Reproduction, however, was fernlike, i.e., spores were produced in sporangia. The fossil record suggests that most species were heterosporous, but some were homosporous.

So far as we can tell, the Progymnosperms were never particularly abundant, nor even particularly tall (12m was the only height that I found mentioned). They were not around for long, only 35 million years (from 370-335 MBP), a flash in the pan, but the evolution of a strong stem structure was critical to the evolution of tall plants that could withstand drought and wind. Any plants that had such a stem had an advantage over weaker stemmed plants in many habitats, although not in mesic lowland forests.

Progymnosperms are also important because some among them moved from heterospory to seed production. This may seem a small step but, judging by the rapid diversification that took place and the current dominance of seed plants, it was one giant step for plants.   Remember that strong stem structure. The potential for greater height that it offered could be more fully exploited by plants that did not require water to enable the male and female gametes to meet. If water were still required, one can envision two kinds of plant that would have made it: those that kept sexual reproduction on or near ground level, and those that settled for self fertilization (so the males had, at most, only to make its way along a branch or two). Or possibly trees that had females on suckers and males on above ground branches. Hmm. How else could one design a plant that has a strong trunk but needs water for fertilization and is adapted to out-crossing?

SEED PLANTS
The earliest fossil seeds are not attached to stems, but they date from around 350 MBP. The earliest attached seeds are associated with plants having fernlike leaves and wood rather like modern cycads. Such plants are generally called Seed ferns, but it is worth remembering that they appear to have had "good wood". Seed ferns lasted from 345-225 MBP, more than twice as long as Progymnosperms. They were also more diverse than progymnosperms.

gymnevol.TIF (97182 bytes)
Of the extant groups of plants, the Cycadophyta are thought to be derived directly from one group of seed ferns; Pinophyta and Ginkgophyta are thought to be derived either from another group of seed ferns or directly from a different group of progymnosperms (see figure above). Note that the origin of the Taxales and Gnetophyta is shown as unknown.

SEEDS

A seed is as an integumented, indehiscent megasporangium that remains attached to the parental sporophyte until after fertilization. This sounds formidable, but take it bit by bit. The diagram may help.
seed.GIF (12738 bytes)
A megasporangium is simply the cell in which megaspores develop as a result of meiosis.   "Indehiscent" means 'not splitting open'.  This means that the megaspores must develop inside the megasporangium, rather than being shed as in ferns. To say that the megasporangium is integumented simply means that it is surrounded by a layer of tissue that is derived from the parental sporophyte. This tissue serves to protect the megasporangium.

To say that the integumented indehiscent megasporangium remains attached to the parental sporophyte until after fertilization means that the megagametophyte must reach maturity inside the sporangium, finally forming one or more egg cells. This is then fertilized and may go through several cell divisions before the seed falls.

The major advantage of seeds is that they enable the genetically susceptible gametophyte to obtain nutrients and protection from its parental sporophyte until after fertilization (sometimes for quite a long time afterwards). This means that it is no longer critical for the megaspore to land in just the right conditions for its survival. In seeds of extant plants, the parental sporophyte packs the seed with a lot of nutrients before it is considered mature

POLLEN
If seeds were to be produced at the top of plants, not just near the ground, a means of getting the sperm to the egg cell had to evolve. Pollen grains and the pollen tube were the means.

A pollen grain is an armored airship for male gametophytes. The pollen wall is the armor. It protects the microgametophyte from UV and desiccation. The pollen tube is produced by the microgametophyte and provides a passage for the sperm to move down through the megasporangium wall and into the egg.  The combination of seed and pollen grain finally freed plants from needing external water to enable their gametes to meet.