Notes for Bio 4420 (Plant taxonomy) at Utah State University
words: Spore bearing plants;
bearing plants; Seeds; Pollen
References: Gymnosperm evolution figure based on Stewart 1983, p. 313 and 348.
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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:
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.
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?
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.
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
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.