In the land plants considered so far there are two limitations that are evident;-
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Water requirement of the gametophyte: The
sporophyte can only grow where the gametophyte can succeed. The gametophyte is the weak
link in the life cycle. |
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Lack of genetic variability: In many of these plants it is quite
possible for male gametes produced by a gametophyte to fertilize eggs produced by the same
gametophyte. In such cases, the only source of variation is the crossing over events of
meiosis during spore production. This is problematic as the fitness of a species relies on
there being variation in a population on which natural selection can act. |
| Heterospory promotes outbreeding and
genetic variability and it is not surprisingly therefore that heterospory evolved
independently in at least four different plant groups - the ferns, club mosses, horsetails
and seed plants. Some of these plants even had mixed sporangia with both microspores and
megaspores in one sporangium! Producing a few large megaspores invested with greater food
reserves would ensure the female gametophyte could not only produce eggs but nurture the
young sporophyte in its early parasitic phase. See Niklas (1997) for a good, though advanced, treatment of this topic. |
Seed plants evolved from heterosporous, endosporous vascular plants
| Imagine such a plant, producing only one megaspore per megasporangium. The megaspore is not released until the female gametophyte has fully developed within it, complete with archegonium and egg. |  |
| Imagine that the whole megasporangium (with its single megaspore) is released. Imagine too that the megasporophyll and ligule (A, B) extend and enwrap the megasporangium, protecting it. This composite structure is an ovule. |  C = megasporangium |
| Imagine that this protected megasporangium (with its single megaspore and internal gametophyte) is retained on the parent sporophyte until the egg in its archegonium is fertilized and forms an embryo. Only when this has happened, and food reserves are laid down with the embryo, and the outer protective integuments harden, is this structure released. This is what we call a seed. |  parent sporophyte seed |
|
This is an oversimplification but it was by this kind of
progression that the seed habit is thought to have evolved. |
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The evolution of the seed reduced the vulnerable gametophyte phase of the life
cycle drastically. |
|
Whereas before dispersal depended on spores taking gametophytes (and ultimately
sporophytes) to new habitats, the seed is the new means of dispersal. |
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The first seed plants were gymnosperms (or "naked-seed" plants) and first
appeared in the Devonian period, soon after the first land plants. |
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Would you
like to look at some sample questions on this part of the course? If so, click the button. |
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We have
now completed our look at the Evolution of Seeds. Click the button to move on to the Gymnosperms. |
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Sean Carrington, April, 1998