Tim Selwyn        Jesus College        13-Dec-05

Heterosporous Plants

(‘hetero-’ = ‘different’)

        An important change took place in many of the major plant groups during the Devonian period.  Until this period, each sporophyte plant had produced spores of one kind: homospores.  Now, some plants began to produce two kinds of spores from two kinds of sporangia, an evolutionary stage which some algae were also to reach in the sea.  These plants were said to be heterosporous.

        In heterosporous plants the two different spores germinate into two small, but distinctly different, gametophytes which remain within spores and never grow into a large green plant.  The two kinds of spores are different in size, for specific reasons.  The larger type (megaspores) becomes a female gametophyte with female sex organs.  It therefore contains the food reserves necessary for the early growth of the future plant.  The smaller spores (microspores) which become the male gametophytes have no food reserves as their sole function is to produce and liberate the sperm before dying.  

Megaspores and microspores are produced are produced in microsporangia and megasporangia, respectively; microsporangia are borne on microsporophylls, and megasporangia on megasporophylls.  Both types of sporophylls usually occur in the same strobilus in Selaginella.  In the megasporangium, usually one cell undergoes meiosis to produce four megaspores.  In contrast, many cells divide meiotically in the microsporangium, producing hundreds or thousands of spores.

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        The sexual part of the life cycle for heterosporous plants becomes less obvious.  The sporophyte plant is increasingly dominant and because the reproductive cycle is speeded up, the next generation can be established more rapidly.  This means that evolution itself can be accelerated as changes can be passed on more quickly and the possibility of new life forms appearing becomes greater.

        Beginning with the primitive, free-sporing, homosporous, monoecious habit, the stages in evolution of heterospory can be summarised as follows:

  • Gradual decrease in the number of spores in some sporangia.
  • Increase in size of remaining spores.
  • Spore ...

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Overall this is a well written and coherent piece of work. Though the lack of an essay title makes it hard to definitively say that the question has been answered, the author has provided a nice summary of the major anatomical and evolutionary features of heterospory, with some reference to notable species such as Selaginella. To improve the work the author should be careful to define all relevant terminology: It is safe to assume that the marker is familiar with the essay, but any key words for the essay (e.g. microsporophyll / antheridium) should be clearly outlined to demonstrate full understanding of their meaning. Lists should also be avoided, except where the information within them is so brief that no expansion whatsoever is necessary. Referencing is more difficult given that there is not an argument to be had in this piece of work, but in text citations should also be used. As a first year piece of work at my (Russell Group) university this would receive 5/5