Alternation of generations revisited
Alternation of generations is a general feature of plants. Liverworts are the simplest true plants and in order to understand their life cycles it is important to know the following terminology:

• the diploid generation is called the sporophyte
• a cell or cells somewhere in its body in sporangia undergo meiosis to form spores.
• If is is homosporous the spores are of one type only.
• If it is heterosporous there are two types of spores microspores ("male") and megaspores ("female").
• The spores germinate to produce the haploid gametophyte(s) that will eventually form the gametes:
  • male gametes or sperm, often in an antheridium
  • and female gametes or eggs, in an archegonium.

Antheridia and archegonia may be on a single gametophyte or on separate gametophytes in different groups of the plant kingdom .

The first land plants
Some time in the middle distance of geological time, in the Devonian (400 mya) there was a green alga that was making the transition to the land. What were the two major problems that it had to overcome?

• Survival out of the aqueous environment - maintaining water in its cells (taking it up and moving it around)
• and reproduction which is particularly difficult for non-motile organisms (how to get the gametes together?)

It took a long time for plants to evolve efficient mechanisms to cope with these challenges, particularly for sexual reproduction. Maybe this is why plants have retained a capacity for asexual propagation which animals lost as they evolved.

Hepaticophyta (liverworts) There are two groups of liverworts, leafy and thallose; we focus on the thallose type which is just about the most primitive true plant alive today.

You may have seen plants like this growing on wet rocks and soil in shady places. The plant that we see represents the gametophyte generation.

The thallus, seen here in section, shows little external differentiation (no stem or leaves) and internally there is little differentiation either.

There are pores in the upper surface which open onto photosynthetic cells beneath, but there is little or nothing in the way of conducting tissue. The pores tend to close as the thallus dries out, but they do not open and close actively like the stomata of other plants. Underneath the thallus is covered in scales and there are unicellular rhizoids which help to hold the plant down but do little else. The plant takes up water over its entire surface and it has no cuticle to prevent desiccation.

The thallus can branch and spread over the available surface and some species give rise to fragments (gemmae) which can detach and blow or float away to found new colonies.

We will not look at the life cycle of this organism (it is a lot like the moss which we look at next) You can tell that we have not moved far away from the watery habitat and it is not surprising that these plants are of restricted ecological range and importance. They can be troublesome weeds in situations where soil is kept permanently moist, such as in propagation of ornamentals.

Copyright © Michael Knee
The Ohio State University
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