Photosynthesis in Poikilohydric Plants: A Comparison of Lichens and Bryophytes
335
land. The fossil record is exceptionally poor for both groups and is little help
in determining their origins. Standard texts consider the colonization of
the land during the Silurian period, over 400 million years ago, as being
by multicellular, telomic (cylindrical) plants with a suite of adaptations,
namely cuticle, stomata, vascular strand, and dispersal spores. These are all
homoiohydric characters and it is interesting that the first fossil records
are of spores (Raven 1977). It is not unreasonable to assume that the
first successful land colonists were the cyanobacteria, which form, today,
the dominant plant communities in the phanerogam-free Antarctic, and
desiccation-resistant chlorophycean algae. Both these groups are involved in
lichen symbioses, and the fungi, which are known as fossils in the Silurian,
can also be desiccation-resistant, and grow using only atmospheric humidity.
Lichens could also be early colonizers, the prime adaptation being poikilohydry but the major advantage being extension of habitat for both symbionts. The earliest lichen fossils are from the 220-million-year-old Keuper
formation in Germany (Ziegler 1991) and it seems quite possible that problems of fossilization rather than their absence, are the reason for their late
occurrence in the fossil record.
What of the bryophytes, since fossil bryophyte-like plants are now known
from the Devonian (Remy 1982). Despite the emphasis in textbooks on the
requirement for liquid water during fertilization, the bryophyte life cycle
is highly suited for colonization of the early land areas. Bryophyte spores
are an excellent dispersal system and continual vegetative reproduction is
possible until the sporophyte can be produced. Combine the spores and
vegetative reproduction with poikilohydry and a successful colonizing lifeform exists. A small sporophyte would be advantageous rather than being
regarded as a reduced structure. Bryophytes are common in the Antarctic
but no mature sporophytes have been reported, so vegetative dispersal has
clearly been sufficient to effect colonization and maintain populations on
that continent.
Perhaps the land was first colonized by algae and cyanobacteria, then by
the lichens and bryophytes. The low A enforced by their simple structure
would not be a disadvantage at that time. If this is correct, then the major
bryophyte and lichen communities in the shade of forests and other higher
plant-dominated ecosystems, would be secondary. Bryophytes and lichens
are successful in these communities because they are poikilohydric and, in
particular, because of their structurally imposed low A which pre adapts
them to shade. Perhaps future research will shed more light on this idea.
16.7 Conclusions
Lichen and bryophytes provide fascinating examples of the interaction between plant structure and photosynthetic metabolism. Both groups have
335
land. The fossil record is exceptionally poor for both groups and is little help
in determining their origins. Standard texts consider the colonization of
the land during the Silurian period, over 400 million years ago, as being
by multicellular, telomic (cylindrical) plants with a suite of adaptations,
namely cuticle, stomata, vascular strand, and dispersal spores. These are all
homoiohydric characters and it is interesting that the first fossil records
are of spores (Raven 1977). It is not unreasonable to assume that the
first successful land colonists were the cyanobacteria, which form, today,
the dominant plant communities in the phanerogam-free Antarctic, and
desiccation-resistant chlorophycean algae. Both these groups are involved in
lichen symbioses, and the fungi, which are known as fossils in the Silurian,
can also be desiccation-resistant, and grow using only atmospheric humidity.
Lichens could also be early colonizers, the prime adaptation being poikilohydry but the major advantage being extension of habitat for both symbionts. The earliest lichen fossils are from the 220-million-year-old Keuper
formation in Germany (Ziegler 1991) and it seems quite possible that problems of fossilization rather than their absence, are the reason for their late
occurrence in the fossil record.
What of the bryophytes, since fossil bryophyte-like plants are now known
from the Devonian (Remy 1982). Despite the emphasis in textbooks on the
requirement for liquid water during fertilization, the bryophyte life cycle
is highly suited for colonization of the early land areas. Bryophyte spores
are an excellent dispersal system and continual vegetative reproduction is
possible until the sporophyte can be produced. Combine the spores and
vegetative reproduction with poikilohydry and a successful colonizing lifeform exists. A small sporophyte would be advantageous rather than being
regarded as a reduced structure. Bryophytes are common in the Antarctic
but no mature sporophytes have been reported, so vegetative dispersal has
clearly been sufficient to effect colonization and maintain populations on
that continent.
Perhaps the land was first colonized by algae and cyanobacteria, then by
the lichens and bryophytes. The low A enforced by their simple structure
would not be a disadvantage at that time. If this is correct, then the major
bryophyte and lichen communities in the shade of forests and other higher
plant-dominated ecosystems, would be secondary. Bryophytes and lichens
are successful in these communities because they are poikilohydric and, in
particular, because of their structurally imposed low A which pre adapts
them to shade. Perhaps future research will shed more light on this idea.
16.7 Conclusions
Lichen and bryophytes provide fascinating examples of the interaction between plant structure and photosynthetic metabolism. Both groups have