Liverworts are, like true mosses, bryophytes, but their form and physiology are quite different, though they also exhibit alternation of generations.
Marchantia growing on a stone
step. The archegoniophores almost totally conceal the green
underneath. The upper part of the step's riser, where it is drier, has been colonised by lichens.
close-up view of the thallus of Marchantia. Note the pores and
the gemma cup containing
several gemmae (asexual spores). The gemmae are loose and waiting for rain splash to scatter them.
a close-up view of the pores in the surface of the thallus which
allow air containing carbon dioxide to enter the thallus for
a developing archegoniophore of Marchantia, in side view. Note
the white frilly membranes hanging down from the underside: This
is the involucre
covers and protects the flask-like egg-containing archegonia within.
an air chamber filled with photosynthetic cells in the thallus
a multicellular scale in Marchantia.
of Marchantia in longitudinal
section (above) and transverse section (left). Each rhizoid is
an extension of a single epidermal cell. Some cyanobacteria associated with the
rhizoids are visible in the photomicrograph at top left.
a section through the thallus of Marchantia sp. Note the curious
shapes of the innermost parenchyma cells: is this due to
mechanical stresses or an adaptation for transport of materials
from cell to cell?
is another liverwort which grows on damp soil in streams and
ditches. It is another non-leafy thallous liverwort anchored by
unicellular rhizoids and differentiated into a photosynthetic
epidermis and cortex and a starch-storing medulla. The thallous
is parenchymatous (comprised of largely undifferentiated plant
cells that non-fibrous)). The antheridia produce the male
gametes (spermatozoa) and the involucre is a flap which shields
the flask-shaped archegonia which contain the eggs. The
sporophyte grows up from a fertilised archegonium, protruding
from the flap as a stalk several cm long bearing a dark sphere
containing the spores. Unlike Marchantia, Pellia
no stomata or air-chambers.
based on information from Plant Types 2: Mosses, Ferns, Conifers
and Flowering Plants - an excellent book by Ruth Miller (Pub.
Above: the life-cycle of Pellia epiphylla. The spermatozoid-producing antheridia are spherical flasks, each inside a pit which opens to the outside via a pore. Each antheridium will give rise to many spermatozoids. The archegonia are flask-shaped structures situated beneath the involucre shield at the tips of the ripe thallus (in mosses the archegonia are usually on the shoot tips, protected by a sheath of leaves) and each contains one developing ovum (oosphere). After fertilisation the sporophyte grows out from beneath the involucre, but is diploid, but produces haploid spores by meiosis, which give rise to haploid gametophytes.
Unlike mosses, the spore capsule of liverworts does not possess a peristome to assist or regulate spore dispersal. Instead, the capsule contains elaters, which are elongated cells with spirally thickened cell walls which cause the elaters to twist and thrive as they drive, breaking up the spore mass and scattering the spores.
through the spore capsule of the liverwort Pellia.
up of the elaters. Note the spiral thickenings which cause the
elaters to twist as they dry.
extending from the lower wall of the spore capsule.
up of the elaters. As the elaters dry and twist, they help
scatter the spores.
up of the spores. Note the elaters entwined amongst them.
up of the spore capsule wall.
or bryophyte-like plants were possibly among the first plants to
colonise the land on Earth some
400 million years ago. (It is possible that today's bryophytes evolved from larger and more complex plants by
miniaturisation, but they are certainly ancient, the oldest known moss fossil is some 360 million years old).
Giant myriapods - millipedes the size of human beings, bulldozed through these primordial forests. In later
times, the bryophytes may have become overshadowed by trees of one kind or another, but they continue to
thrive, covering soil, rocks and trees. Indeed, those that grow on trees are helped off the ground, toward the
light and the fresh air which brings in carbon dioxide for photosynthesis and dispersal spores. It is easy when
walking in a wood not to pay too much attention to these ancient and miniaturised plants, but closer inspection
reveals amazing diversity and beauty of form in the tiny, and often overlooked, forests that these plants often
form. It is remarkable how such small plants have made such diverse use of chemistry and physics in so many
ingenious ways in order to solve life's problems.
growing on a post on a stream bank.
Although it is hard to tell without a closer look, the larger
thalloid liverwort is possibly Conocephalum (Snakeskin Liverwort)
whilst the form with smaller lobes could be Lunularia.
Mosses and Introduction to Bryophytes
An excellent online book on bryophytes, detailed and with photographs: http://www.bryoecol.mtu.edu/
29 Oct 2016
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