Download 1 2006S Bio153 Lab 4: Seedless Vascular Plants July 11th / July

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2006S Bio153 Lab 4: Seedless Vascular Plants
July 11th / July 13th
After the appearance of land plants, the next big evolutionary step was the
development of vascular tissue. Vascular tissue allows for the movement of water
and nutrients around the plant body, meaning that plants could be larger. Vascular
tissue also provides support, which means that plants could grow upright. This is
important, because upright growth allows a plant to escape competition for space
and light by overtopping its competitors. Plants that could keep part of their body
in contact with moist soil and have access to sunlight would have a great
advantage over those with a sprawling growth form (such as is seen in the
bryophytes). To colonize land and compete with other plants, it was necessary for
plants to be able to grow into large structures that could anchor themselves in the
ground, capture light efficiently, and extract water and nutrients from the soil and
transport it around the body. This has involved the evolution of leaves, stems,
(collectively called the shoot), and roots. Thus, in the seedless vascular plants, we
see the evolution of vascular tissue, true roots, and tracheids.
The earliest known vascular plants appeared in the late Silurian period about 420
million years ago. These were simple plants with a dichotomously branched
horizontal stem system bearing erect dichotomously branched stems. Fossil
evidence shows that these early plants contained elongated cells organized into
tissues. These cells had walls thickened with an extremely strong substance called
lignin. These lignin rings would have allowed these plants to support erect stems
and conduct water throughout the plant. By about 380 million years ago, fossil
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plants show an elaboration of these elongated cells into more complex tissues
known as tracheids. Tracheid cells die after maturing, leaving no cytoplasm and
only cell walls. This increased the efficiency of water transport in these primitive
plants.
The aboveground and belowground parts of early vascular plants showed
little differentiation; in later groups we see the evolution of highly differentiated
plant parts. It appears that these structures have evolved independently in
different plant lineages – current evidence suggests that roots evolved at least
twice and leaves perhaps six times.
Leaves: Simple vascular plants such as Psilotum do not have true leaves, but
instead possess small projections called prophylls. Prophylls differ from true
leaves in that they lack vascular tissue. The two types of true leaves found
vascular plants – microphylls and megaphylls – are thought to have
independent evolutionary origins. Microphylls are found in Lycophyta and contain
a single unbranched strand of vascular tissue. It is hypothesized that
microphylls arose from sporangia (spore-bearing structures). Most other vascular
plants have leaves with an elaborately branched vascular system. These leaves are
known as megaphylls. The currently accepted hypothesis is that megaphylls
evolved from branched stems. Originally, most plants branched dichotomously (the
stem apex divided to produce two equal branches). In some Devonian plants the
two branches became unequal, so only one continued the growth of the main axis
of the plant, while the other remained short and spread horizontally. This
photosynthetic tissue eventually joined the parts of the short branch. Primitive
megaphylls often show dichotomous branching of the vascular tissue.
Reproduction: In early vascular plants we see the shift from the dominance of
the gametophyte generation to that of the sporophyte generation. Rather
than being a temporary structure permanently dependent on the gametophyte, the
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sporophyte eventually emerges as a separate, independent entity. However, the
early vascular plants are still dependent on water to complete their life cycle –
sperm must swim through water to reach the egg and produce the diploid embryo.
Most early vascular plants are homosporous. In homosporous plants, the
sporophyte produces a single type of spore that gives rise to a bisexual
gametophyte. This structure bears both the female (archegonia) and male
(antheridia) sex organs, which produce female (egg) and male (sperm) gametes.
A later evolutionary trend is the emergence of heterosporous plants. In
heterosporous plants, 2 types of sporangia (spore-bearing structures) give
rise to 2 types of spore - microspores that make male gametophytes, and
megaspores, that make female gametophytes.
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Early vascular plants:
1. Psilophyta (whisk ferns)
The living genus Psilotum is
remarkably similar to early fossil land
plants although there is much debate
over whether there is a direct
relationship. Examine a specimen of
Psilotum and note that it consists
only of a horizontal underground stem
(called a rhizome) without roots and
an erect over-ground stem. It bears
small projections on the over-ground
stem resembling small leaves.
However there is no vascular tissue
in these projections so they are
named prophylls to distinguish them from the leaves of
other vascular plants. Examine a cross-section of the
stem of Psilotum. Find the xylem tissue in the centre
(stained red) and locate the tip of the stars. The small
blue cells between these tips are the phloem tissue.
Xylem transports water from the roots, while phloem
transports the products of photosynthesis form the
leaves.
2. Lycophyta (club mosses)
Lycophyta is a division of vascular plants that originated early in the Devonian
period, about 390 m. y. ago. Plants very similar
to modern Lycopodium occur in the late
Devonian period about 350 m. y. ago. Many
fossil lycopods were enormous plants up to 40m
tall, but living lycopods are all small. This group
of plants shows two important advances; they
have leaves with a strand of vascular tissue
and they produce rather small relatively simple
roots. Roots differ from rhizoids in that they
contain vascular tissue. In the Lycophyta the
leaves are spirally arranged and branching is
dichotomous. The leaves are usually long and
narrow and do not have branching vascular
tissue. Simple leaves of this type are known as
microphylls. Lycopodium is homosporous,
meaning that meiosis in the sporangium forms
spores that give rise to bisexual gametophytes bearing both archegonia and
antheridia. The bi-flagellated sperm swim to the egg, and early development
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of the embryo occurs in the archegonium. The sporophyte in club mosses grows a
root and becomes an entity separate from the gametophyte.
Another member of the
Lycophyta is Sellaginella. is
the only genus in this group,
but includes 700 known
species. Most live in moist
places, but a few are desert
plants that lie dormant during
the driest parts of the year
(such as the “resurrection
plant”). Selaginella is
heterosporous; its
megaspores and
microspores germinate to
form separate male and
female gametophytes.
Heterosporous reproduction in Selaginella represents an evolutionary step
forward. Selaginella needs water for reproduction, as its sperm must swim to the
egg in the archegonium. The young
embryo is nourished by the
megagametophyte, but eventually
emerges from the gametophyte and
becomes independent. The
sporangium is able to produce microor megaspores depending on its
nutritional status. The slides of
Selaginella cones that you will
examine will show the random
distribution of micro- and megaspores
within a cone. Examine a
longitudinal section of a
Selaginella cone (known as a
strobilus). Note the presence of
microspores and megaspores, the
micro- and megasporangia, and the
sporophylls (the modified leaves that bear the sporangia).
3. Sphenophyta (horsetails)
Sphenophyta is another division of plants originating in the Devonian period.
Basically similar to the Lycophyta in organization, but in the Sphenophyta the
stem is organized into nodes and internodes. In the Sphenophyta the leaves
are arranged in rings (whorls) at each node and branches arise at the same
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point. The branches arise alternately with the
leaves, an arrangement unique to this division.
Observe a stem of the horsetail, Equisetum and
note the arrangement of the tiny leaves and the
origin of the branches at the nodes.
4. Pteridophyta (ferns)
Ferns are an enormously successful group with over 12,000 species. They are the
largest and most diverse group of seedless vascular plants. Many of them are
epiphytes, growing on other plants (particularly in the tropics). The have large well
developed megaphylls called fronds. Most ferns are homosporous, bearing
spores in sporangia that are
often clustered in sori on the
edges or undersides of leaves.
The gametophyte resembles a
liverwort, while the sporophyte is
usually structurally complex and
large. In ferns, the sporophyte
depends upon the gametophyte
during the early phases of its
development, and in some species
the gametophyte is capable of
supporting several young
sporophytes. Reproduction again
requires water, because the motile
sperm must swim to the egg. Examine a fern and note the sori, which contain
the sporangia and the spores. Make sure that you see gametophytes. You may
see young sporophytes growing on them.
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Fig. 4. Life cycle of a homosporous fern.