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The microevolution of structural change
Hydrostasis and the evolution of the upright plant
The evolution of plant transport
hydrostasis and evolution of the
upright plant
Cell Mechanics
some problems to overcome
•
One of the most perplexing questions that need to be asked and
answered in relation to plant growth, is how do terrestrial plants
stand up?
Unlike vertebrates, land plants have no obvious hard skeleton to
help resist the pull of gravity.
Land plants do not benefit from the buoyant effect of water as do
aquatic plants.
We therefore need to question the nature of the structures and
physical principles that allow land plants to withstand gravity.
The answers lie in understanding cellular biomechanics
the aquatic habit
•
Comparisons with aquatic plants are
necessary.
•
Many of these organisms consist of a
single cell, and some may attain large
size and complexity.
one giant cell for all?
Leaf-like
appendage
An example here would be Caulerpa,
which has a one-celled thallus. The
thallus is without cross walls, but
has numerous trabeculae of callose
and pectic materials.
Mechanical support of the
erect shoots is due to turgor
and thickness of the cell
wall.
Specialisation for a holdfast area
interlinked with a horizontal
rhizomatous unit, linking this
structure to other ‘leaf like’
structures.
“Any plant body that is not divided into true leaves, stems, and roots. It is often
thin and flattened, as in the body of a seaweed, lichen, or liverwort, and the
gametophyte generation (prothallus) of a fern.“
http://www.tiscali.co.uk/reference/encyclopaedia/hutchinson/m0007018.ht
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the multicellularity of land plants
It is very important to realise that all successful
terrestrial plants are multicellular.
The primary benefit of multicellularity is internal
compartmentalization of the plant body, thus
facilitating physiological specialization.
The second benefit is often overlooked -multicellularity confers structural support (which is
so crucial to land plants).
Multicellularity thus allows for a more rigid
form, through the alteration of the thickness,
chemical composition, layering and spatial
distribution of cell wall materials within tissues.
land transition
Land transition meant that unicellularity was lost.
•Sheer size would preclude a single cell from attaining
any mass.
•The force of gravity as well as environmental issues
such as solar radiation, desiccation and the plethora of
physiological conditions attendant upon these, would
negate the possibility or probability of a massive single
cell.
•They would simply flatten out, like a plastic bag full of
water due to loss of support!
hydrostasis
•
In their operational state, living plant cell are hydrostatic units.
•
The cellular aggregates that comprise land plants, act as
composite hydrostats, capitalizing on the spatial
configurations of cell wall
•
Concept of hydrostasis first discussed it in the 19th century.
Defn: hydrostasis: concerned with fluids that are not in motion
example: hydrostatic pressure
hydrostats 1
By definition, a hydrostat is, a structure whose mechanical
behaviour depends on internal water pressure. Osmotic potential
is generated because of plasmamembranes
(noun
a device that detects the presence of water
)
Since all living cells contain water, each can be considered to
be a functional hydrostat.
The unicellular thalli of aquatic plants such as Caulerpa and
Bryopsis are thus, single-celled hydrostats in which the cell wall
operates like an inflatable, semi rigid sac; within which the
protoplast exerts an internal pressure that in part maintains the
shape of the cell wall.
hydrostats 2
•
While the cell wall contributes to the rigidity of the organism by virtue of its
own material properties, turgor pressure creates "hoop" tension in the
internal fibrous network of the cell wall, further strengthening the whole cell.
The cell wall can vary its response to changes in turgor pressure
(Preston 1974). In growing cells, the wall has a metabolically
controlled ability to yield. This allows for the differential expansion or
elongation, which is essential to cell morphogenesis.
hydrostatic behaviour
The hydrostatic behaviour and composite nature of cell walls are
as essential to unicellular hydrophytic plants such as Caulerpa and
Bryopsis as they are to growing multicellular plant systems.
For aquatic plants, water is not a limiting resource, and turgor pressure varies
little.
On land, the availability of water may change continuously, and a unicellular
hydrostatic structure can become a liability.
Terrestrial plants do not rely solely on this mechanism.
the need for hydrostatic mechanisms 1
•
The variability of water availability changes continually, and an
all-encompassing hydrostatic structure would be a liability in land
plants. Terrestrial plants do not & cannot therefore, rely solely on
turgor pressure for their principal mechanical support.
Cell wall infrastructure incompletely partitions the protoplasts of
terrestrial plants. This allows cellular compartments to differ in size and
shape.
heterogeneity in cellular compartment geometry is ensured by
the deaths of some cells during developmental programming.
the need for hydrostatic mechanisms 2
Differences in size and shape
of cell walls within living
tissues and organs and
differences in orientation of
cellulosic fibrils within cell
walls provide a system in
which shearing, tensile and
compressive forces can be
preferentially resisted.
cell wall mechanics
The cell wall is composed of a fibrous
network of cellulosic strands is
embedded in a plastic and ductile
matrix. Research has shown that
preferential yielding is the result of the
composite nature of the cell wall matrix
itself.
conclusions
•
•
Cell walls impart a degree of rigidity
wall pressure (changing solute concentration)
imparts hydrostatic force
•
leads to a less [or more] rigid structure
the evolution of plant transport
engineering a plant
Engineering theory leads to a
blend of interrelated
conceptual synthesis, which
provide insight into the
ecology and evolution of
plants, as well as indicators of
the the construction materials
that need to be used.
multicellularity leads to the
formation of remarkable
structures
increasing complexity –
domains and transport
complexity related to function 1
Unicellular domains
Single vegetative cells (as in
this post cell division example)
contain all components
needed to sustain growth and
metabolism. Cell division will
occur when the cell surface
area to volume ration
becomes unfavourable. Yet,
within this cell control and
regulation system occur, due
to compartmentation (akin to
domains).
complexity related to function 2 –
multifunctioning domains
RC
RC
Complex multicellular and
supracellular organisms
contain many differing
domains. In higher plants there
are domains involved in
transport, storage, retrieval
or metabolism. This is
exemplified in this illustration of
a cross section of a young pine
stem, where resin canal cells
(RC) occur in proximity to
tracheary elements as well as
radial parenchyma.
transport in hydrophytes
Transport in hydrophytes is in some instances,
simpler than that in their land plant counterparts.
Yet, in others, being immersed in water presents a
series of complex physiological problems, mostly
to do with ion exchange processes (such as Na+ :
K+ pumping) or aeration, to prevent anaerobic
processes becoming dominant.
Energy expenditure (ATP use) will be high and
respiration outputs less efficient?)
the two transport domains
In lower plants, including some of
the giant Kelps, we find evidence of
transport systems composed of
hydroids (water-conducting
elements) and Leptoids
(carbohydrate-conducting
elements).
Hydroids (left, above)
occur associated with
Leptoids (left, below) in
the central region of the
stem and evolved
simultaneously.
trumpet cells
Trumpet cells are found in
many of the large brown
algae – here we see an
example of a three files of
trumpet cells, within the
stalk of the algae.
Below this, a scanning
electron micrograph
showing the details of the
porous sieve plate platelike end wall structure.
These are thought to be
nearly as advanced as
Angiosperm sieve tubes!
evolution of
vascular tissues
vascular systems
First, simple central
xylem core
protostele
siphonostele
eustele
atactostele
Hollow-centred
Vascular bundles (young
Gymnosperms and
Dicotyledons)
Spiral arrangement
Poaceae
multifunctioning transport domains
In this example, one can see a
multifunctional transport system. The
xylem (an apoplasmic domain) is
adjacent to the phloem (a
symplasmic domain).
Both have different functions, yet
they co-exist in the same plane and
axis.
domain control & regulation
domain control requires communication pathways. These are
usually provided by plasmodesma such as these, seen in a
scanning electron microscope image of a surface view showing
plasmodesmal fields in the bundle sheath interface in
Themeda triandra.
domains - summary
•
A single celled organism has one DOMAIN;.
•
A multicelled organism may have two to many
DOMAINS;
•
DOMAINS have different functions in the land plant,
•
DOMAINS can regulate activities of other DOMAINS
simple beginnings
Amongst the earliest vascular plants, there was little
distinction between the arrangement of the water
conducting (xylem) and carbohydrate-conducting
(phloem) in stems and rhizomes or roots.
the protostele
The fossil record has many examples to offer of these
early cellular arrangements. These first-formed structures
were called protosteles (first-seen or formed) are
evidenced in the seedless vascular plants, which arose
some 400 to 350 million years ago. Today, these structures
are evidenced in groups such as the division
PSILOPHYTA.
(Psilotum, Tmesipteris) Psilotum is unique as it lacks roots
and leaves. Tmesipteris is an epiphyte of tree ferns. Both
have protostelic vascular tissue.
the protostele 2
The LYCOPHYTA (Lycopodium) 200 species;
vascular system is more advanced with a dissected
protostele.
The xylem in the example below is dissected into
three pieces, with four “points” to the xylem, thus
four groups of phloem exist between the xylem
poles.
the plectostele
further division, into a plectostele (plate like) structure,
see the xylem and phloem occurring in alternating sheets.
Note large-diameter sieve cells in close proximity to the
tracheary elements.
the actinostele
The vascular tissue is more dissected, with some of the
xylem strands showing almost complete dissection from
one another. Phloem form finger-like projecting mass
between the xylem
dichotomy and anastomoses
- prelude to leaf and branch traces
Fusion of separate dichotomous strands of vascular
tissue led to anastomoses of originally separate
vascular strands (individual protosteles). From this
simple beginning, the evolution of the megaphyll
reduction in complexity, cheaper structures
Diagrams illustrating the
difference between a hollowcentred (A) and solid (B, pithcontaining) core in which
vascular bundles are
scattered – This is an
Atactostele.
A
B
microevolution - 1
•Engineering theory relevant to the design of plant stems,
can be used to predict which anatomical configurations
are most likely to succeed in different environments and to
test aspects of (successful) evolutionary theories.
•For example, genotypes that maximize their net carbon
gain are favoured through natural selection over those
less efficient.
microevolution - 2
•Plant ”economics” suggests that that
metabolic investment made in the
construction of an organ should be minimized
provided its functions are not impaired.
•One would expect a reduction in the
relative density of tissues or organs
involved in vertical growth for example
‘pannelboard’ organs, hollow sterns, and
hollow leaves.
microevolution – 3
•The ratio of the living protoplasm to the living and
dead cell wall infrastructure in an organ would be
expected to be greater in environments where the
tissue’s depravation of water is less likely, as
hydrostatic support mechanisms are energetically
"cheaper" than those based on thick cell walls and little
or no protoplasm.
•Thick cell walls prevent collapse
microevolution - 4
In contrast, plants growing in habitats with cyclical
or unpredictable episodes of water stress would
be expected to "invest" in thick walled cells or waterinsensitive tissues, such as wood or lignified fibres..
microevolution - 5
An advantage of a living tissue with thin cell walls
and turgid protoplasts is its resistance to
deformations due to rapid water cycling out of cells - it takes time for water to move out of cells. Thus,
"quick" stresses are resisted whilst "slow" stresses
produce "slow' strains
end