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Supervisor: Justin Gerlach, Evolution and Behaviour
Describe the problems generated by life on land for the green plant lineage. How
have these problems been overcome?
There were many problems generated by life on land for the green plant lineage;
the transition from water to land very difficult, as illustrated by it occurring only
once. The problems resulted from no longer being submerged in water, and were
solved by adaptations that allowed the plants to avoid desiccation and carry out
essential functions such as gas exchange, reproduction, and supply of water and
nutrients to all parts of the plant, without being submerged.
The first problem overcome by plants resulting from life
on land was finding a way to survive in the highly
dessicating aerial environment. Plants evolved to
overcome the problem of dessication in a variety of ways.
The first way is through avoiding drying out. For
example, the liverworts live on moist ground, are very
flat so remain in stiller, moister air close to the ground,
and have a large surface area for absorption of water.
However, this does not prevent the plants from drying
out completely, and liverworts, along with some
Lunularia cruciata hornworts and mosses, have evolved the ability to suspend metabolism,
Crescent-cup
Liverwort
growth and reproduction when they dry out, and revive when
rehydrated; an ability described as dessication tolerance. Hornworts
and mosses also have an alternative means of avoiding dessication that is shared
by most other terrestrial plants; a
waterproof cuticle, containing the
hydrophobic polymer cutin, which
prevents water loss from the plants.
However, the acquisition of a waterproof
cuticle resulted in a further problem; they
are impermeable to gases, including
carbon dioxide required for
photosynthesis. The solution to this
changed requirement for gas exchange
was stomata: pores in the cuticle between
pairs of guard cells in the epidermis that
Scanning electron microscope image of stomata
can open or close the hole dependent on
the conditions and requirements of the plant, in order to regulate water loss and
diffusion of gases, closing for example when the air is dry. In addition to this,
inside the plants there are spaces through which air can flow close to spongy
mesophyll cells, providing a large surface area for diffusion of gases into cells, a
feature lacking in water plants. These adaptions enabled the plants to survive on
land without dessicating, or allowing them to tolerate dessication, but in order to
become established on land plants needed to evolve altered mechanisms of
reproduction.
A major problem caused by the transition from water to land was that there are
different conditions required for successful reproduction in each environment.
Sexual reproduction in water plants worked by release of eggs into the water,
Supervisor: Justin Gerlach, Evolution and Behaviour
and fertilisation by swimming sperm. This is less possible on land, although it is
still carried out by liverworts and to some extent in the gametophyte stage of
Embryophytes (that includes all land plants) when flagellate sperm are released
from antheridia and swim along a film of water to reach an archegonium, where
they fertilise the egg to produce the diploid sporophyte. To enable this the
gametophytes of vascular plants are small and live close to the moist ground.
Reproduction required a great deal of modification to decrease dependency on
water, the main solution to this problem being the evolution of a thick walled
haploid spore stage that is resistant to dessication, coated with the decay
resistant polymer sporopllenin, produced by the diploid sporophytes in meiosis.
These spores germinate and grow to produce the gametophyte. Reproduction in
seed plants is even less dependent on water, for example in angiosperms
fertilisation occurs inside the maternal tissue, since the egg is buried inside the
sporophyte, and fertilisation occurs when a pollen grain germinates after
reaching a stigma, and pushes a tube through the
sporophyte tissue and megaspore wall through to
the egg, then releases sperm nuclei. The
gametophyte living in a moist environment is not
required. The order in which these adaptations to
terrestrial life evolved can be followed, as for
example Bryophyta such as moss, from which
much of the rest of the terrestrial plant lineage are
descended, have spores, but not extensive vascular
Bryophyte spore tetrad
tissue for transport.
As upward plant growth increased the lack of water and nutrients present in the
air became more of a problem, and the need for transport systems and a system
for uptake of water and nutrients from the soil increased. This requirement
resulted in the evolution of xylem vessels, the precursors of which were found in
Bryophytes. Xylem vessels are continuous tubes lined with lignin, which is
waterproof and antibiotic, and triggers apoptosis in the cells that initially make
up the xylem to leave a hollow tube. The xylem vessels carry water using the
transpiration stream; evaporation of water through stomata creates a low
hydrostatic pressure, which pulls water upwards, along with capillary action and
root pressure. The xylem also provides structural support to the plants. This
enabled the primary sites of photosynthesis to be moved upwards towards the
light, resulting in further upwards growth. The taller plants needed anchoring in
the ground, and greater nutrient and water uptake from the soil, resulting in the
evolution of a root system, first found in the club mosses: Lycophyta. The roots
had greatly increased efficiency due to mutualistic symbiosis with soil dwelling
fungi; a relationship known as mycorrhiza. The hyphae break through epidermal
cells in the roots to associate closely with the cortical cells, and increase mineral
nutrient uptake from the soil, a relationship on which early land plants may have
relied on for supply of phosphate. A problem with the use of roots is that they
have no sunlight available to them, therefore cannot photosynthesise. The active
cells require products of photosynthesis, therefore there was a need for
transport systems in plants to supply the products of photosynthesis to the roots
for their metabolism and growth, in addition to the water and nutrients
absorbed from the roots to the rest of the plant. A second transport system, the
Supervisor: Justin Gerlach, Evolution and Behaviour
phloem, also first found in the club
mosses, evolved for transport of
phytosynthates such as sucrose from its
sources; the sites of photosynthesis, to
the rest of the plant.
In conclusion, as solutions to the
problems faced by the transition of
plants from water to land, including the
dessicating aerial environment,
requirement for sexual reproduction without submergence in water, and the
requirement for transport systems, plants evolved a variety of complex
adaptions. These include dessication tolerance and cuticles, an altered
mechanism of sexual reproduction involving dessication resistant spores, as well
as vascular tissue. An observation common to many of these adaptions to life of
land is that they led to further problems, that had solutions resulting in even
greater complexity. For example, the gas exchange systems needed to be altered
due waterproofing from the cuticle, resulting in the evolution of stomata.
Additionally, the development of roots led to the increased requirement of the
phloem transport system. Conversely, the adaptations also allowed evolution of
new features without posing problems, such as transport systems providing
mechanical support enabling upright growth, and the altered reproduction of
plants allowing them to inhabit even drier environments. The solutions to the
problems of plant life on land enabled terrestrial plants to become the dominant
life form on earth, making up the vast majority of biomass on the planet, and
being the primary producer for most terrestrial ecosystems.
Bibliography
Glover, Beverly “The origin and evolution of plants” Lecture notes
Mark Ridley (2004) ‘Evolution’ 3rd Edition Blackwells: Oxford
Wood, Andrew J. "The nature and distribution of vegetative desiccationtolerance in hornworts, liverworts and mosses." The Bryologist 110.2 (2007):
163-177.
Kenrick, Paul, and Peter R. Crane. "The origin and early evolution of plants on
land." Nature 389.6646 (1997): 33-39.
Sperry, John S. "Evolution of water transport and xylem structure." International
Journal of Plant Sciences 164.S3 (2003): S115-S127.
Image 1 source:
http://www.aphotoflora.com/liverwort_lunularia_cruciata_crescent_cup.html
Image 2 source: http://www.ammrf.org.au/innerspace/img15.html
Image 3 source: http://www.cpbr.gov.au/bryophyte/life-cycle-sporophytedev.html
Image 4 source: http://www.deanza.edu/faculty/mccauley/6a-labs-plants04.htm