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Describe the problems generated by life on land for the green plant
lineage. How have these problems been overcome?
All land plants are monophyletic: having evolved once from Charophyceae, freshwater green algae
around 450mya. .As a result of this, All organisms on the green plant lineage share the same
reproductive cycle: sporic meiosis. This involves a haploid gametophyte producing gametes via mitosis
which fuse to form diploid sporophytes, these sporophytes then undergo meiosis to make haploid
spores. Originally in water based plants and the early land plants, these cycles were heavily haploid
dominant, however over time this haploid phase has reduced and the diploid phase become more
dominant in a process known as the alteration of generations. The evolution of land animals occurred
many times independently and so the fact that plants only managed this transition once suggests that
there were many problems that they needed to overcome.
The first, most obvious of these problems is the fact that the plants must move from an aquatic
environment, in which the whole of the organism is surrounded by water, to an aerial environment
where extreme desiccation will occur if the plant doesn't adapt to cope. Loss of water from the plant's
cells would cause almost immediate death, this does not allow any time for a plant ancestor to move to
land and then evolve a waterproofing mechanism to stop desiccation.
Instead, the earliest land plant species, Liverworts, tend to live on the moist ground and have a very
large surface area to volume ratio. Consequentially, they are able to absorb water into their cells via
osmosis. The Liverworts do not have any waterproofing mechanism and are said to be amphibious
plants as they still rely on water from the ground and the high in water saturated air near the ground in
order not to desiccate. One adaptation they do have and share with the mosses, is the ability to suspend
metabolism when conditions become too dry, and recover quickly once water is returned to their
surroundings.
Hornworts heavily resemble liverworts, however they have developed a waterproof cuticle made of
cutin: a hydrophobic polymer. Now found in all vascular plants, this cuticle stops water escaping from
the organism's cells. However, it also poses an issue with gas exchange as the cuticle is impermeable to
gases such a water vapour, oxygen and carbon dioxide. Hornworts were the first to overcome this
problem with the evolution of stomata, pores which are controlled by two guard cells with flexible walls.
These stomata not only allow the flow of oxygen and carbon dioxide in and out of the plant's cells but
can also minimise the loss of water. When the plant cells contain a high concentration of water, the two
guard cells swell outwards and open the pore, whereas when the cells are dehydrated they shrink and
close the pores, so no more water can be lost. For further gas exchange efficiency, the cells inside the
plants form a ramifying open pathway of cells alike to alveoli in mammal, allowing gases to easily diffuse
to required areas of the plant.
The improvement in efficiency of gas exchange and regulation of water loss allowed plants to grow
more vertically, away from the water and nutrient rich soil. They needed a specialised unidirectional
transport system to deliver water and nutrients from the roots to the rest of the plant. Mosses were the
first plants to develop a rudimental transport system, which was improved later in the vascular plants.
The xylem was formed: a continuous tube of dead, water conducting cells. Their cellulose walls are
strengthened by lignin, a complex polymer of aromatic alcohols which had been present in the plant
lineage since early algal ancestors. Previously lignin had acted as an antifungal coating to protect the
algae, now the lignin coated the xylem vessel causing apoptosis of the xylem cells. This pre-adaption of
lignin was vital to create the hollow tube which allows water to be transported around the plant, this is
driven by the evaporation of water through the stomata and the cohesion of water molecules to each
other by hydrogen bonding. Also by adhesion of the molecules to the walls of the xylem cells
themselves.Plants experienced another problem with transport though, as their roots were growing
underground there was no light available and so root cells could not carry out photosynthesis as the
light independent reaction could not occur. Phloem vessels were developed in the vascular plants which
transport photosynthates such as sucrose to the root cells. Photosynthates are vital to the root cells as
they are used in respiration to produce ATP to drive the active transport of ions in through the root hair
cells.
Once plants had resolved the issues of desiccation and gas exchange, they were becoming very
successful as an evolutionary lineage, however they many plants still relied on some amount of water
for reproduction. Aquatic plants simply release their gametes into the water allow them to fuse together
independently, this method continues in the Bryophytes, but in order to allow plants to survive in less
wet environments, they must evolve a new method of reproduction. Reproduction without relying on
water did not have to evolve straight away however, due to plant's ability to utilise clonal propagation
which allowed them time to evolve effective reproduction methods despite variation staying low.
The general concept of gymnosperm and angiosperm reproduction is that the gametophytes become
embryophytes: the gametophyte contains structures called archegonium, which contain and protect the
gametes. Eggs are held in archegonia and sperm within antheridia, the two structures often release their
gametes at different times in order to ensure plants breed with separate organisms to increase
variation. Sperm must swim along a film of water to reach the eggs, where the gametes then fuse to
form the zygote. The sterile cells of the archegonium nourish and protect the young sporophytes
produced from the fusion of gametes. Transport cells with high surface area to volume ratios transport
nutrients such a sucrose from the gametophyte to the sporophyte. Plants have become diploid
dependent.
This method of reproduction is seen in Psilotum, a genus of vascular fern-like plants which are spore
producing, the large vascular sporophyte produces haploid spores which are coated with a decayresistant polymer called sporopollenin. These spores are distributed by wind and germinate
underground where a small embryophyte forms and the archegonium can then release their gametes.
The developing sporophyte grows up into the light where they can then start photosynthesising for
themselves and stop relying on nutrients from the embryophyte's transport cells.
The next development in the decrease in dependency on water for reproduction of land plants was the
evolution of seeds in gymnosperms and angiosperms. Ovules are enclosed in a carpel, a sporophyte
layer with an tubular extension called a style with a stigma, a receptive area for pollen. In the ovule, one
of the four haploid spores forms a larger megaspore whose nuclei divides by mitosis to leave 8 nuclei.
This multi-nuclei cell is a coenocyte, the female gametophyte. Three of the nuclei group together to
form the egg.
Meanwhile, in the stamen microspores form and are coated in sporopollenin to form pollen grains.
Inside, 3 cells are formed by mitosis which form the male gametophyte, two of those cells become
gametes. The pollen grains are then transported usually by wind or animal vectors to a stigma, here the
plant can either reject or accept the pollen depending on the outcome of a series of chemical exchanges
to determine some of the genetics of the gametes. First evolved in the conifers and angiosperms, a thin
tube grows from the grain and pushes between layers of plant cells to the megaspore, in which the polar
nuclei cause the tube to bend and grow up into the carpel. Growth of the tube relies upon the
sporophyte of the plant supplying it with nutrients and so rejection can occur at any time.
Angiosperms then developed double fertilisation, in which two sperm cells are released, one undergoes
mitosis with the egg to produce an embryo. The other enters mitosis with 2 other cells in the coencyte
with a single spindle and so 2 triploid nuclei, these then mitose rapidly to produce 64 nuclei called the
endosperm. The endosperm has two purposes, one is to provide nutrients to the developing embryo
and the other is to act as a genetic buffer between the female and male genes in the embryo. In this
way, the endosperm is similar to a placenta in animals.
After considering all the problems plants had to overcome when moving to the land, it is worth
considering the huge advantages to plants becoming terrestrial which explain why the transition was
evolutionarily advantageous. For example, there is a much higher availability of light on land that under
the water where many frequencies of light would never reach the plants, this allows much higher rates
of photosynthesis to occur. Higher rates of photosynthesis allow higher protein synthesis and so growth
becomes faster. Thus, larger plants could exist which would come to dominate the majority of the
landscape of the earth.