Download Describe the problems generated by life on land for the green plant

Describe the problems generated by life on land for the green plant lineage. How
have these problems been overcome?
Travel back in time nearly half a billion years and the planet would be
unrecognisable. Atmospheric CO2 levels were 15 times higher than today and terrestrial life
was rare – although fungi were common, which, as we shall see, is of significance. In water it
was a different story: in the oceans, most of the present day animal phyla had appeared, and
freshwater was home to primitive plant species similar to the Charophytes (stoneworts) of
today. These ‘plants’ were faced with a great opportunity – to leave the water and make use
of the abundant atmospheric CO2 and direct sunlight. Adapting to this new environment
would convey a large selective advantage, although many large obstacles would need to be
overcome.
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Plants first appear on land in the Ordovician (450 Ma), with spores similar to those of
Liverworts. These are the most water-dependent plants around today, although they
can suspend their metabolism during dry periods. Gas exchange – simple air pores.
Next major breakthrough was the development of a waxy hydrophobic cuticle –
waterproof. However its impermeability makes gas exchange difficult. Stomata
developed in the clade comprising all green plants except liverworts. Cuticle and
stomata enabled movement into slightly drier areas – although the hornworts and
mosses still rely on water for reproduction.
Need an extensive root system to cover a large surface area to get water/minerals
from the soil. This necessitates a transport system to carry water up the plant.
Vascular system developed: lignin (phenylpropanoid polymer) and cellulose. Lignin
was probably already used in primitive charophyte-like ancestors for its antifungal
properties. This same feature means cells cannot survive with a lignin coating so
undergo apoptosis. A hollow tube is left, through which water can be transported by
cohesion-tension. Gives compression resistance but with lateral flexibility.
Plants grew bigger, anchored by roots and supported by lignin, and so the phloem
evolved to transport large quantities of photosynthate (mainly sucrose) to both the
roots and stem tips. Suddenly plants could exploit the world in three dimensions,
and light competition would now drive plants to become bigger and bigger.
Even with extensive roots, many minerals (especially phosphate) are found in very
low concentrations in the soil, as well as being difficult to solubilize and mobilise.
Plants evolved a way around this early on (420Ma, in the Devonian). Symbiotic
relationship with a group of fungi (Glomales) to form the Vesicular Arbuscular
Mycorrhiza (VAM): fungal hyphae force in between epidermal cells of the plant and
into the outer cortical cells, without penetrating the plasma membrane. Highly
branched hyphal mass has an enormous contact area between the plant and fungus.
The fungus gains certain carbon compounds, and returns the favour by mobilising
phosphate from the soil, producing plant growth hormones, and protecting from
pathogens.
Vascular plants well established by the late Palaeozoic, with vast forests of large club
mosses and horsetails producing the Carboniferous coal measures we exploit today.
As Pangaea broke up in the Mesozoic, ferns became very successful, especially on the
damp Jurassic forest floor. Major constraint is requirement of water for eggs and
sperm to fuse on the gametophyte. Hence all vascular plants have reduced this
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water-dependent haploid stage to a minimum, with the sporophyte form being
dominant.
Major step forwards arrived with the development of seeds. This was achieved by
keeping the gametophyte attached to the sporophyte. The main part of the plant is
the sporophyte, which produces haploid spores. These grow into gametophyte tissue
still attached to the sporophyte, and produce gametes. These gametes undergo
fertilisation, and the tissue that surrounds the embryo that develops hardens to form
the seed. This is desiccation resistant and can remain dormant for tens or hundreds
of years.
Crucially, the male spores (microspores) are coated in sporopollenin to prevent
desiccation, and in the case of angiosperms are transported by pollinators to the
female organs of other flowers, where the gametophyte forms to produce gametes.
Water is not necessary because the egg and sperm meet in an internal environment.
This internal fertilisation makes the gymnosperms and angiosperms much more
independent of liquid water, and the seed means that in extreme drought conditions
the plant can wait for an improvement before it germinates – so seasonally arid
climates can be colonised.
The first successful seed plants were the cycads, forming much of herbivorous
dinosaurs’ diets. Later other gymnosperms like conifers would become dominant –
and still are to this day (covering 1/3 of Earths land surface as the Taiga forests – one
of the driest habitats). In the last 140My though angiosperms have appeared and
have found niches in almost all ecosystems, with cacti for example in some of the
world’s driest places.
It is not usually helpful or accurate to see evolution as making progress – but the extant
green plant lineages seem to represent just that, at least in terms of moving away from
water: from the stoneworts and liverworts right up to the angiosperms. We are very
fortunate to have such a series of relict species, for example the one remaining Gingko, to
fill in the gaps in this vital evolutionary transition. In botanical research we are in the
process of a rather different and exciting transition: from a mainly morphological
approaches to a phylogenomic one. More readily available DNA sequence data will no
doubt enable us to figure out the more intricate detail of how land plant evolution has
literally changed the face of our planet in the last 450 million years.