Question
Describe the evolutionary developments that enabled the successful colonisation of terrestrial habitat by the early plant?

Plants evolved from an aquatic algal ancestor during the Devonian period. The transition from water to land had some useful advantages. Land provided a good substrate with nutrient rich soils and there were no predators in form of herbivores and microbes. Carbon dioxide concentration in air was very high and readily diffuses in plants than when dissolved in water. Sunlight levels in air were very high and these conditions favoured photosynthesis (Raven et al, 1992). However life on land presented challenges that led to morphological adaptations and to the subsequent evolution of present day plants.
Life on earth presented problems of acquisition and distribution of nutrients, preservation of water support and reproduction. The earliest evolutionary adaptation was multicellularity, differentiation and specialisation of structures. The plants evolved an aerial structure that is photosynthetic and functions in gaseous exchange. They also developed an anchorage to the substratum, soil. Meristems evolved and provided the much needed cells that differentiated and specialised into useful structures (Campbell and Reece, 2005). In bryophytes the cells differentiated into rhizoids, antheridia and archegonia, setae and capsule. In Pteridophytes they developed into xylem and phloem, fronds, roots and root hairs (Solomon et al, 2012). This differentiation led to division of labour which in turn results in high productivity and thus contributed to the successful colonisation of land by plants.
Water was now a scarce resource on land and plants were on risk of drying out. To prevent desiccation, the early plants synthesised cutin, a secondary metabolite that is used to form a thin cuticle covering the plant. This is very obvious in Pteridophytes. Bryophytes have a similar covering but it is not made from cutin. This is because a cuticle is waterproof yet bryophytes specifically the Liverworts rely on simple diffusion for water acquisition. The thin cuticle however interfered with gaseous exchange and this led to the development of stomata and accompanying guard cells. Bryophytes have pores that control gaseous exchange but these are not as developed as stoma and guard cells (Raven et al, 1992). Both the bryophytes and the Pteridophytes inhabit areas where moisture is abundant. The liverwort for example Marchantia species have developed desiccation resistance such that they turn brown and brittle under dry conditions but quickly gain water and return to a healthy green state once water is available. This means that they do not die under dry conditions but can lie dormant until enough water is available. Another mechanism developed by the peat mosses the Spaghum sp is to pack densely in mounds to create high humidity zones that have a greater water holding capacity (Russell et al, 2012). These mosses are even used in horticulture to conserve moisture around plants.
The second challenge experienced by early plants was reproduction without desiccation of both the gametes and the embryo. In water gametes swim freely in the water without drying and the resulting zygote can float in the water. On land both gametes and the zygote will dry if not protected. The early plants adapted by developing a multicellular antheridia and archegonia for the production of gametes. The sperm is the only mobile gamete and it waits until there is enough dew or raindrops to splash it onto the archegonia where it swims to the ovum and fertilise it. The resultant zygote is dependent on the mother plant for both water and nutrients. It is also protected from desiccation and infection by microbes. Because the zygotes of these plants develop inside the parental tissue, the land plants became known as embryophytes (Solomon et al, 2012). The early plant also synthesised sporopollenin a resistant polymer that is found in the sporangia that produces spores. It protects spores from desiccation and other damages.
Plants on land were prone to mutations since the amount of UV light is high because air does not filter light. Water acts as a filter reducing the photo spectrum of sunlight such that plants in water are not exposed to the mutagenic effects of UV light. To protect themselves plants developed alternation of generations. This is when plants exhibit two life phases with a haploid gametophyte that produces gametes and the diploid sporophyte that produces spores. The significance of alternation of generations is that it allowed genetic variation that produced environmentally fit organisms that were better adapted to life on land. During meiosis in the gametophyte generation, genetic variation is brought about by crossing over and recombination. The gametes fertilise to produce a haploid sporophyte. Here deadly mutations are masked if they are recessive such that they do not get expressed but are lost along the way (Raven et al, 1992). In addition to alternation of generations, the early plants biosynthesised secondary metabolites called flavonoids. These absorb the extra UV light hence they are called internal shields. The cuticle is also shiny aiding to reflect some of the sunlight.
Plants in water rely on buoyancy to withstand gravity. This is different to plants on land that had to develop a mechanism of support so as to counter gravity and extend vertically to increase photosynthetic advantage. The bryophytes relied on turgidity and have no specialised support systems. This has limited their height to around 20cm (Campbell and Reece, 2005). However the Pteridophytes synthesised lignin, a component of the secondary cell wall that provides support. The ferns developed a vascular system made up of tracheids elements hence their name as the early tracheophytes. The tracheids and vessel elements are highly lignified and lie end to end in long rigid pipe like structures that provide support. This allowed the ferns to grow taller since water could be transported to aerial parts of the plant while products of photosynthesis are translocated to all parts of the plants. Along with the vascular tissue plants evolved roots that aided in anchorage and absorption of minerals and water from the soil. The early bryophytes had no roots but had rhizoids for anchorage. The rhizoids did not absorb water. Some mosses like Spaghum evolved elongated cells called hydroids that aids in water transport and leptoids that aids in nutrient transport (Raven et al, 1995). The Lycophytes had fully developed roots as those found in Saginella sp that connects with the vascular system to provide support. Pteridophytes have a well developed vascular systems that is well branched to the leaves thus accounting for their success to grow to heights up to 20m (Russel et al, 2012).
Upon their colonisation of land plants had to absorb nutrients. In water these nutrients are readily available as dissolved minerals in water. On land the nutrients were not available, existing either as gases or compounds in the soil. Plants developed mutualistic symbiotic micorrhizal associations with early fungi. The plants provided a habitat to the fungi while the fungi provided nutrients that were readily absorbable by plants. An example of such symbiotic association is that found in between root nodules and Rhizobium, the nitrogen fixing bacteria. This enabled both fungi and plants to survive on sterile soils (Solomon et al, 2012).
These body forms and physiological functions later evolved in higher plants which led to the diversification of land plant. As the plants evolved further they successfully adapted to arid conditions and fully utilised the advantages of life on land despite water being a limited resource.

References
1. Campbell, N. A and Reece, J. B. (2005) Biology. Benjamin Cummings. New York
2. Raven, P. H; Evert, R. P and Eickhorn, S. (1992) Biology of Plants. Worth Publishers. New York
3. Russel, J.P; Hertz, P. E and McMillan B. (2012) Biology: the Dynamic Science. ( 8th ed) Thomson. Brookes/Cole USA
4. Solomon, E.P; Berg L.R and Martin D. (2012) Biology (8th ed) Brookes/Cole USA