Plant Science Portal 
Hurdles of Land Colonisation
 Plant Evolution Tour: Part II of XV

The ability for plants to emerge from the water and colonise land required major changes in the environment and to the algae and sea lettuces that would evolve into terrestrial plants.

Before plants could venture onto land, it was important that certain environmental changes be made to the Earth's surface. Firstly the near-shore environments needed to be of adequate size and stable. Secondly, soils needed to develop. Thirdly, the climatic conditions needed to be appropriate to support terrestrial plant life.

There was significant tectonic activity during the Cambrian and Ordovician (543-443 Ma). Rodinia fragmented and East Gondwana collided with West Gondwana, before they both collided with Laurasia to form the Pangea supercontinent about 320 Ma. There were also significant changes in sea levels due to the ending of ice ages at around 650-590 Ma, and this led to widespread flooding and the creation of shallow straits. Then, at about 440 Ma, there was another period of glaciation (another ice age) and the water levels dropped again, with ocean levels decreasing by 70 metres. It is this period of time that indisputable evidence for the first land plants can be dated back to.

By 440 Ma, a soil profile has been created which contained nutrients essential for plant growth such as phosphorous (P) and iron (Fe). These soils are thought to have been created through a weathering of rocks (some of which would have contained Fe2+) caused in part by the secretion of organic acids by microorganisms (such as alga and lichens) and acid rain produced by the high concentration of CO2 which was about 18 times the concentration of CO2 in the atmosphere today.
Well Preserved Yuknessia, Cambrian Explosion Fossil Algae
A fossil of Yuknessia, an algae from the Middle Cambrian era. Photo by PhotoMuseum.net
The problem with the high concentrations of CO2 at this time was that it limited the colonisation of land by plants, because it contributed to a warm atmosphere. It is estimated that during the Cambrian and Ordovician that summer temperatures could be as high as 40°C although there was substantial cooling of some regions by about 450 Ma.

The first fossilised evidence for terrestrial plants dates back to the middle Ordovician to the Silurian (470-430 Ma) where fossils contain evidence of specialised cells which would function for carrying water and nutrients as well as other morphological features such as structural support and protection from desiccation. Since terrestrial plants are not living in an aqueous environment, it is critical that they have a vascular system and be able to control water loss. Significantly also, fossils from this period showed a sexual reproduction system which no longer relied on external water.
Moss
Mosses are a group of primitive non-vascular land plants which reproduce via spores.
Spores were the means by which plants were able to sexually reproduce without the requirement for external water.

This requires a concept called the Alternation of the Generations which is clearly seen in algae but exists in all plants which reproduce sexually.

The Alternation of Generations

A sporophyte diploid alga (that has two copies of each chromosome) produces gametes which are haploid (have only one copy of each chromosome) through a process called meiosis. These grow into gametophyte (haploid) plants which release spores (also haploid). These spores which are male sperm and female egg cells fuse and form new diploid plants and so the cycle continues. The process is called the 'alternation of generations' because one generation is haploid, and the next diploid and so on.

The challenge for plants that were to colonise the land was to adapt this process to a drier environment. For algae which live in the water, the process is easy because the sperm and eggs swim in the water, come into contact, fuse, and a zygote (which becomes a new organism) is formed. On the land, there is no watery medium for the sperm and eggs to travel in, and so spores evolved. Spores basically miniaturise the process in a water-filled structure. However spores still require a watery environment in order to be successful.
Alternation of the generations
Alteration of the Generations. (Illustration from Willis & McElwain (2002), fig. 3.6, p.50.)
In modern vascular plants (called tracheophytes) the vegetative sporophyte generation is the plant that you can see, the gametophytic generation is the male pollen or female eggs. In non-vascular plants (bryophytes such as mosses and algae) it is the gametophytic generation which is seen.

The gametophytic generation requires more water for survival, and for the travel to and fusion of sperm to egg and initial survival of the new sporophytic zygote. Hence Bryophytes which have a dominant gametophytic phase tend to occupy consistently moist environments while tracheophytes occupy many environments and grow much larger as it is the sporophytic generation which is dominant.
Moss
Macro detail of moss.
In addition to the modified reproductive systems, plants also needed a number of other adaptations before moving onto the land, such as cuticles and structural support.
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