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Pioneer Plant Adaptations in Dune Systems.Colin Smith, Edge Hill College
Any visitor to the local beaches and dune systems can't help but notice the way in which the vegetation changes from the strand line at the top of the beach, where almost nothing grows, to the first signs of a plant community as the dunes start to build. In considering this it might help to give some thought to the problems facing such plants, pioneer species, in this fairly hostile environment. For plants in this environment hostile means a shortage of fresh water, the chances of being regularly inundated with seawater and constant battering with wind and scouring effects of windblown sand particles. Add to this the fact that moving air will de-hydrate a plant and all in all "hostile" fits the bill. But, and this is important, there are no other plants to compete with you for whatever resources there are around.
The basic requirements needed for a plant to flourish are light, carbon dioxide and water. No surrounding plants mean unimpeded irradiation. The atmosphere has all the CO2 that the plant needs, too much in fact if we are to heed the warnings of increasing greenhouse gases. This brings us to the need for fresh water. How to acquire it? And just as important, how to hold on to it? The pioneer plants we are talking about, Marram Grass and Lyme Grass, do not have deep reaching root systems but rely on condensation from freshwater in the upper sand layer to supply their needs (Crawford, 1990).
At this point it is useful to look inside the plant leaves to try and identify those physical adaptations that enable them to exist where they do. In order to do this we prepare them in the following way.
The leaves are cut perpendicular to their long axis in slices 30 microns thick. A micron being a unit equivalent to one millionth of a meter. They are stained with two dyes one having an affinity with those tissues that are lignified and give mechanical strength to the plant, the red areas. The other, the green dye, goes to the thinner walled cells that are involved in photosynthesis and gaseous exchange.
Figure 1 is a cross section of Ammophila arenaria (Marram Grass), it shows a smooth, curved outer face with a thick cuticle to resist abrasion from windblown sand and a deeply fissured inner face. The whole section has an almost circular shape. The important parts of the plant anatomy are situated on the inner face, Figure 2. We can see in more detail the numerous pits lined with photosynthetic cells. We can also see the stomata, those organs concerned with the intake of carbon dioxide, the specialised hinge cells at the bottom of the pits and the numerous hairs that line the inner face of the plant leaf. The plant's strategy to deal with life where the collection and retention of fresh water is problematic is to roll up in times of stress and hold on to what it has acquired. This is where the hinge cells in Fig. 2 are used. They can be inflated and deflated thus furling and unfurling the leaf. Furled, the hinge cells are flaccid and the plant rolls into a cylinder. The flexible hairs interlock and the movement of water molecules from within the plant through the stomata is virtually stopped. Unfurled, with turgid hinge cells, the plants can collect carbon dioxide, photosynthesise and manufacture their food that is needed for growth. Of course being rolled up prevents photosynthesis but the hinge cell adaptation is an evolved trait that enables the plant to live successfully where it does. It is an acceptable trade-off and these two grasses benefit from it.
The hairs are involved in collecting water that evaporates from the sand. They provide a surface upon which the water vapour can condense. Figure 3 shows how dense and flexible these hairs are. This is another adaptation that helps the plant make best use of a scarce resource.
The red stained woody tissue (lignified) provides the plant with the mechanical strength to withstand buffeting in unprotected surroundings. As can be seen this tissue forms girders that connect the inner and outer faces of the plant.
It is clear that two separate plant species have solved the problems of living in a hostile environment by using similar strategies.
These are just two of the plants that have evolved over time and developed adaptations that make them able to flourish in areas where life for a plant can be difficult but where competition with other plants is not an issue.