(Reading time: 5 mins)
In Part I, we remarked that “Plants are pioneers, colonisers, innovators and builders at least comparable to the animal kingdom, to which they are related and …. with a superior record.” We followed their emergence from the waters and their colonising of land, along with various strategies they developed for their new environment. Now we watch them constructing their very own environments and adapting to some of the most challenging climes of the earth.
Most plants have leaves, which is where the photosynthesis takes place; they are in fact sunlight collectors and the plants deploy them to best effect to catch the available sun. Quite a late development, they were flanges on the stems first before becoming appendages further out of the plant’s main body. Most leaves are intricately veined and contain many different layers and parts and although it is within them that photosynthesis takes place, strangely they are mostly short-lived and in cold seasons, even in perennial plants, all but the conifers let them fall.
The greater the volume of material created by plants, the more there was to decompose with their deaths or seasonal decline. Bacteria, already long existent on the planet, evolved to feed on this detritus and break it down into soil, which the same plants or others could turn to their advantage as a medium in which to anchor but also from which to draw nutrients. Other organisms evolved to live on and break down cellulose too, the main building material of plants: fungi, gastropods like snails and slugs, woodlice, termites …..
The plants, with the help of bacteria and other organisms, were creating the environment below them!
But they were and are doing more than that: they are also creating an environment immediately around them. The most concentrated examples are perhaps rain forests, tropical, temperate or cold-climate, retaining a surrounding moisture-laden air, in which not only the local tree species thrive but also providing ideal environments for ferns, algae, orchids and epiphytes and, of course, mosses.
Away from forests, sphagnum moss creates a mini-atmosphere around itself and as generations die, their bodies create a spongy moisture-laden medium. This bog is quite capable of existing on an incline, with much of the water being retained by the vegetation and ‘soil’, as may be seen in a number of examples in Ireland, such as parts of the Dublin and Wicklow Mountains.
Plants, especially trees, discharge oxygen into the air and consume carbon dioxide during the daytime, for which reason they are sometimes called “the lungs of the world”. They have not only created an environment for themselves, below, around and above but also for so many other life-forms – including ourselves.
LEARNING TO LIVE IN DIFFERENT CLIMES
Creating one’s climate isn’t always possible and, when it’s not, adaptation is the other option. Plants that adapted to grow in arid areas developed fleshy ‘leaves’ and often stalks, in which to store water and also sometimes long tap roots to find that water. But extensive shallow root networks are good too, to collect the occasional rain water that is quickly absorbed into the soil or otherwise evaporates. The “pores” on leaves through which plants absorb carbon dioxide and allow the gas-exchange necessary for photosynthesis (stomates) also permit evaporation of water, hence many dry-condition plants have fewer of them. Some only open to collect carbon dioxide in the cool of the night and store it for use on the following day. Plants grow trichomes, tiny bristles, underneath their leaves but some arid-dwellers grow them also on top of their leaves; these ‘trap’ a layer of air that prevents or slows evaporation.
In very wet areas, plants learned to remain active by a number of strategies. Of course they originally came from aquatic environments but for some of them, returning there again after adapting to dry land, produced challenges (think of the changes necessary for land mammals to evolve into seals, otters, dolphins and whales). Nevertheless we have lillies growing in shallow water with wide floating leaves, reeds with upright blade-like leaves growing inside the water margins, thin spears of rushes in damp and water-logged land. That too is the preferred environment of some other plants and grasses, including the rice plant. And of the willows, alders and hazels growing on the banks and stabilising them. In the tropics and semi-tropics, mangroves do a similar job to willows but on a much grander scale – and they tolerate seawater too.
The alder, a tree with a high toleration of water around its roots, is thought to have been the major post-glacial coloniser of Ireland, following the retreating ice across the land. It is the only native tree which though not an evergreen produces cones, an indication of its early adaptation to cold climate. Cones, when closed, protect the seeds inside against continual freezing and thawing and, when the cones begin to dry and automatically open in spring and summer, allow the seeds inside to drop out to the ground, to be carried by river or on the wind. A closed cone collected and brought home will open as it dries; shake it then and the seeds will fall out. Alder timber, incidentally, remains waterproof for centuries, witness the wooden piles in Venice.
Adapting to cold seasons required protective materials, structures and timing. The deciduous trees (and it is worth noting that many trees have both a deciduous and an evergreen version for different climes) shed their leaves and close down for the winter, the sap retreating down to the roots. Were the sap to remain in the exposed branches it would freeze, expand and destroy them. The leaves drop because they no longer receive anything from the tree; it is going into a kind of hibernation, in preparation for the coming winter.
Many of the conifers have downward-sloping branches, to allow most of the snow to slide off, rather than break the branches with its weight. People who live in areas with heavy snowfall also tend to live under sharply sloping roofs. The “leaves” of the conifers are small, narrow and hard so that most snow falls through them and are also covered in a waxy polymer to withstand freezing. The plant cells can be emptied of water to prevent freezing but a dense waxy residue keeps them open for refilling. So, of course, they have to be tolerant of dehydration. Concentration of sugars also lowers the freezing point and small flexible conduits for water resist the formation of large ice bubbles that can burst those “pipes”.