Talk Report – Sept 2018
Plants and light
Living and gardening in the fynbos, we are very aware of the impact of the specific soil conditions of our gardens and the need to provide protection from the wind, but we may not give much thought to the amount of light that various plants require. That is unless your home is close to the mountain, where you are in shade for a large part of the day, particularly during the winter months when the sun is low on the horizon.
Sunlight and its effect on plants was the topic of the interesting talk given on Saturday 15 September by Chris Whitehouse, who runs the Philipskop Mountain Reserve, near Stanford.
Chris began by reminding us why the leafy natural world around us is predominantly in harmonious shades of green. It is difficult to imagine what it would look like in any other colour, but since plants absorb light, how is it that leaves are not dark or even black? Why green?
Plants get energy for growth from light during the process of photosynthesis, during which chloroplasts in the chlorophyll of the leaves absorb light waves and convert them into energy. Two kinds of chlorophyll in the chloroplasts absorb light – from the red (longwave) and blue (shortwave) regions of the visible colour spectrum. The green light waves, however, are not absorbed, they are reflected, thus leaves appear green. Seasonal colour changes occur as chlorophyll levels in the plant decrease towards autumn and as fruits ripen.
Chloroplasts visible in the cells of Bryum capillare, a type of moss.
Chris illustrated his talk about how plants use light with pictures and examples of many different species and diverse growing conditions, from evergreen fynbos to kelp forests in the ocean. All plants need light to grow, but their needs differ and all light is not the same. In the absence of natural light, fluorescent light waves can be absorbed because they are in the blue spectrum, but yellowish tungsten light is not.
The diversity of red algae. Red algal chloroplasts are characterised by phycobilin pigments which often give them their reddish colour.
The diversity of green algae. Green algal chloroplasts are characterised by their pigments chlorophyll a and chlorophyll b which give them their green colour.
Leaf shapes vary immensely, because they maximise adaptation to conditions of light and heat. Leaf types range from broad and feathery to thin and spiky, and when a plant lacks leaves altogether, light is absorbed through the stem. We could say ‘form follows function’ in the leaves of plants just as it does in good architecture and design.
In temperate climates, or jungle conditions where light may be diffused, leaves have large leaves to capture maximum sunlight. Having a large surface area ensures more light hits them. To survive in arid desert conditions such as the Knersvlakte or Namib, however, plants have found ways to keep themselves cool (and avoid predation) by developing tough, leathery or spiny leaves. Some even alter their orientation to minimise the onslaught of direct and reflected light.
The bababoudjies (Argyroderma) in the Knersvlakte are prominent dwarf compact plants resembling pebbles and grow throughout on the quartz gravel. Their silver-green or grey leaves reflect the sunlight.
In this age of energy conservation, we are acutely aware of the value of sunlight and the benefits of harnessing it effectively. We capture its rays for heating and orientate and design our homes to give us maximum light while keeping the interior comfortably cool. Learning to live off what nature provides is called ‘going green’!
As Chris’ talk illustrated, the green world can still teach us a thing or two!