It is often brief glimpses or incidents that leave the most indelible impressions on human minds. Springbok are so named because of the occasions they are observed ‘springing’. The name ‘trekbokke’ was used to describe the relatively few occasions they were observed migrating in aggregations of millions.
The accounts of early observers (of ‘mega-herds’ three miles wide and three and a half days long - estimated at a density of twenty one thousand animals per acre) left an impression that is preserved in the name to this day. The great herds are long gone, destroyed by human carelessness and greed, like the bison herds of America and the wildebeest herds of Botswana.
The east African migrations continue because of the lack of veterinary and international boundary fences and are the last of these large animal aggregations on the African continent. As with animals, plants also produce brief but lasting impressions.
Many of our trees are named after seasonal shows of colour or occasional use by animals. Jackalberry, pigeon-wood , camelthorn and others by their association with such animals. Red bushwillow and other trees by their brief colouration before losing their leaves in the dry season.
If you have not seen the Combretum apiculatum in its ‘autumn’ colours it is hard to understand where it got the name red bush-willow for it is neither a willow nor is it normally red. The name ‘willow’ or ‘wilge’ was originally given to the Combretum erythrophyllum which was first encountered in the Cape, normally growing along river lines and looking like the European willow tree.
The name willow has stuck to all of the genus Combretum - even to those that look nothing like a willow tree. The beginning of the names Erythrina and erythrophyllum mean ‘red’ - the former to the flowers and the latter to the brief, autumn leaf colour.
Although very few, if any, African examples can compare with the ‘fall’ or autumn colour displays of the deciduous forests of North America, Canada and parts of Europe, we do have some impressive shows. The spring colouration of the new ‘msasa’ (Brachestegia spiciformis) of the ‘miombo’ woodlands to our north are some of the most extensive. Of the autumn shows, our lowveld species of Combretum probably give the best of the brief displays and now is the time to appreciate it.
Depending on the temperature and moisture of the various various areas, many trees are now showing beautiful colour - especially with the sun back-lighting the leaves. In the mountain forests the Combretum kraussii (forest combretum) has bright red leaves. The tambotie has brilliant red leaves near the crown.
The Kirkia wilmsii (mountain seringa), which is endemic to the Limpopo province and, although scattered near the base of the escarpment, can best be seen on the Able Erasmus dolomites south of the Strijdom tunnel, gives a wonderful show of red, orange and purple leaves as one views them towards the afternoon sun. The colourful display does not last for long. Many other plant species are colourful in their own time.
Green - Yellow - Red
Many botanists will understand the mechanisms involved in the leaf colour changes but other folk may be interested in a brief account of the process. Within the leaf cells there are organelles called chloroplasts. These chloroplasts contain grana which are the exact location of chlorophyll and other pigments that capture light - the primary energy source of plants and , ultimately, of all living things.
The leaves of higher plants contain various types of photosynthetic pigments. Chlorophylls exist in two forms - chlorophyll A and B - both of which are green. Carotene is an orange-yellow pigment that is also abundant in carrot roots, and several xanthophylls range from shades of yellow to almost colourless, depending on their molecular structure.
In the light reaction, chlorophyll B, carotene, and xanthophylls absorb and channel light energy to chlorophyll A, whose electrons (negatively charged) are boosted to a high energy potential. In such an energised state, chlorophyll’s electrons are diverted into a system that extracts and stores their energy for later use in the synthesis of substances such as sugars.
A complicated chemical sequence of splitting molecules and exchanging electrons is all completed within a fraction of a second. During the second phase of photosynthesis, carbon-dioxide fixation, carbon dioxide from the atmosphere unites with a sugar.
Although carbon-dioxide fixation is slower than the light reaction, millions of molecules are synthesised within minutes after light enters a leaf ’s tissues. Apart from starch and cellulose, thousands of products are formed in details of enormously complex chemistry.
When leaves turn yellow, they simply lose chlorophyll that had previously masked the appearance of the orange and yellow carotene and xanthophylls. A purple-red pigment, called anthocyanin, may also be present in the same species but does not participate in the photosynthesis because it is stored in cell vacuoles. A rainbow’s spectacle reveals that sunlight is composed of several colours.
Of these, red and blue are captured by chlorophyll, whereas carotene and xanthophylls intercept only the blue-green part of the visible spectrum. At wavelengths represented by these colours, the energy of light is transferred, via the pigments, into the synthesis of foods. Auxin, gibberellin and another hormone named cytokinin maintain the functions and structural integrity of cells - including the existence of chlorophyll and other pigments.
Chlorophyll, among other cell structures, is built up with assistance from various minerals introduced through moisture from the roots. Acting antagonistically against the auxin and other hormones, two other growth regulators promote the ageing process. External environmental cues, such as seasonal changes in temperature, day length (plants actually measure lack of light), lack of moisture, and internal (biochemical) signals shift the balance between the two sets of hormones.
The senescence-promoting (breakdown) substances are ethylene, a gas, and abscisic acid, so called because it was originally thought to promote leaf abscission in all species; later, it became clear that most often ethylene controlled the separation process.
When fruits ripen various biochemical events take place. Colour changes result from the breakdown of green chlorophyll and an increase in the yellow, orange, and red pigments. Leaf senescence, prior to abscission (cutting off), includes the breakdown of chlorophyll and weakening of the cell walls at the base of the petiole, in a narrow band of cells called the ‘abscission zone’.
In spring and summer, auxin produced in the leaf keeps the abscission zone intact - but low temperatures and short days in a u t u m n (and in our case - lack of moisture) cue the leaves to reduce auxin production and increase the liberation of ethylene.
The latter stimulates an enzymatic breakdown of cellulose walls and pectin in the middle lamellae - the glue holding cells together. The restriction of ethylene’s destructive effects, only to the cells in the abscission zone, illustrates the precise control plants exercise over their hormone systems.
Prior to leaf abscission, some of the nutrient elements (including nitrogen, potassium and magnesium) are released from their bound form in protein, chlorophyll and other molecules and transferred from the leaves to the plant’s growing tips for reuse. This mineral relocation is still in progress when the older leaves turn yellow.
So in layman’s terms - when plants receive signals of low temperatures, short day length or reduced moisture conditions, they change their chemistry, withdraw nutrients from the leaves, the leaves change colour and eventually die.
Sealed off at the abscission zone by a corky layer, the leaf is weakly attached and ready to be blown off by the (August) winds. After the dormant period, rising temperatures and soil moisture and longer day length stimulate the tree to grow again. These dormant and active periods give rise to the ‘annual’ rings in the woody parts of trees.
Key To Life
Autotrophic plants hold the key to life on Earth. They alone are the intermediaries between the sun and all other creatures. It is their leaves’ microscopic chloroplasts that have the awesome responsibility of making the system work.
It is about time that humanity woke up to the fact that wherever they destroy green plants by overgrazing, burning or clearing - they are partisan to the destruction of the Earth and their own future existence. The red leaves of autumn may only be around for a short time but they give such pleasure to all who bother to appreciate their beauty.
The impression of their brief appearance is indelible in their names and in the minds of all who encounter them. People, like leaves, can choose to join the mass of green in their invaluable but routine daily tasks - or they may choose to be colourful and pleasing to others for a moment.
The ‘Red Leaf’ people will understand that a happy smile, a kind deed or a word of encouragement will live on in the minds of those they encounter long after their nutrients have returned to the dust of this earth. “We blossom and flourish as leaves on a tree - and wither and perish but nought changeth Thee”.