Tuesday 1st February. Presentation to Mr. Bob Drummond. We are combining with the staff of the Herbarium to make a presentation to Mr. Bob Drummond at 4 p.m. at the Herbarium in the Botanic Garden. While tea is served there will be an opportunity for members to chat to Bob. Contributions of cakes or the occasion will be much appreciated.
Sunday 20th February. We are pleased to be re-visiting one of our favourite haunts in Chegutu – Ijapo Farm, as guests of Brian and Kerry Welburg. This is a wonderful spot on the Mupfure River full of botanic interest. We look forward to meeting our Chegutu members and friends again. Brian and Kerry have kindly offered to serve tea at 9.15 before we walk down to the river and gorge.
Saturday 26th February. Botanic Interest Walk. The Blatherwick Avenue entrance to the Mukuvisi Woodland at 3.00 p.m.
Tuesday 1st March. Botanic Garden Walk
February 4th – 6th. At Gwayi River. We will have contacted you by now about the details if you are coming.
Sunday 20th February. This is an extra, which we would like to put in the calendar, to give an outing for those who will not be going to Gwayi. There is still so much more to see at Roy Stevens’ property, especially after this good rainy season, that we have asked if we could return. He has kindly agreed to our request so we will leave Girls’ College car park at 8.30. Please prepare for an all-day trip.
Sunday 6th March. North of Bulawayo in the Insiza area to the Brownlee-Walkers. Details next month.
April 1st Easter. West Nicholson area for Easter. Will anyone in Harare be interested, if you are please, phone Maureen Silva-Jones on 739711 so that accommodation can be arranged?
The Urban Trail. After three years at the Hillside Dams I feel we have more or less exhausted the potential of the area, in terms of my own limited expertise, and I feel it is perhaps time to move on to new things. With this in mind I am also thinking of a new time, a new venue and a new voice and therefore float the following ideas for: –
a) Venue – the Circular Drive area
b) Time – the second Saturday or Sunday of the month at 9 a.m.
c) Subjects – succulents, annuals and grasses.
Please let me know your feelings. So there will be no Hillside Dam walk in February.
Saturday 1st January 1994: The Annual New Years’ Day Social in Bulawayo. For the second year Doc and Mary Raub opened their lovely home in Khumalo to us and for this, their generosity and warm hospitality we thank them most sincerely. The violent squall that interrupted our pre-lunch drinks sent one and all scurrying for shelter on the wide verandah, but otherwise failed to quench the jolly spirit of the occasion. We thank Angela Sykes, Mary Raub and Margaret McCausland for coordinating the event and laying the foundations for a very successful party.
Sunday 2nd January.
A glorious morning dawned for our trip to Roy Stephen’s Shumba Shava Mkulu farm, nestling in the eastern Matobo. This unique area is one of the most consistently high areas in Matabeleland, with the granite kopjies rising to over 5000 ft and then swooping down to the Umzingwane and Mtshabezi valleys. The rainfall is above average as it is driven onto the sheer rock faces and winter mists and heavy dews are not uncommon. These wet conditions create unusual microclimates and perfect niches for a wide variety of plants and there are many sponges, vleis and perennial streams on the farm. Shumba Shava Mkulu is home to breeding pairs of Peregrine falcons, Black-breasted and Brown snake eagles, migrant Wahlberg’s eagles and Yellow-billed Kites. The persistent calls of the Red-chested cuckoo and Rufous-naped lark were heard all day long.
A leisurely walk up a granite dwala revealed a spectacular view, a remnant gravesite from a bygone era, Faurea saligna in blossom, Ficus abutilifolia, Canthium lactescens, Brachylaena rotundata , Maytenus heterophylla and some earnest discussion on Elephantorrhiza sp. by the more learned amongst us!
An interesting and scenic walk, winding through perennial streams, sponges and woodlands was a botanists ‘ delight. Among the most unusual trees noted were Ficus verruculosa growing very close to a Halleria lucida (very rare indeed), Turraea obtusifolia with its lovely honeysuckle white flowers and quite unlike Turraea nilotica, Boscia angustifolia and Calodendrum capense in magnificent pink flower. A superb Albizia tanganyicensis with its orange bark, Catha edulis (used prodigiously by the Somalis and Kenyans as a narcotic). Clerodendrum myricoides in pretty blue blossom, Euphorbia matabelensis, Heteropyxis dehniae, Olea europaea, Pavetta eylesii, Pittosporum viridiflorum, Protea gaguedi, Securidaca longipedunculata and Vepris reflexa were also interesting finds.
Lunch was taken amid raindrops of a passing shower (Matopos recorded 70 mm in parts) and followed by a walk into a vlei area to look at wild flowers. Anacampseros rhodesiaca with its fragile pink and white flowers, Moraea sp., scarlet pimpernels, orchids, irises, yellow stars, stud thorns and African foxgloves added colour and beauty to a wonderful day.
Our sincere thanks to Roy Stevens for his hospitality and generosity and for sharing his intimate knowledge and enthusiasm of the Matobo.
THORA’S HERBAL HINTS
Lavender Scented leaves.
LAVENDER – Lavendula spica (Lavendula officinalis) and Lavendula dentata.
These herbs look good in the garden and smell divine. For thousands of years they have been used not only for their fragrance but also as a disinfectant, vulnerary, sedative, stimulant, tonic and carminative. The Romans used lavender in their bath water (its name in fact derives from the Latin lavare – to wash), satchels of lavender were placed in chests and cupboards and lavender oil was brushed on bedsteads to get rid of bugs and rubbed into children’s hair to kill lice
A dog if bitten by a viper was rubbed with a handful of lavender to save its life – the essential oil is a powerful antiseptic and a remarkable neutralizing agent of venom. In the 16th Century laundry women dried their washing on lavender hedges and lavender water was always sprinkled on washing before ironing. How lovely!
It is the flowering tips, picked before fully in bloom, that form the basis of various preparations. A mild infusion is sedative and anti-spasmodic and therefore prescribed for disorders of a nervous origin – insomnia, poor digestion, migraines and irritability.
A stronger infusion is stimulant, sudorific, tonic, disinfectant and diuretic. Can be prescribed for laryngitis, bronchitis, whooping cough, asthma, chills, influenza and feverishness.
Lavender oil can be made by macerating a handful of fresh flowers in a litre of olive oil in a transparent jar in the sun for three days. Strain and press, then repeat the operation till the required strength is reached. I think Vitamin E oil works equally well and is excellent for the skin and not quite as expensive. This oil can be used for migraines and headaches rubbed on the forehead and taken internally – 6 drops on a lump of sugar. Also a good remedy for vertigo and digestive disorders. The lotion is good for burns, eczema and congestion of the lungs. Rubbed into the hair it strengthens and cleans. Lavender is said to promote hair growth, is an excellent rinse and a quick setting lotion. Use 2 cups lavender flowers and leaves
2 litres of boiling water
Boil then strain.
Use this lotion in the bath as well. It is stimulating and acts as a deodorant.
Make a small lavender pillow to induce sleep – fill with dried flowers and foam chips.
Lavender flowers and leaves chopped up and mixed, impart a special subtle flavour to many sweet dishes, in tea, fruit drinks, cake icing, pancake and sponge cake mixes.
This is equally true of the scented geranium leaves.
Try making lavender syrup – boiling flowers and sugar together then adding it to your favourite ice cream recipe. Delicious!
All old woody stems of your lavender plants can be put in a tub and covered with boiling water then allowed to cool in order to make, when strained, a delightful fragrant foliar spray for your precious pot plants and garden flowers and vegetables.
THE LAVENDER TREE – Heteropyxis dehniae
This tree was renamed in honour of Mrs. Gertrude Dehn who made extensive studies of it in the Rusape and Marondera areas. It is a fragile dainty tree, lovely at all times of the year and deserving an honoured place in the garden.
African tribes have used it in herbal medicine and have also appreciated its fragrant perfume.
A scented tea is made by pouring boiling water over the leaves and sterns, and then strained to relieve heartburn, colic and flatulence. It is a tonic tea, strengthening and reviving, given to old people, to refresh weary travellers, and to help a new mother regain strength. It also aids digestive upsets and colds.
A strong brew is used as a wash and deodorant – very good in the bath – fragrant and invigorating.
Crushed leaves added to Vaseline treat cracked heels and dry skin on hands and feet – relieve tiredness and soften calluses. Rubbed on to pillows, the crushed leaves help to keep mosquitoes away. Dried and powdered leaves are used as talc.
The leaves and twigs boiled make an excellent inhalant to clear the nose and chest.
This brew is also used as a mouthwash for gum infections and toothache.
A strong brew is made to keep ticks off dogs and cattle, goats and donkeys, and for treating bites and scratches on these animals. The dried leaves make an excellent ingredient for potpourri.
LAVENDER CROTON – Croton gratissimus
The branches of this tree when crushed are pleasantly aromatic and are dried and powdered by the Bushman ladies to make a perfume. The charred bark is used to treat bleeding gums but this tree is believed to be toxic, a Euphorbiaceae.
It is also known as the hairy lavender fever berry.
The palm tree, according to the Rev. E Cobham Brewer, LL.D (Dictionary of Phrase and Fable, 1906) is said to grow faster for being weighted down. Hence it is the symbol of resolution overcoming calamity. And there is an Oriental legend that the palm tree sprang up from the residue of the clay of which Adam was formed.
Palmy days are prosperous, happy days, as they were for the victorious Roman gladiator who went to receive the palm branch as the reward for his prowess.
Sad Palm Sunday, 29 March 1463, was the day of the battle of Towton (near York), the bloodiest of all the battles in the War of the Roses, when more than 37 000 Englishmen were killed.
The palms in all of these tales undoubtedly referred to the date palm, Phoenix dactylifera, but there are more than 1110 species of palms in the world and we saw a fair sample of them during the Botanic Garden walk with Tom Muller on Tues. 7 December. A note on some other notable palms in Harare might be of interest.
The well-known Washingtonia robusta in Julius Nyerere Way (formerly Kingsway) seem to have got there more by accident than by design. The first decade of this century saw a tremendous struggle by Salisbury’s Town Engineer, Mr. de la Poire Crawford Lindesay, to tame an unnamed stream that divided the Kopjie section from the Causeway section. This he eventually achieved by channelling the stream into an underground drain and building a dual roadway above it with a four-metre boulevard in between, and the boulevard was to be planted with linden trees for shade. What the linden trees were is not clear, perhaps the English linden, Tilia x europaea (Tilia vulgaris), and it is interesting to speculate on how well it would have done here.
The road was named Broadway, in keeping with its great width, but in November 1910 it was renamed Kingsway during the visit of the Duke and Duchess of Connaught. On 27 January The Rhodesia Herald reported that tree planting in Kingsway was going ahead and that convicts were preparing the holes for the trees – presumably the lindens. There is no record of what happened to these trees, but in the early 1920s H.E.V. Pickstone and Brother Ltd., of South Africa, presented to the City Council the Washingtonia palms that now grace Julius Nyerere Way. Fifty palms stand in the two-block stretch between Baker and Samora Machel Avenues, but gaps left here and there by missing palms suggest that as many as seventy might have been planted originally.
Washingtonia robusta is a Mexican palm from Baja California and Sonora, where it reaches heights of 25 metres or more. The closely related Washingtonia filifera, from the United States is somewhat shorter, but with a thick trunk of around one metre in diameter, it is a less popular ornamental than the slender Mexican species.
There are 20 or more species of Livistona from tropical Asia and Australia. The genus was named after Patrick Murray, Baron of Livistone, who founded the Edinburgh Botanic Garden. One wonders why it was named Livistona and not Livistonea. Both Livistona chinensis and Livistona australis are fairly common in Harare but, sadly, one of the finest specimens of Livistona australis in the City was removed a few years ago to make way for the new offices of one of the parastatals.
Livistona australis comes from the east coast of Australia from the Victoria/New South Wales border northwards to Fraser Island off Queensland. It reaches heights of around 25 metres and diameters up to 35 cm. It is known as cabbage-tree palm from the edible cabbage-like material at the apex of the stem.
The pink-flowering palm that we saw is Archontophoenix cunninghamiana, known in Australia as bungalow palm or picabeen palm, both bungalow and picabeen being Aboriginal names. This palm has much the same natural range as that of Livistona australis, perhaps extending a little further north and not quite so far south. The wood of the trunk is very hard and heavy, with a density of around 1000 kg per cubic metre, so it will just about sink in water. The wood is used for small ornamental articles, inlay work, walking sticks, umbrella handles, batons, etc.
Archontophoenix cunninghamiana is known outside Australia as the King Palm, probably because of its regal appearance when full grown to its mature height of 20 metres or so.
On his Zambezi expedition of 1858-1863 David Livingstone occasionally used Borassus aethiopum for firing the boilers of his steamboat. He called the species Palmyra, the name for Borassus flabellifer from India and Sri Lanka. Livingstone even left us a record of a measurement of one specimen – his journal entry for 9 January 1859 reads; “Palmyra 75 ft. long, 19 inches diameter at two feet from the ground, 21 in. at bulb.” And on 10 August 1859: “Got three Palmyra trees, the cutting up of which, into small pieces detained us till 1 p.m. and they steamed us for three miles or to about a mile short of Shupanga house.”
Borassus flabellifer is cultivated in India and Sri Lanka, and its uses are legion; an old Tamil song lists 801 of them! Our word toddy is probably derived from the Hindi tari, the juice of the Palmyra palm, from which an alcoholic drink is made.
Palms in Manica Province of Mozambique
It was with interest that I read about Tom Muller’s garden walk in the January Edition of Tree Life. Tom’s walks are always interesting and it prompted me to start writing about some of the trees and vegetation that I have seen in the Manica and the Sofala provinces of Mozambique over the past three years in the course of my various wanderings. I have been meaning to write some of what I have seen for Tree Life, but due to a somewhat chaotic existence I never got around to putting pen, or word processor, to paper. What better trees to start with than palms and pandans!
Like in Zimbabwe Phoenix reclinata has a very wide range of habitats; I have seen it growing from the mountains along the border to coastal dunes and practically right up to the beaches along the coast. It is very common along the Pungwe Flats, in the Mossurize and Buzi Valleys nearly always near water.
Raphia farinifera is probably more common in Mozambique, than in Zimbabwe, though it is not wide¬spread but locally common along rivers, and its seed appears to often be spread along river courses after it has flowered and died. A few Mozambican farmers actually plant it near their machambas or farms – presumably for raffia to use in basketwork or similar purposes. There is a very good stand of Raphia farinifera by a river about half way between Vanduzi and Catandica (formerly
Borassus aethiopum – known in Mozambique as the Palmeira – is wide spread and is often planted near villages. It is particularly common in Barué District, but can be seen around Manica, Bandula – near the main Beira Road, in the Mossurize Valley and in numerous other places. It appears to be adapted to a fairly wide range of environments. The easiest way to tell the difference between Borassus and Hyphaene is that in the latter the fan leaves have a very bluish cast, whilst in the former the leaves are normally quite a darkish green. The fruit is also much larger. Lastly Hyphaene normally only grows in fairly dry areas – by Mozambican standards that is – whilst Borassus grows in both areas of high and low rainfall.
Hyphaene coriacea is not so common; I have seen it in Mossurize, Caia, Sena, Tambara, and the Pungwe Flats and in the Gorongosa Game Reserve. The differences between it and Borassus are described above.
Around Gondola, Amatongas and Doroe the Oil raffia, Elaeis guineensis, appears to have escaped cultivation and occasionally can be seen growing in the woodland forest mosaic that is characteristic of that area. There are some fine specimens – planted – on the northern margin of the road by the Amatongas Mission, and it is commonly planted around the City of Beira. From about this same area the Coconut Palm – Cocos nucifera – starts to look respectable and can occasionally be seen around villages, though it does not become proliferous until just before Dondo is reached.
Lastly something special – although not a palm a close relative and member of the Pandan or Screw-pine family Pandanaceae – Pandanus livingstonianus. These are very common along certain stretches of the Lucite (Rusitu) River, between the Sitatonga Mountains and Dombe, and probably further down stream. They are gregarious along the river bank near the site of the old bridge on the Gogoi to Dombe road, which is a very beautiful spot with magnificent view towards the Chimanimani Mountains some 30 km to the west. The Pandans reach considerable heights. Having only seen pandans in parks and botanical gardens before, I was never really sure why they were called screw pines, but the large specimens definitely take on the appearance of a large pine tree, but of course with very different leaves. Pandanus livingstonianus also occurs along the Munhinga River near Sussundenga and Old Mavita.
Nyarupinda catchment January 1994 -1993 retrospect
Phragmites mauritianus has thrived in the shallow places in the Nyarupinda River now that it carries more water to top up the Nyarupinda dam where the frogs croaked in winter; perhaps this is the reason for more frogs on our premises. The shores of the dam have been submerged for a longer time than ever before, which has resulted in the death of coarse grasses and tough vegetation. Irrigation has lowered the level and during the past three months the shores have become clothed with shorter finer tufted grasses such as Digitaria, Paspalum and a short grass with soft spreading panicles that look misty at a distance. On higher ground very few orchids and red flame lilies have emerged to flower, the climbing Gloriosa is rampant and continues to bloom. Many geophyllous orchids have not yet made their leaf. Beautifully pleated erect solitary leaves of Nervilia sp. have appeared for the first time in the vicinity of the civetry nearest the garden; elsewhere another species with purple on one side of its leaf may be found when the ground becomes sodden. The yellow flowers of Pterocarpus angolensis and Pterocarpus rotundifolius have lasted, a long time in the absence of rain and storms, the Wattle, Peltophorum africanum has been spectacular, it is followed by the less attractive heavily scented flowers of Acacia amythethophylla. The star-like yellow blooms of Grewia monticola are almost over now. Lonchocarpus capassa is weighed down with pods.
This district is very short of rain and sharing water for irrigation is a problem. There is an alarming increase in the rat population; they are attacking crops such as maize, groundnuts and paprika. In the garden they have climbed the mango trees to eat the fruit in situ, the pips hang, all forlorn, on the fruit stalks. So far there are no reports of damage in the young plantations of fruit trees.
Today January 14th there was a Giant Rat in the shelter which houses the pool filter; this visitor seems to highlight the recent changes in the occurrence of rodents. Whilst this rat resisted removal, a yellow parasitic cockroach was seen on the fur on its belly. A new burrow excavated since there was a shower on January 11th will be investigated, its entrance is well hidden with undergrowth, and the opening is about the right size for the Giant Eat. A large quantity of soil has been moved, about two bucketful’s, on top of it amongst fine gravel there are several black droppings typical of a rodent. This burrow is some 100 metres from where the rat was found.
Surprise, surprise, a few minutes ago on returning from the rat hole something made me look back and there was a black and white flowered Ceropegia on a shoulder-high Combretum molle. It had up to forty flowers of all ages in each cluster at the nodes. Miles have been trudged to find a sight like this; here it is almost on the doorstep!
With reference to the bark spider mentioned in Tree Life 167, this species or a similar one is described by Alistair Chambers in the book entitled African Seasons, Wildlife at the Waterhole which is illustrated by Craig Bone. There is a pencil drawing of the spider (Caerostris) in repose. Further observation of the spider in the box at Tinto showed that it was nocturnal, made temporary webs in which insects entangled themselves; it reared up supported only by its two pairs of hind legs when threatened. After a week in the box the spider walked with a sprightly step to freedom spinning a thread of silk as it descended the trunk of a Mnondo tree in the garden. Almost immediately it retraced its steps and settled head downward in the base of a fissure in the bark, at eye level. This tree has been visited frequently, to catch another glimpse of the spider – wishful thinking, because it can resemble aborted branchlets, knots in wood and paired thorns that deceive predators at large in the daytime, chiefly Spider Hunting Wasps. The spider is the size of a twenty cent piece surmounted by a pair of woody prongs, this is what to look for, good luck. It had been fortunate to find this rarity on a path beside the house.
Grewia monticola and the Carder Bee.
Early in November there was a spindle-shaped woolly growth on a branch of Grewia monticola. It was a cocoon, the nest of a Carder bee probably Serapista sp. (family Megachilidae) so called because of their habit of combing plant material to produce cotton wool-like fibres that they use to construct their nest. These bees collect pollen; they look a bit like a bumble bee/carpenter bee but smaller about 2 cm long.
Mrs. Moira FitzPatrick Curator of Invertebrates at the Natural History Museum in Bulawayo remarked that there were still larvae in the nest that she had received from me; they will try to rear them and then give a definite identification. For further information on the Carder bee see S.H. Skaife’s African Insect Life.
My apologies for bad spelling in my previous letter, it must have been due to domestic blindness/drought lethargy, Aborigines and arthropods are not difficult words to spell.
This project has not caught on in Zimbabwe, what is the good, there seems to be no future in it. The field card of the trees and shrubs of Zimbabwe lists three species of Faurea and a subspecies. The Protea amount to five, possibly six species so that there is not the necessity of conserving biodiversity which is the most important aim of the Project in South Africa, where some species have become extinct and others seriously endangered. Down South there are 80 Protea species, which can be atlassed by beginners, and 350 Proteaceae species for enthusiasts.
The data recorded in the field will be of use to taxonomists who catalogue and describe species and to plant biogeographers who study the history of why species occur where they occur. To evolutionary biologists who study how plants speciate; to ecologists who study what factors limit the distribution of species; and last but not least it benefits every Atlasser who enjoyed contributing to the conservation, and understanding of their country’s floral wealth. There are 136 active atlassers in South Africa, Swaziland and Zimbabwe, by December 1993 the Botany Department at Cape Town University had received 6739 Sight Record Sheets which provided 30 321 records of Proteaceae. Data collecting began between June and September 1991. To date there are only 29 known species of this family which have not been atlassed. In two years of atlassing two new species have been discovered. Herbaria records indicate that the discovery of new species has dropped since 1950 from one species every 18 months to one species every five years.
In Zimbabwe there are some relatively unexplored places in the mountainous regions where a new species might occur. Another way to find one is when the Atlasser provides the material necessary to describe an unnamed species collected by the herbarium whose material is inadequate to distinguish the species as different. This is as good as discovering a species!
Whilst searching through the Protea Atlas Newsletters (21 of them) one important fact has evaded me, it is the total number of species to be atlassed, probably nearly 400.
KENT ESTATE 16 JANUARY 1994
For the small that party braved the rainy conditions the dry turned out to be somewhat different to what was expected; the area surrounding the ridge we intended to walk along was the territory of a large male ostrich.
Oscar (for wont of a better name) made our acquaintance early in the morning. Oscar was armed with an excellent pair of claws and was bursting with curiosity, but his presence was not really welcome although it was fascinating to watch his courtship display at very close range. We moved our vehicles to the other side of the ridge but within ten minutes the ostrich once more appeared on the scene! It took a good deal of effort to entice our curious, stubborn long-legged friend out of the fenced enclosure. Mervyn then parked his truck across the entrance – resulting in a highly frustrated bird.
A saunter up the ironstone ridge revealed a number of Ochna puberula (the granite Ochna) and a few droopy-leaved Tarenna neurophylla. Being mainly Brachystegia woodland the usual highveld species were well represented as well as a few large specimens of interesting hybrids. Some exciting finds were a fruiting Hexalobus monopetalus, Rhoicissus revoilii with tiny, almost petal free flowers and a small but striking red flowered member of the Iris family Anomatheca grandiflora.
Another interesting find was Garcinia buchananii also in fruit, whose leaves exude the typical, yellow sap when damaged.
Incidentally, we saw many little spiny spiders – later identified as Kite Spiders. According to Dr. Martin Filmer’s book Southern African Spiders, the family is Araneidae and the sub-family is (wait for it) Gasteracanthinae. These spiders are easily recognised by the bright colouring on the abdomen – usually red, black and yellow – and also by flattened spiny projections arranged laterally and posteriorly. As with the other Orb Weaver spiders the Kite Spider is sedentary and prefers to wait at the centre of its web for prey to fly up into the snare threads.
To escape the approaching rain we beat a hasty retreat down the ridge passing a heavily browsed Elephantorrhiza goetzei and a cluster of Dichrostachys cinerea. Strangely enough a plucked leaf of Elephantorrhiza goetzei smelt very similar to crated peaches!
Our feathered friend welcomed our return, fortunately from the other side of the fence by inflating his neck and uttering the strange booming sound peculiar to his species. We escaped ostrich and rain by a few minutes but after an hour’s downpour decided to head home. Many thanks to the Manager and Derek Lenton for arranging and allowing us to wander about this interesting part of Kent Estates.
WHAT IS BUTTRESSING?
Sizwe Cawe replies:
Buttresses are modified lateral roots that form wing-like expansions at the base of large tree trunks.
Although buttressing is characteristic of tropical and sub-tropical trees, it also occurs in the temperate regions. The phenomenon is not confined to any one family or group of families, but occurs in a wide range of taxa. Buttresses are of two main types; stilt and plank buttresses. The distinction between these buttress types is not absolute; both being modifications of lateral roots and structures intermediate between them are common. Some species, for example Grewia coriacea have both stilt roots and plank buttresses.
These are woody roots that spring from the main tree trunk above the ground and curve downwards and enter the soil. In southern Africa stilt roots are associated with mangroves such as the red mangrove Rhizophora mucronata, but in more tropical areas stilt roots also occur in many species of freshwater swamp forests and of normal well-drained rain forest. Some of the plant families in which stilt roots occur Include Combretaceae, Euphorbiaceae, Annonaceae, Tiliaceae, Myrtaceae, Moraceae, Burseraceae and Araceae.
Stilt roots are common in the tropics and subtropics, but are very rare in temperate regions. Within the tropics and subtropics the occurrence of stilt roots appears to vary with soil type and individual species show variation with regard to this character. Some species that are stilt-rooted in swamp forest lack these structures in well-drained soil.
The adaptive significance of stilt roots is uncertain. They appear to be of value in anchorage, especially under swampy conditions, but the fact that they are commoner in small trees, mainly growing in the sheltered undergrowth, than in tall trees with heavy crowns to support, suggests that the true explanation lies elsewhere. There is, however, a correlation between moisture and the occurrence of stilt roots. In some parts of the world the height on the trunks to which stilt roots occur coincides with the highest level reached by floods in the district.
Also known as tabular roots, plant buttresses are flat, triangular plates subtended by the angle between the trunk and lateral roots. Unlike stilt roots, which spring from the main axis 1 m or more above the ground, plank buttresses arise at, or just above, ground level. Plank buttressing occurs when the upper surfaces of lateral roots grow faster than the lower surfaces (epinastic growth) and is much more common than stilt buttressing, occurring in many different plant families. Most tree with buttresses exceed 30 m in height and often form the forest canopy or are emergent, but buttressing in lower strata is not uncommon.
The lateral roots from which plank buttresses form begins with a normal structure and acquire their peculiar modifications later. The onset of plank buttressing varies between different species, occurring much earlier in some species than in others. Although the size, shape and thickness of plank buttresses vary between and within species, these can be of diagnostic value. In some species buttresses extend up to 9 m up the trunk and for an equal distance away from it. Such large buttresses are sometimes used for making dining tables.
Why do buttresses form?
No satisfactory explanation for the formation of buttresses has been put forward. It used to be thought that buttresses help the tree to withstand the stresses due to wind and gravity. However, their paucity in temperate regions where violent storms are common, suggests that support is not a primary function of buttresses. Furthermore, buttresses are absent in the eucalypts of Australia and the giant sequoias of California, which are much taller than tropical forest trees. Tropical rain forest trees are usually held up by a network of lianas far more effectively than buttresses. It has been suggested that in trees with shallow lateral roots the transpirational stream bringing in dissolved nutrients from the soil encourages growth in the sectors of the cambium on the same radii as the lateral roots at the expenses of the intervening sectors and leads to the formation of ridges, which in time become buttresses. According to this explanation the frequency of buttressing in the wet tropics is due to the waterlogged condition of the soil and all species are not buttressed because in some species the taproot may be tolerant of water logging. This explanation is discounted by the fact that buttressed trees with taproots are known and it does not account for the absence of buttressing in shallow rooted trees in temperate regions. Other workers suggest that buttress formation is the direct response of the tree to mechanical stimulation by the wind. According to this view the wind stimulates cambium on the upper side of the proximal end of the lateral roots and in the adjoining part of the trunk, causing buttresses to form. In line with this explanation, significant correlation between buttress orientation and the direction of the prevailing winds has been recorded in temperate regions. Such a correlation is absent in tropical areas.
In general, buttressing appears closely related to the type of root system and the nature of the soil. It occurs most frequently in shallow-rooted trees growing on poorly drained soils.
Sizwe Cawe, University of Transkei
Veld & Flora Sept. 1993. Acknowledged with thanks to Author and Publisher.
ANDY MACNAUGHTAN CHAIRMAN