General Info

Tree up to 14m high. Small Leaves are short-lived. Inflorescence is a Cyathium. unisexual Flowers are much reduced and lack both calyx and corolla. Fruit is a 3-lobed capsule.

Description

RSA Tree No. 354.

Common names: Honey Euphorbia, Rubber tree.

Family: Euphorbiaceae (Euphorbia family e.g. exotic poinsettia – Euphorbia pulcherrima). This family has about 275 genera and 7 500 species. Our local genera containing trees include Alchornea, Croton, Euphorbia, Macaranga and Spirostachys. This family has plants that may be herbs, shrubs or trees and latex is often present. Leaves, when not rudimentary, are usually alternate and simple with free stipules. Inflorescences are situated terminally or axillary. They are mostly in cymes (a broad, more or less flat-topped, determinate flower cluster, with central flowers opening first). They may be grouped in spikes (simple indeterminate inflorescence with sessile flowers on a single unbranched stalk), thyrses (the main axis grows indeterminately, and the branches have determinate growth) or pseudanthia (inflorescence with many flowers appearing as a single flower – like a sunflower). In Euphorbia they appear as cyathia (where they usually have 5 joined bracts outside: up to 10 brightly coloured nectar glands which may have petal-like appendages or brightly coloured bracts followed by 5 much reduced male flowers at the base of each bracteole. In the centre is a much-reduced female flower). Plants may be monoecious or dioecious and the regular, unisexual Flowers may or not have a perianth. Male flowers have 1-many stamens with free filaments and stamens with up to 4 pollen sacs. Female flowers have a superior ovary with 1-many locules – each with up to 2 ovules. Fruit is a capsule or nut(s). Seeds may have a caruncle (a fleshy structure attached to the seed) and may be poisonous.

Name derivation: Euphorbia – after Euphorbus (50BC-AD19): physician to the king of Mauretania (NW Africa). tetragona – 4 angles: refers to the number of angles on the branchlets. There are more than 300 species of the Genus Euphorbia in the RSA.

Conservation Status: L C. (Least Concern). Assessment Date: 2009 (Raimondo et al.). The impact of trunk destruction of the tree by the black rhinoceros may have to be investigated.

Tree

This robust, spiny, succulent Tree is candelabra shaped and up to 14m high. The Trunk is either round or slightly angled with grey Bark. It may be single, but is frequently divided from near ground level into 2-5 stout, stem-like branches topped with small crowns 1,5-2m in diameter. Young trees are deep green. Branchlets (a small branch or division of a branch – especially a terminal division; usually applied to branches of the current or preceding year) are distinctly square or 5 angled but flat sided appearing square or concave. They are slender and up to 5cm in diameter, shallowly constricted with edges slightly extended. Spines are in pairs, which are up to 1,2cm long. Adjacent paired spines are on separate spine shields. Young trees are conspicuously spiny. The horny margin is not continuous. Old branchlets are sometimes spineless. Latex (milky sap) is released from damaged parts. An underground Tuber may exceed 4cm in diameter.

Leaves

The usually small Leaves are short-lived. Stipules (basal appendage of the petiole) may be transformed into spines. Some Euphorbia, in arid regions have an additional method of photosynthesis called CAM photosynthesis. This involves closing the Stoma (structure utilising 2 guard cells which, unlike lenticels, can control the gaseous exchange between the plant and the surrounding atmosphere) during the day – the opposite of most plants. In this way, they substantially reduce loss of water due to evaporation. The stomata then open at night to absorb carbon dioxide that is stored until daylight and is then used to complete the photosynthesis process. CAM photosynthesis also occurs in some other plants including pineapples.

Flowers

The yellow-green Flowers are part of an Inflorescence (a group or cluster of flower arranged on a stem). The apparent single flowers are a combination of separate, unisexual male and female flowers, bracts and nectar glands. This group of flowers is called a Cyathium and cyathia are solitary. They are located near the tips of fleshy stalks. The exterior of each cyathium is surrounded at the base by 5 bracts (not sepals) which may have a petal-like appearance. The nectar glands are brightly coloured. Male and female flowers are extremely reduced – lacking both sepals and petals and they do not produce their own nectar. The separate nectar glands produce this nectar. The groups of Male flowers are attached at the base of each bract. Each male flower consists of a single Stamen. The solitary Anther has 2 thecae (pollen sac – microsporangia of an anther. They produce microspores – pollen grains). Inside the group of male flowers is a single Female flower that consists only of a short stalk that holds a single Pistil containing a single Ovary. Nectar glands produce a great deal of nectar, which attract many insects – that are the agents of pollination. (Jul-Sep).

Fruit

The somewhat rounded, 3-lobed Fruit is a Capsule (a dry fruit resulting from the maturing of a compound ovary which usually opens at maturity by one or more lines of dehiscence) that is about 9mm in diameter and becomes woody before releasing the seeds by dehiscing. (Sep-Nov).

Distribution & Ecology

This succulent plant is Found in dry tropics and sub-tropical areas in southern Africa specifically Eastern Cape (can be seen on the banks of the Gamtoos River) and KwaZulu-Natal. It is fairly frost hardy and drought resistant. Black rhinos are said to target and push over the trees.

Ethnobotany

Latex of this plant is toxic if ingested. It is highly irritant to the skin and eyes and may cause blindness. Wash affected areas of the skin with soapy water or milk. Dried plant material retains toxicity. Propagation is by planting seeds or truncheons. Do Not collect and use the wood for making a braai.

References

Boon, R. 2010. Pooley’s Trees of eastern South Africa. Flora and Fauna Publications Trust, Durban.

Coates Palgrave, M. 2002. Keith Coates Palgrave Trees of Southern Africa, edn 3. Struik, Cape Town.

Lawrence, G. H. M, 1951. Taxonomy of Vascular Plants, The Macmillan Company, New York. Tenth Printing 1965.

Palmer, E. & Pitman, N. 1972. Trees of southern Africa, Balkema, Amsterdam, Cape Town.

van Wyk, B. & van Wyk, P. 1997 Field guide to Trees of Southern Africa, Struik, Cape Town.

http://www.thefreedictionary.com)

http://practicalplants.org/wiki/Euphorbia_tetragona

http://operationwildflower.org.za/index.php/component/joomgallery/euphorbias/euphorbia-tetragona-thabo-5-1682

https://www.researchgate.net/publication/40118069_Will_tree_euphorbias_Euphorbia_tetragona_and_Euphorbia_triangularis_survive_under_the_impact_of_black_rhinoceros_Bicornis_diceros_minor_browsing_in_the_Great_Fish_River_Reserve_South_Africa

http://redlist.sanbi.org/species.php?species=574-350

https://www.khanacademy.org/science/biology/photosynthesis-in-plants/photorespiration–c3-c4-cam-plants/v/cam-plants.