San Francisco State University
Department of Geography

Geography 316:  Biogeography 
The Biogeography of  Euphorbia obesa

by Robin Heuver, student in Geography 316, Fall 1999

Kingdom:Plantae
Phylum:Magnoliophyta
Class:Magnoliopsida
Order:Euphorbiales
Family:Euphorbiaceae
Genus:Euphorbia
Species:Euphorbia obesa

Description of Species:

The plant that will be described is a succulent euphorbia, globular in shape, approximately five inches in height, predominantly a green/gray color with purplish longitudinal and transverse striping, creating an almost plaid effect.  There are eight distinct, broad, vertical ribs that are slightly raised  with shallow furrows in between. (Haage 1963)  Visible on top of the ribs, are mminute, brown, ruglose tubercles in a single series. ( Euphorbia Journal Vol. 1 1983)  This plant grows in clusters that can be propagated by division, or easily started by seed.

  Habitat:

Euphorbia obesa is an endemic species to the Northern Cape region of South Africa.  These hard to find succulents are found growing in what are called the regions of Karoo within the Northern Cape.  Typically they are found growing under the protection of low shrubs in sandy soils in the presence of small boulders.

Natural History:
        The first documented description of E. obesa was made in 1897 by Professor Pete MacOwan. (Euphorbia Journal Vol.1 1983)  MacOwan began as a curator of the Capetown Botanical Gardens.  He worked himself up the professional ladder and became a governmental botanist.  During this time, he did extensive work with the renowned South African botanist, Harry Bolus (1834 - 1911).  It was during the time that MacOwan worked with Bolus that he discovered Euphorbia obesa
MacOwan sent this plant to the Royal Gardens, Kew mislabeled as a close relative, Euphorbia meloformis.   A man by the name of Joseph Dalton Hooker identified it as Euphorbia obesa.  He described this plant after the drawing was completed.  The description and illustration was then submitted and was published at Tab 7888 in Volume 59, 3rd series of Curtis’s Botanical Magazine, in April of 1906 (Euphorbia Journal Vol. 1 1983).
     As plant enthusiasts, collectors, traders and sellers became more familiar and fond of this plant, natural populations began to rapidly decline due to exploitation.  By 1915, the plant was considered endangered for two reasons.  One was exploitation, physically removing large amounts of plant material from the Cape Province for redistribution.  The second important factor to consider is that E. obesa is treasured by other inhabitants of the Cape, baboons and most four legged creatures.  Locals call this plant “Springbok Ko’s - and monkeys love it!”  Baboons particularly sought this plant in times of drought, and grazing animals simply could not help themselves from indulging on this plant.  These factors lead to the imperative embargo that was created in the 1920’s to prevent this species from going extinct.  Plant material was no longer allowed to be removed from the Cape Province area, which encouraged their survival. (Euphorbia Journal Vol. 1 1983)  Distribution continued, but a new method was put into place, seed collecting.  If treated properly, this plant can produce up to 150 seed per year, 95% of which are viable. (Euphorbia Journal Vol. 3 1983)  Seed collecting and distribution has been steady for close to seventy years now, and E. obesa can be found in greenhouses and botanical gardens around the world.
     The family Euphorbiaceae incorporates a wide range of plant types, from trees, shrubs, herbs, vines to succulents with and without spines.  There are diversities among these plant types both chemically and morphologically, but there are a at least two unifying features almost throughout the entire family.  One, is there complex floral arrangement that is called the cyanthium.  This refers to the unit of inflorescence where one female flower is surrounded by many male flowers that are enclosed in one involucre. (Court 1981)  The second almost unanimous feature among Euphorbias is the white, milky latex substance that is secreted when a leaf is torn or part of the plant is punctured.  This latex juice is often poisonous and acts as a defense mechanism against predators.  Clearly, not all Euphorbias are poisonous, E. obesa is savored by the fauna of the Cape Province.
     Many Euphorbias have substantial economic importance.  Castor oil, Para rubber, tuberous roots that are eaten in tropical regions and the ever popular poinsettia that is sold for its ornamental decor are all part of this incredibly diverse family. (Cronquist 1981)  A wide variety of Euphorbias are also used for medicinal purposes such as, curing boils, worms, fever reducing, aiding with bronchitis and asthma, relieving toothaches, even remedies for venereal diseases have been acquired from this family.
     Euphorbia obesa’s contribution is purely aesthetic.  A beautiful contribution to the family lineage at that!
 

Evolution:
        The Euphorbia family was first described by Linnaeus in 1753.  The name was created in honor of the physician who treated King Juba of Mauretania.  More important than his reign were the nobility’s of his studies.  King Juba and his physician discovered and explored the medicinal properties of E. resinifera which still occurs on Mt. Atlas today.  (Haage & Dutton 1963)  This occurred very long ago, sometime around 46 B.C.
     Prior to our need for classifications and creation of systematic methods for learning, Euphorbias were continually evolving starting as far back as the Eocene.  Macrofossils that represent the Euphorbs are also found in more recent deposits.(Stebbins 1950)  There has been considerable evidence to support that Euphorbiaceous pollen can be traced even farther to the Paleocene.(Cronquist 1981)  Clearly, this plant has substantial roots in the history of this planet.
     Higher land plants are descended from aqualtic ancestors just like higher animals.  It is probable that the flowers of Euphorbiacea became apetalous and unisexual as an adaptation to wind pollenation.  Their ancestors most likely were pollenated by insects. (Stebbins 1950)  The present Euphorbias with brightly colored bracts and obvious nectaries indicate an evolutionary reversal to insect pollenation.  Their path of evolution has required succulent Euphorbias to develop thick stems to deal with deser aridity and protection from animals ( Euphorbia Journal, Vol. 1 1983).
     Euphorbias in general, grow in many forms and can tolerate almost all climates except very cold, which has lead to their widespread distribution.  Natural distribution of succulent Euphorbias are somewhat confined to South and East Africa, which is another indicator that this is a plant from the Old World.  Of all the succulent plants, Euphorbias closely resemble cacti of the new world most significantly.  An excellent example of convergent evolution is the fact that when examining Euphorbia succulents of the old world, to cacti of the new, it is possible to match their forms of growth when not in flower.  There are many examples of this today, one would be that  E. obesa and E. meloformis  resemble the wide variety of barrel cactus.  For all of the similarities present in Euphorbian succulents and various cactus, you can easily differentiate the two by the milk that flows with the slightest prick from the Euphorbia.  The flowers are also clear evidence that the two are different.  Cactus flowers tend to be very showy, large and colorful whereas the flowers of Euphorbias are small nectaries with colorful bracts, and the cyanthas actually consist of multiple flowers.(Haague 1963)  These are the two most predominant features that differentiate a cactus from a succulent Euphorbia.
     Euphorbia obesa is endemic to the Cape Province of South Africa.  More specifically, it is native to a region known as the Karroo, which is divided into three major areas.   The great Karroo, lies north to the Cape belt which is in the south.  It averages to be 2,000 - 3,000 feet above sea level, a place where there is no significant frost.(Riley 1963)  Dominant plants found in this area are succulents and low bushes ranging from 8 - 10 inches high. (Riley 1963)
     The little Karroo lies between 1,000 - 2,000 ft above seal level, but the boundaries are vague.  Shrubs and succulents are also present in this area, but not as many.
     The upper Karroo is north of the great karroo region.  The elevation predominantly exceeds 3,000 feet above sea level.  This area is also filled with succulents, shrubs and small undershrubs.  These plants are mostly woody, perennials ranging from 3 - 12 inches in height and on to several feet apart. (Riley 1963)  Also present are annual grasses that appear shortly after rains, they seed quickly and die.
 
 
Distribution
     This particular species is considered, according to R. Allen Dyer, to be one of the most localized species within this genus in regards to natural distribution.  Only a handful of areas within the Cape Province of South Africa provide homes for this euphorbia.  Those areas include:  Graaf Reinet, the Kendrew district, Charlwood and Karoo.  These locations are all found within the Cape.  One must look very closely to find this succulent.  Their preferred habitat is a place that provides sandy soils and the presence of small boulders.  “E. obesa grows either under the protection of Karoo shrublets in loose sandy soil, or in hard sandy soil among boulders from which it is often very difficult to distinguish the plants.” (Dyer 1940)
     South Africa has a Mediterranean climate, much like our own here in San Francisco.  Flora of the area have evolved so that they can endure distinct dry periods in the summer and moderate amounts of water in the winter.  Ideal temperature for this plant is fifty degrees Fahrenheit or more.  Seasons are not clearly defined, some winters are wetter than others and some dryer than others.  At no time is there consistent rain.  Frosts do not occur in the Cape Province, maximum temperatures ranges would be 40 - 107 degrees Fahrenheit.  There is little shade and high, drying winds occur frequently. (Euphorbia Journal Vol. 1 1983)
     Due to their hardiness and aesthetic quality, this species is highly sought after.  Collectors and dealers did an excellent job exploiting this species in the early 1900’s.  By 1915 this euphorbia was endangered, and shortly thereafter, an embargo was put into effect so that no euphorbia obesa’s left the Cape Province.(Euphorbia Journal Vol.1 1983)  Because of this conservation effort, the plant population has recovered, and current, primary distribution is from seed, not divisions or mother plants.  Botanical gardens, and green houses throughout the world provide a safe haven for this species.  It is commercially available through seed, trade and sale.  To find this plant in its natural habitat is much more of a challenge.  If you are headed to the Cape Province of South Africa, you just might catch a glimpse at this euphorbia.

Map of Distribution:
 

Bibliography
Court, Doreen.  Succulent Flora of South Africa.  Grace Doreen Court Publishing, 1981.

Cronquist, Arthur.  An Integrated System of Classification of Flowering Plants - Angiosperms and Dicotyledons.  New York:  Columbia University Press, 1981.

Haague, Walter.  Cacti and Succulents.  USA:  Dutton and Co., Inc.,  1963.

Hutchinson, J.  Evolution and Phylogeny of Flowering Plants.  New York:  Academic Press, 1974.

Hutchinson, J.  The Families of Flowering Plants.  London:  Oxford University Press, 1973.

Riley, Herbert.  Families of Flowering Plants of South Africa.  Kentucky:  University of Kentucky Press, 1963.

Stebbins, G. Leddyard.  Variation and Evolution in Plants.  New York:  Columbia University Press, 1950.

Turner, Roger.  Euphorbias.  Oregon:  Timber Press, 1981.

The Euphorbia Journals:  Vol
 

send comments to bholzman@sfsu.edu
 

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