Socotra Cucumber Tree (Dendrosicyos socotrana): Weird and Fantastic Look of a Vulnerable Tree on an Isolated Island

The Socotra cucumber tree is the only tree member of the family Cucurbitaceae, which includes gourds, melons and squashes.

It also is the only member of the cucurbit genus Dendrosicyos.

Additionally, it is the sole member of that genus’ lone species, socotranus.

Where is the Socotra cucumber tree found?

The species is endemic to the Indian Ocean island of Socotra (سُقُطْرَى‎ Suquṭra), where it is called qamhiyn. Socotra is not an Arabic word. Instead, it is thought to derive from the Sanskrit phrase dvipa sukhadhara (द्वीप सुखाधार). The original meaning therefore is "island of bliss."

Socotra is the largest island within the Socotra archipelago. The archipelago is made up of the two rocky islets Ka'l Fir'awn and Sābūnīyah as well as four islands, of which the three smaller are the sparsely populated Abd-al Kuri and Samhah and the uninhabited Darsah. Abd-al Kuri, Ka'l Fir'awn, Samneh, and Darsah are commonly known as The Brothers.

The island Socotra is about 150 miles (240 kilometers) east of the Horn of Africa. It is 240 miles (380 kilometers) south of the Arabian Peninsula’s Republic of Yemen, of which it is a part.

The isolated island measures just over 77-1/2 miles (124.72 kilometers) long by just under 28 miles (45.06 kilometers) wide.

The area of the remote island totals 1,415 square miles (3,665 square kilometers), which represents about 95% of the archipelago’s total land mass of 1,465.6 square miles (3,796 square kilometers). The island claims all but a few hundred of the archipelago’s estimated total population of 50,000.

According to the botanical field surveys of the Centre for Middle Eastern Plants of the Royal Botanic Garden in Edinburgh, Scotland, the Socotra archipelago is host to 825 plant species, of which one is the Socotra cucumber tree. The Socotra cucumber tree also is one of a 307 member subgroup of that number to be found nowhere on Earth except on Socotra. About one-tenth of that subgroup additionally meets the International Union for Conservation of Nature (IUCN) criteria for inclusion on the Red List of Threatened Species. The Socotra cucumber tree numbers among the threatened members of the archipelago’s flora. In fact, it has the specific conservation status of vulnerable to threats to survival and reproduction because of climate change and habitat loss/destruction.

In fact, the ratio of endemic to total plant life throughout the Socotra archipelago is high, at 37 percent. The figure reflects both the geologic history and the historic isolation in which life is carried out on these Indian Ocean islands. Geologically, the archipelago is not volcanic but continental in origin since it is considered a detached piece of the supercontinent Gondwana (510-200 Mya [million years ago]). Climatically, the archipelago’s geographic isolation is made even more inaccessible by the fierce, windy monsoon weather conditions that prevail between June and September.

Because of its unique environment, the Socotra archipelago is a United Nations Educational, Scientific and Cultural Organization (UNESCO) World Heritage Site as of July 2008. Part of the uniqueness lies in the archipelago’s estimated age of 10 Myr (million years). Molecular analysis indicates that the archipelago’s cucumber tree may be more than twice that of the archipelago, at 22 Myr. The Socotra cucumber tree therefore is not only a window onto the archipelago but also onto the mainland home of the plant’s ancestors.

Who found the Socotra cucumber tree?

The Socotra cucumber tree first was described by Sir Isaac Bayley Balfour FRS FRSE (March 31, 1853 – November 30, 1922) in 1882. The Scottish botanist in fact led an expedition to Socotra in 1880 to 1882. Sir Balfour's interest in endemic plants previously had led him to investigate flora further south on the mid-Indian Ocean island of Rodrigues in 1874.

"a weird and fantastic look": Impressed with the unusual tree's "weirdness" and "peculiarity", Sir Balfour's description noted:

"The tree never attains any great height, but its soft, bare, and stout stems, surmounted by a tuft of few slightly pendant branches, give it a weird and fantastic look possessed by only one or two other plants on the island, e.g., the Adenium multiflorum, Klotzsch, and the Dorstenia gigas, Schweinf." (p. 101)

What does the Socotra cucumber tree look like?

Bark, branches, and trunk: White is the color of the Socotra cucumber tree’s bark. The branching is limited and therefore responsible for the tree’s small crown. The trunk is bulbous, conical and enlarged. The swollen look of the trunk causes the tree to be called a “bottle tree” and to be described as podagric (Greek: πούς, pous, “foot” + ἄγρα, agra, “trap"), or swollen-stemmed. The trunk typically will mature to a diameter of 3-1/4 feet (1 meter) on a 6-1/2 foot (2 meter) tall tree.

Leaves: The leaves are petiolate in that a stalk attaches the blade to the stem. They grow to a diameter of 1-1/2 to 3-1/4 inches (4 to 8 centimeters) in an alternate arrangement on the branch. Their cordate or heart shape divides into five palmate lobes with slightly toothed margins. Their bristly, rough surface is covered with fine, short thorns that become less prickly with age.

Flowers: The flowers of the Socotra cucumber tree grow in clusters in the axils (Latin: axilla, diminutive of ala, "wing" and therefore "upper arm," "armpit"), which is the angle formed by the meeting of the individual leaf stem with the main leaf stalk.

The tree's flowers may be yellow or orange yellow in color. They measure no more than about 1-1/4 inches (3.175 centimeters) in length.

Cross-pollination is possible, because the Socotra cucumber tree's flowers are considered complete. Specifically, all of the parts necessary for pollination to be achieved without pollinators are found on each of the tree's flowery clusters. In fact, each floral cluster is centered on one female flower surrounded by male flowers.

Fruits: The fruit looks like a cylindrically shaped berry that is pointed at both ends. Each Socotra cucumber tree fruit may measure 1-1/4 to 2 inches (3.175 to 5.08 centimeters) wide by 2 to 4 inches (5.08 to 10.16 centimeters) high. The color of the skin changes from green to brick red as the fruit ripens and begins to split open. The inside flesh is orange to reddish orange.

Seeds: The seeds measure just under ¼ inch (6 millimeters).

Seeds are orange and covered with a very fine, translucent, velvety layer.

There may be as many as five to a half section.

What does the Socotra cucumber tree need to grow?

In its native habitat, the Socotra cucumber tree is accustomed to high heat in a context of extended drought, fierce winds and regular sunlight.

1. Temperature: Island temperatures reach average highs of 90+ °F (32.2+ °C) from April to June and from August and September. They fall to average lows of 62+ °F (16.66 °C) from October to March. January is the coldest month.

Away from its native habitat, the Socotra cucumber tree nevertheless can adapt to just about any climate whose temperatures do not dip below 68 °F (20 °C).

2. Precipitation: Rainfall reaches a yearly total of just over 10 inches (254 millimeters). It is more or less evenly distributed, in amounts of less than one inch (25.4 millimeters), throughout the year and by way of at least two rain days per month. Amounts in excess of one inch are reached only in May, September and October, which are not the months with the most rain days. Instead, the Socotra cucumber tree experiences quadruple the number of rain days in July and August, both of which average 9+ rainy days.

3. Humidity: The moisture content of the air hovers in the teens during more than half of the year. It averages in the twenties in January, April and September. The average is 37 to 38% only in May and October.

4. Soils: Calcareous soils characterize the coastal plains and low inland hills where the Socotra cucumber tree grows, from sea level up to an altitude of just under 1,641 feet (500 meters). Such soils are gritty and strong in delivering the nutrients calcium and magnesium to Socotra cucumber tree roots. Their pH is highly alkaline because the acids present in such soils are so weak that the level cannot drop down to the neutral range of 6.5-7.0 or lower into the acidic levels below.

But Socotra cucumber trees also grow on the shrub land of the island’s coastal plains. The plains offer a bit more hospitable of an environment in terms of alluvial soils and fast draining sand. There, the Socotra cucumber tree finds protection from predators and weather extremes by growing in the midst of spiny (Lycium sokotranum) and succulent (Cissus subaphylla) shrubs.

Specifically, Lycium sokotranum, which also is known as Lycium europaeum sensu auct. and Lycium socotrana, is a species of flowering plant in the nightshade family, Solanaceae. It is endemic to the Socotra archipelago. It grows throughout the coastal plains and limestone plateau of the island Socotra, from sea level to altitudes just under 2,625 feet (800 meters). It also may be found on the central plains of the island Abd al Kuri.

This spiny shrub matures to a height under 6-1/2 feet (2 meters). Its leaves are obovate, or gently egg-shaped, and measure up to ¾ inches (2 centimeters) in length. Its yellow flower is just under 2/5 inch (1.016 centimeters) long, which also is the maximum diameter of the plant’s red fruit as well as the length of each of the sharp spines which grow on the shrub’s many branches. What with its menacing spines, it may come as no surprise that the plant's conservation status is of least concern to the International Union for Conservation of Nature and Natural Resources.

The succulent Cissus subaphylla, which also is called Vitis subaphylla, is a plant in the family Vitaceae, whose plants flower from two embryonic leaves.

On Socotra, it grows on the island’s dry plains as well as on calcareous slopes up to 984 feet (300 meters) above sea level and on the western limestone plateau up to 2,460.6 feet (750 meters) above sea level.

Sir Balfour remarked:

"Over all, and giving quite a character to the whole landscape, is the glaucous-grey colour, due either to a waxy bloom on the more leathery and fleshy forms, as . . . Vitis subaphylla . . . and others . . ." (p. xxxiv)

"This is quite a distinct species in the simple entire-leaved group of vines . . . as found in Socotra, it is nearly leafless, only the younger twigs showing leaves, and these soon fall off. It is one of the commonest plants on the limestone plain about Galonsir, forming small clumps with intertwined branches." (pp. 58-59)

It also may be found in the open shrub land of the island of Samhah at 328 feet (100 meters) above sea level.

On both islands, its conservation status also is of least concern.

5. Drainage: Drainage varies, from excellent on the coastal plains to obstructed on hill slopes.

6. Nutrients: Generally, tree roots need 17 nutrients in order to grow and to support above ground growth as well. The Socotra cucumber tree is no exception even though it is adapted to the nutrient deficiencies that may develop in a context of an arid climate and limestone geology. Away from the archipelago, the soil regularly should be analyzed for nutrient uptake, toxicity or deficiency.

7. Propagation: Seeding is the only method for propagating the Socotra cucumber tree, in the wild and in cultivation. No other method works. Budding, cutting and grafting all have been tried, with dismal results.

Plant each seed about ½ inch (1.27 centimeters) down in a starter pot filled with a pumice mix to about one inch (2.54 centimeters) from the lip. Water just enough to keep the contents moist, but not waterlogged. The seeds should germinate within 14 days.

The Socotra cucumber tree most likely will need to be repotted every 3 to 4 years. At least half of the soil should be pumice. An effective mix for the other half is compost since the Socotra cucumber tree requires good drainage. Watering can be adjusted from a regular 7 to 10 day schedule any time that the soil feels and looks dry or waterlogged. Even though temperatures need to hold steady at 68 degrees F (20 °C), the Socotra cucumber tree handles occasional drops to 50 °F (10 °C) and even down to 30 degrees F (-1 °C) without injury.

What are the Socotra cucumber tree’s uses?

Medicine, nutrition, ornamentation and revenue are the four main uses of the Socotra cucumber tree. In its native habitat, it is its roles as a food source and a medical treatment that traditionally prevail.

Specifically, the tree in its seedling form is a popular food source for the island’s plenteous population of grazing goats. In its mature form, the tree often is cut down, made into pulp and fed to livestock.

In terms of medicine, the Socotra cucumber tree’s leaves make an important contribution to traditional island culture. The leaves are used to treat an array of serious health conditions: burns to constipation, cystitis, diabetes, and liver and urinary problems. Additionally, Socotra cucumber tree extract --- which contains polysaccharides, proteins and terpenoids --- is a traditional treatment for malaria.

But the revenue possible through eco-tourism is overcoming the traditional lack of attention to the historically ornamental role of the Socotra cucumber tree in its native habitat. The tree's odd appearance as a sentinel of isolated coastal plains and remote inland slopes in fact is a major draw for eco-tourist activities on Socotra. For example, the tree's locations cooperate with nature classes that are followed by celebratory forays into pristine mountain pools and sparkling sun kissed beaches as well as thrilling quests for treasure rumored to be buried in area caves by Indian Ocean pirates drawn to the remoteness of the Socotra archipelago.

Socotra cucumber tree: a window into the past, a fragile view of the present

It is indeed the ornamental role that accompanies the export of the Socotra cucumber tree outside the archipelago. Its unusual bottle shape and the ancient botanical legacy that it preserves serve to remind us of earlier times in Earth’s history and of fragile membership in today’s world.

My special thanks to Philippe Corman/Au Cactus Francophone and Leo J. Song Jr./California State University-Fullerton Department of Biological Science for superb closeup photos of Dendrosicyos socotrana.

My special thanks to talented artists/concerned organizations who make their fine images available on the Internet.

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