Euphorbiaceae is a large family of plants with many variations in plant habits, floral structure and ecological niches. They are also known as spurges and include the commonly known species like rubber (Hevea brasiliensis), castor (Ricinus), jatropha – oil seed (Jatropha), tapioca/manihot (Cassava), candlenut/buah keras (Aleurites moluccanus), cat’s tail (Acalypha), crotons (Croton) and poinsettia/Christmas plant (Euphorbia pulcherrima).
Recently (March – April 2024), I was in Kluang, Johor, Malaysia for a short holiday and noticed small, brown pumpkin-looking fruits on some trees growing by Sungei Mengkibol. Remembering these as sandbox trees (Hura crepitans, Euphorbiaceae family), I scrutinised the trees and noticed some bright red, cone-shaped structures (male inflorescences) dangling from long stalks on the trees. I repeated my visits a few more times and stumbled across strange burgundy-coloured growths (solitary female flowers) which were always pointing skywards. I then noticed whole and exploded fruits, and the seeds scattered around the grounds. Below is a photo gallery of the tree that piqued my curiosity and made my holiday interesting.
Introduction to the sandbox tree
Photo 1: The two trees on the left are the beautiful sandbox trees growing by the banks of the Sungei Mengkibol. A paved path runs below the trees.Photo 2: The tree has a straight bole from the ground up to about 2m, three branches grow around each other and upwards and then branch repeatedly. The figure on the right of the picture is for scale comparison.Photo 3: The second tree is smaller with a lot of low-hanging branches. I studied these branches, the male inflorescences, female flowers and fruits which piqued my curiosity.Photo 4: Five brown fruits circled red and one green fruit circled yellow. Photo 5: Two mature brown fruits. The lines of dehiscence of these 14-16 carpellate fruits are showing. The seeds and fruits are poisonous. Like the rubber trees, the fruits expel the seeds with explosive force. During my few visits, I did not hear or encounter any fruits exploding even though day temperatures hit 35°C. Photo 6: The tree trunk has vicious thorns growing out horizontally, deterring animals from climbing it. This tree has a few nicknames: monkey no-climb, dynamite tree, possumwood and jabillo. It is a North and South American native and introduced to Africa and the tropical regions around the world. Photo 7: Heart-shaped leaves with drip tips reminding one of Bodhi trees (Ficus religiosa).
Male flowers
Photo 7: A young male inflorescence still enclosed within a membranous skin known as an indusium. Indusium is commonly used to refer to the membranous structure that protects developing sporangia in ferns.Photo 8: The red, petal-less male flowers at the base of the pendulous structure burst through the indusium first. The waters of the Sungei Mengkibol can be seen in the background.Photo 8: In this male inflorescence, the male flowers bursting through the indusium leave an untidy membranous covering in its wake.Photo 9: This uncharacteristically short male inflorescence leaves shreds of the indusium (r) still clinging onto the spike.Photo 10: This inflorescence, length of ~ 6 cm, bear the male flowers.Photo 11: Each red, disc-shaped structure is a male flower bearing two rings of pink-coloured stamens. Lobes on the disc-like surface of each flower could be glands (function: release the smell that attract pollinators?). The lower flowers are still enclosed within the indusium.Photo 12: This ultra short inflorescence thrust the male flowers in different directions. Copious pollen grains can be shed in all directions.Photo 13: A more highly-magnified view of an inflorescence. As the flowers break through the indusial covering, jagged tears can be seen (i). The stamens are arranged very close together in two rings on each flower.Photo 14: Four male flowers teased out of an inflorescence.Photo 15: A male flower held in position by a sewing needle. The relative positions of two rings of stamens and the sessile flower can be discerned.Photo 16: Picked up a dried male inflorescence further away from the two trees. The individual flowers dried to a crisp and the rings of stamens are visible as whitish rings. In the background are tents erected to support the Ramadhan Bazaar.
Female flowers
Photo 17: A very young, axillary, female flower with a green, multi-fingered stigmatic disc (s) and a young male inflorescence (m) growing from the same terminal shoot. There are 15 fingers in this stigmatic disc. Does this figure correspond to the number of carpels in the fruit that will develop? 2 April 2024Photo 18: The same stigmatic disc has acquired some pigments two days later. The female flower develops in the axis below the young male inflorescence. Both male and female flowers are devoid of petals and known as incomplete/naked flowers.Photo 19: A mature female flower. The stigmatic disc has turned burgundy with some lighter patches near the mouth of the style column.Photo 20: A mature flower. The stigmatic disc is uniformly burgundy in colour.Photo 21: Distance between stigmatic disc and base of ovary ~ 4 cm.Photo 22: Looking directly into the style column.Photo 23: The burgundy colour spreads to two-thirds the length of the style column. The ovary is well defined.Photo 24: The stigma has shrivelled whilst the male inflorescence next to the female flower is only just blooming. The indusial membrane is separating from the inflorescence.Photo 25: A young fruit/capsule with the dry remains of the stigmatic column. Fruits point skywards.Photo 26: The dried stigma did not fall off this fruit.Photo 26: This ripe fruit measures ~ 8 cm in diameter. The remains of the stigma column has a lumen in it.Photo 27: The female flower (f) on one growing shoot seen near the male inflorescence of another shoot.Photo 28:The mature female flower grows next to a developing male inflorescence spike.Photo 29: The female flower has been pollinated, the stigma disc and style have dried up but the male flowers are freshly emerged. This points to pollination by pollens from another male inflorescence.Photo 30: While this female flower is about to mature, the male inflorescence still has a lot of growth to complete.Photo 31: Two growing shoots (gs). A female flower is preceded and followed by growth of male inflorescences. Thus, a female flower can be accompanied by a very young or well-developed male inflorescence during my random observations.Photo 32: Three different views of the seeds.Photo 33: From right to left: whole fruit, split fruit, seed lying in its carpel and a seed out of its carpel. Note the thick carpel wall.Photo 34: A carpel splits from top downwards, exposing the single seed in it. The wall between carpels splits from bottom upwards. This pattern is maintained over a number of carpels. This resulting spiral structure is commonly seen under the two trees.Photo 35: The above split fruit seen from the external surface. Reminds one of slinky toys.
Discussion
In the paper[4]
Floral development in Hura crepitans (Euphorbiaceae): a bat-pollinated species with multicarpellate gynoecium by Larissa Machado Tobias, Ines Cordeiro and Diego Demarco, Brazilian Journal of Botany, 2019
the authors consider the sandbox tree a bat-pollinated species. The reasons are that the flowers bloom at night and the male spikes release a fermenting smell to attract pollinators while the female flowers release an unpleasant smell. Male spikes are fleshy and serve as food for the night pollinators. The visitors to the male spikes thus pick up pollen grains during their meals.
Generally, bat-pollinated flowers are white or cream-coloured and comparatively large in size e.g. the dragon fruit plants (Selenicereus sp.) and the petai (Parkia speciosa). The sandbox tree flowers intrigued and challenged me to find out what functions they serve.
I only visited the two trees in the day time and did not notice any smell around the flowers nor saw any pollen grains on the male spikes or female stigma discs. A Chafer beetle, Trichochrysea hirta, burrowed into a male spike that had been partially eaten and deposited a mass of droppings on a leaf. Specks of white, possibly pollen grains, were caught on the hairy elytra (front wings of the beetle which forms a protective shield over the hind wings and body). I conjecture that this beetle could also be a pollinator. The upward pointing female flowers and funnel in the middle of the stigmatic plates could function to channel pollen grains raining down from above into the style column, facilitating pollination. The trees produce many male and female flowers at different stages of maturity.
A cockroach nymph also dropped out from amongst the plant shoots. Could this also be one of the pollinators?
Photo 36: Chafer beetle Trichochrysea hirta feeding on the fleshy spike and leaving lots of droppings behind it. Feed and poop at the same spot!Photo 37: Cockroach nymph fell out from the shoots.Photo 38: A female flower sectioned at two levels “a” and “b”. At level a, the cut surfaces exuded a clear, colourless and slightly viscous liquid. I did not taste it as this is a toxic plant. Could this be the sugary sap that aids the growth of pollen tubes, phloem sap or nectary liquid? Level b shows a style with a lumen in it.Photo 39: Cut end of style at level “a” shows an apparent hollow centre but is actually solid when tested by poking with a needle. When cut at level “b”, the female flower stalk is definitely hollow, as shown by light passing through it (see photo 40) and the needle passing through it with no resistance. Hence, the funnel of the female flower leads into a hollow tube that probably extends 3/4 of the length of the flower stalk and ends in a pool of sugary liquid?Photo 40: Peering down the cut style from the stigma disc end. A distinct lumen is evident. The cut at level “b”.
The fingers of the stigmatic disc are fleshy, firm and slightly waxy. Could the finger-like structures comb the fur of bats/mammals that visit the trees causing pollen grains caught on the fur to fall off and drop into the styles of female flowers below?
Could insects small enough to crawl into the style lumen also pollinate the flowers? Could crepuscular long-tongued moths from the Sphingidae family (hawk moths) also pollinate these trees?
A lot of articles about the sandbox trees mention the toxicity of the fruits and seeds, and skin allergy some people develop from walking beneath the trees. I am glad I did not develop any skin irritation from staying around and touching the leaves and flowers. Empty cigarette boxes and drink bottles were seen under the trees where people had sat, contemplating their lives and admiring the river scenes as well. A number of locals also rested on deck-chairs in the shade of other sandbox trees in the vicinity while waiting to get the bazaar running in the late afternoons.
The articles I found describe the morphological structures of the trees, leaves, flowers, fruits and seeds but not the reproductive details: pollinators, pollination, duration of growth of flowers, seeds and fruits. I did not stay long enough at Kluang to understand more about this tree. Most importantly, I did not have the laboratory chemicals and equipment.
Floral development in Hura crepitans (Euphorbiaceae): a bat-pollinated species with multicarpellate gynoecium by Larissa Machado Tobias, Ines Cordeiro and Diego Demarco, Brazilian Journal of Botany 2019 https://link.springer.com/article/10.1007/s40415-019-00543-0
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