Floral Complexity of Strophanthus caudatus

Strophanthus caudatus, a liana, flowered recently in MacRitchie. It belongs to the family Apocynaceae - the same family to which belongs the towering Pulai tree at Kampong Melayu, Pulau Ubin.

In Singapore, we have on record two species of Strophanthus, and possibly, an unknown third yet to be identified. Strophanthus spp. are well-known in medicine, yielding Strophanthins, glycosides used to treat heart disease. In their countries of origin - basically of the Old World, stretching from Africa to Asia - these climbing shrubs or lianas are famed for their lethal poison for hunting. There are currently about 38 species known.

Today, we find a much bigger Apocynaceae expanded under the relatively new APG II system of classification. It has been combined with the closely allied family Asclepiadaceae, the family of the familiar Hoya plants; both families having exquisitely intricate floral designs if one care to examine closely.

Besides the beguilingly long slender red 'tails' (some measuring about 14cm), the corolla of Strophanthus caudatus is crowned with a corona of 5 paired creamy lobes. Projecting from the middle one might assume at first glance to be the stigma and style, but no, it is not.

Take the flower apart and you will see it is actually the distal long extensions of the anther-tips joined together; the anthers being short, arrow-shaped, huddled together to form a cone arising from the throat of the corolla tube (see photo above).

Going further along, try taking the anthers apart. You will realized immediately they are also joined... yes, to the stigma. From here, your investigation compels you towards the complexity of such a floral arrangement and how you might wonder the kind of pollination strategies or pollinators are involved here and what advantages there are to advance such a seemingly successful plant group.

Looking further into the inner cryptic chamber of the corolla tube, I found the special structure called the clavuncula. It is an enlarged drum-shaped stigma of which the sides and lower surface are the receptive zones. The clavuncula is found to be coherent with the anthers, stuck together with sticky white exudates (third from the left, the rest being stamens).

I am tempted to think the pollens are to be found at the pointed top of the anthers which is facing inward around the sterile apex of the clavuncula. As of now, I cannot confirm it. The rotten mass that I found there is undefinable at the time of my investigation. Come next flowering season, I will definitely head straight to MacRitchie to look for fresh specimens and fresh pollens if I can. Wish me luck.