Friday, 9 December 2011

A Mid-Night Horror Story

Imagine you’re walking around in the forest at night and suddenly you find a pile of thin bones, you look up and you see a tree with dangling sickles and bats flying over… Creepy, uh? Well, maybe if you look a little bit closer the bones are not real bones, they are fallen branches and there are no sickles, they are just the fruits of this bat-pollinated tree – but that was scary enough! Today’s story is about the “Midnight Horror”!
The midnight horror tree, Oroxylum indicum, is a very popular tree in Southeast Asia due to their weird appearance, but despite of this, there is no reason to fear it - in fact most of the plant is used for medicinal purposes (leaves, seeds, bark and roots).

Oroxylum indicum. On the left side: the tree with hanging fruits; on the right side: the flowers.
Oroxylum indicum belongs to the Bignoniaceae family, a family with tropical distribution, belonging to the problematic Lamiales order. Lamiales encomprises some closely related and big families such as Lamiaceae, Acanthaceae, Oleaceae, Gesneriaceae, Scrophulariaceae, Plantaginaceae, Verbenaceae… The relationships between these families are not well understood and in many cases the morphological characters are not strong enough to determine the families’ delimitation, especially floral morphology. The typical Lamiales flowers have a well developed and basally fused calyx, a monosymmetric bilabiate corolla (upper and lower lips) and a superior bicarpellate ovary.
*Wow, botanists’ sophisticated dialect alert!*
Ok, ok… It is easier if I show some pictures, but what I want to show you here are the fluctuations on the number of stamens.

Typical Lamiales flowers. On the left: Lamiaceae, upper lip formed by 2 fused petals and lower lip formed by 3 fused petals; Dashed line representing the monosymmetric corolla. On the right side: Orobanchaceae, upper and lower lips fused forming the corolla tube; the calyx is fused on the base. 

In Lamiales the androecium varies from one to five stamens, including reductions of stamens into staminodes (sterile stamens) which are not well understood. This character is highly variable, and in my opinion is due to the current evolution of the group. This means that the characters for families’ circumscription are not well established yet, giving this morphological fluctuation as a consequence.
However, Bignoniaceae, has some characters restricted to the family, which makes it easy to spot in the field. The flowers in this family are very big, colourful and showy – hard not to see. They usually also have big nectarines in the base of the flower, releasing a strong and sweet fragrance – no wonder they are pollinated by big animals such as bats! Usually they have 4 fertile stamens and 1 unfertile stamen (staminode), but this species in particular has 5 fertile stamens and no staminodes. I have also found Bignoniaceae flowers with more variation, but as I explained before, I believe the presence or absence of staminodes is possibly related with a transitional evolutionary state. The stigma (the female part which receives the pollen) is usually wet and relatively broad.

Typical Bignoniaceae flower with 2 pairs of stamens (long and short) and one staminode

 The essential vegetative character of the Lamiales is the simple-opposite leaves, but luckily Bignoniaceae has compound opposite leaves, and this makes our lives easier! Bignoniaceae are usually represented by trees, shrubs and are also well represented in tropical America by climbers, having the terminal leaflet differentiated into a tendril.
So don’t forget, whenever you see a tree or climber with big showy flowers with the petals fused into a tube, check for the leaves, if they are compound, you probably found a Bignoniaceae! In the case of the midnight horror tree, they have big white or whitish flowers, blooming and releasing a sweet scent at night – all perfect characters for efficient bat pollination.

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