Solving the mystery of Myriocarpa flowers

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Composite image of the base of a female flower showing what we now believe are bracts at the base. Note the small stalked glands spaced evenly along each bract. You can see each cell in the flower thanks to the amazing imaging facilities at the Natural History Museum.

For over a hundred years the genus Myriocarpa in the nettle family which comprises ca 15 tree species in South and Central America has been impossible to place within the family. This is largely because of the very unusual shape of the part of the female flower that receives pollen, known as the stigma (see image below) and the fact that neither of the two great experts could agree over whether the petal-like structures at the base of the flower were petals associated with the flower, or bracts associated with the stalk. Whilst this might not seem like the stuff to keep a botanist awake at night it has become of interest again as using DNA data we have identified as sister to another small group of trees, Gyrotaenia, found in Cuba, Hispaniola, Jamaica and Central America that has a flower which consists of the petals fused to form a tube which is fused to ovary.

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The tip of the female Myriocarpa flower showing the very unusual forked stigma, the part of the flower which receives the pollen. You can see that it is covered in multicellular hairs, which are characteristic of the nettle family, and serve to capture pollen form the air and guide it to the stigma.

I was therefore very curious to see whether Vladimir Blagoderov, Manager of the Museum’s Sackler Imaging Suite could help me generate an image that would help us resolve the mystery. He could! The two images above are each composed of about 20 images which ‘slice’ through the sample which was of young flowers collected in alcohol in Belize over 10 years ago. The resolution was amazing, each cell being visible. In fact you could even make out the rough crystalline structure on the surface of the hairs! Both of these images also helped us to answer the question, revealing that this flower does indeed consist of a tube composed of fused petals that is subsequently fused to the ovary. This we could see in both images where the clearly visible ovary is enveloped by another distinct tissue, as in the case of Gyrotaenia. It was also confirmed by the petal-like bracts at the base of the flower having stalked glands, structures not known to occur on the ovaries of Urticaceae. So a morning’s work and an idea of evolutionary relationships enabled me to answer a question that had been frustrating an albeit very small group of botanists for over 100 years!

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Photograph of the string-like flower clusters of Myriocarpa longipes taken in Panama where it occurs as a small tree growing near rivers in tropical forest. Each flowers cluster consists of thousands of tiny flowers



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