Ghost flowers in the nettle family

Boehmeria-zollingeriana-AM7498 15-05-07_103123_M=A_R=8_S=4 v4
A pickled Flower of Boehmeria zollingeriana, 1.2 mm in length viewed under a Zeiss Axioskop microscope. You can clearly see the two stigmas (ling filament like structures) and if you look carefully the two overlapping eggs within the ovary (dark egg-shaped structures)

Nettles are characterised, amongst other things, by having flowers with a single egg in their ovary and a single stigma, the structure which conducts the pollen to its target. Work by developmental biologists almost a century ago suggested that the ancestor of nettles probably had two eggs per ovary after discovering that at a very early stage of development nettle ovaries contain two eggs one of which disappears as the flower develops resulting in the single egged flower which characterises the family. It was therefore a great surprise when plant collections from Costa Rica examined in the 1990s were found to have flowers with two or three eggs and stigmas per ovary. These very unusual plants were described as a new species: Boehmeria burgeriana  by colleagues Melanie Wilmot-Dear and Ib Friis.

How the tiny Boehmeria zollingeriana flowers look like in the flesh. The flowers are borne in small clusters and number many thousands per plant. The plant itself is a small nondescript straggling herb from South East Asia, here photographed in Guangxi, China in 2014

Since that time, however, there have been several examples of plants with one or more of these two-egged and two-stigma flowers cropping up in the family. One of these was material I collected with Chinese colleagues in Guangxi last October. It was not until I started to look at material that I had collected in alcohol under a transmission light microscope that I saw a couple of these very unusual flowers. These unusual flowers suggests that the mechanism whereby one of the eggs and associated stigma are lost somehow fails allowing the second egg to develop normally. In the case of the material from China only a few flowers had this very unusual arrangement but in the case of the Costa Rican species they consistently develop in this way.

 

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