Tag Archives: Stigma

Exciting images of nettle flowers

Pouzolzia zeylanica3
Partially dissected female flower of the subtropical shrub, Pouzolzia zeylanica showing the ovary exposed (left) and the elongated style and stigma (right) protected by tusk-like hairs. Image by Jia Dong, RBG Edinburgh

Working with Royal Botanic Gardens, Edinburgh student Jia Dong and plant anatomist Louis Ronse De Craene has resulted in some exciting and thought-provoking images of nettle flowers. The aim of our collaboration is to understand how nettle flowers develop and in the process work out what parts they have in common and which they don’t. The samples used were from living collections at Edinburgh and RBG Kew, together with my own collections in alcohol made over several years. The results are some beautiful and very informative scanning electron micrographs which show that the part of the female flower which recieves pollen (stigma) and conducts it to the egg (style) is characterised by two classes of hairs, one comprising defensive tusk-like hairs (above) and the other receptive tubular like hairs (above & below).

Pilea grandis3
Stigma hairs at the tip of the ovary of Pilea grandifolia, a succulent herb from Jamaica. Stigma hairs in the nettle family are characterised by their rounded obtuse tips and cylindrical shape. Image by Jia Dong, RBG Edinburgh.

Tubular hairs associated with the stigma are characteristic of all nettle flowers. They also appear very early in development. Combined this makes us think that they might have a role in pollination. Specifically in the reception of pollen. Being wind-pollinated, nettles don’t have a lot of control as to whose pollen reaches their female flowers and so there needs to be a way for them to control which pollen grains develop and fertilise the single egg. It seems likely that these hairs play a role and hopefully Jia will be able provide some more great images to test some hypotheses about this.

Cecropia3
Female flower of Cecropia sp., a tcommon ropical tree from the Americas. In addition to stigma hairs you can see an apparent fold in the ovary. Image by Jia Dong, RBG Edinburgh.

 

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. Continue reading Ghost flowers in the nettle family