Over to Taisha again for today's entry. She writes:
Today's images are of Urtica dioica, or stinging nettle. Recently, I've been obsessed with looking at various "best of" lists of photomicrography, or light microscope photography. While perusing the Botany Photo of the Day Flickr Pool, I came across some work by Eckhard Völker (aka Tatcher a Hainu@Flickr). He has some great microscope shots! I really liked the sprout cross-section of Aesculus hippocastanum, but this species has been twice been featured on BPotD, once by Douglas Justice and another by Daniel, so I thought I'd choose another species. The first photo above is a rhizome cross-section of Urtica dioica at 10x magnification. To give this close-up some context, I've complemented his image with one of stinging nettle in habit by Tim Hicks (aka corvinundrum@Flickr). Thanks Eckhard and Tim!
Urtica dioica (Urticaceae) is a herbaceous perennial that varies widely in morphology, yet is still immediately recognizable throughout its distribution range, i.e., much of the northern hemisphere. This species has a vast system of rhizomes and stolons, so it can form dense populations. Plants can reach 3 meters in height in ideal conditions. Densely covered in stinging hairs, plants can be a hazard to those not paying attention.
The sting from stinging nettle is caused when the stinging hair (made up of a large stinging cell embedded in a multi-cellular base) comes into contact with the skin and releases a chemical mixture. The stinging cell and the main part of the cell are pulled into a brittle tapered shaft that is capped by a hollow terminal knob. When contact with a surface like skin is made, the terminal knob breaks off, leaving the hollow shaft tip. With the downward contact force on the flexible basal bulb, fluid is forced upward and expelled onto the surface. The compounds responsible for the burning sensation have been proposed by Thurston & Lersten (1969) to be histamine, acetylcholine, 5-hydroxtryptamine, serotonin and formic acid, however it has been noted more recently that despite this citation being in various texts, negative findings are ignored (see: Taylor, K. (2009). Biological Flora of the British Isles: Urtica dioica. Journal of Ecology 97(6):1436-1458).