PLANT HAIRS

by Jean-Marie Cavanihac, France

In the evolution story, hairs on mammals were a useful adaptation to protect them against cold weather and also against flies' stings. But plants also have hairs: although their function can be different, their microscopic aspects are interesting and often intriguing. The pictures below, display some hairs from common (not exotic!) plants which we can find during a walk in our local area, or even growing in our garden or on a balcony!

Some plants defend themselves against bugs which can eat them; hairs trouble their advance, like obstacles on a cross-country race. At the scale of an aphid, a hair becomes a tree and a hairy leaf, a forest! Other hairs may glue attackers with sticky excretions... but it's not the only reason for the utility of hairs. They also provide a larger area on the stem or leaves, which can help to stabilize tissue temperature. Often the undersides of leaves are very hairy and can help the plant to store water, restrict evaporation and slow down drying. Hairs can also break the strength of the wind...

There's no need to travel to distant countries to find several astonishing shapes of hairs. But it is winter, and Nature is sleeping, so we can take out our old prepared slides in order to travel into the botanical world.

First, how can you collect hairs and observe them? Look at the picture above: you can use a razor blade (but be careful for your fingers!) or a knife blade, to make a little incision parallel to the axis of the stem. With small forceps, take the fragment of epidermis (with hairs) and pull it smoothly perpendicular to the stem. Put it into a drop of water or glycerin** on a slide and cover with a coverslip. Make several samples and choose the thinnest; if only one layer of epidermis cells is removed, hairs are clearly visible on it. You must not shave the stem. Hairs stay attached to the layer and they should be observed both from the top and in side view where they can show diffferent aspects of their structure.

NETTLES: NOTE. Be careful when you collect them because they can sting on contact! (Use gloves for example or a thick piece of rag ...). The image below shows a nettle hair.

It looks like a syringe because it's hollow. The hair is a large cell full of liquid under pressure. The liquid, which is injected when the tip of the hair is broken, is composed of formic acid (the same acid that ants produce). This is an example of an active defense strategy (... and very efficient!)

The image above shows the details on the tip (x150).

Another plant with repellent hairs is the thistle: its defences are spines which are clearly visible to and deter large animals (sheep, cows...) and microscopic hairs which, at the tip, are rounded but held tight to the stem.

The image above shows the microscopic hairs in dark-field.

MORE PLANT HAIRS ON PAGE 2

Comments to the author Jean-Marie Cavanihac are welcomed.

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Published in the March 2000 edition of Micscape Magazine.

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