Big Hairy Deal!
By Bruce Rottink, Volunteer Nature Guide and Retired Research Forester
If you’re reading this, there’s an excellent chance that you have hair! Well, you’re not alone! Amazingly enough, many species of plants have “hair” too. Technically, plant hairs are called “trichomes” and they’re different from mammalian hair in many ways. They come in a variety of shapes and sizes, and perform many different functions. The plants at Tryon Creek State Natural Area (TCSNA) have a variety of different kinds of trichomes. Based on their physical properties, trichomes can protect plants from a variety of herbivores, reduce the loss of water and help spread the plant’s seeds.
Simple Plant Hairs
Let’s start out with the basic plant trichome. These simple trichomes can be found, for example, on the petiole (“petiole” is the little stem that attaches the leaf blade to the branch) of TCSNA’s own red elderberry (Sambucus racemosa L. var. racemosa), as shown below. These are simple, straight, unbranched hairs.
Plants, like most things in nature, are rarely frivolous. If they expend the energy to produce plant hairs, those hairs most likely have a purpose. One of the benefits of plant hairs is that they help reduce the rate of water loss from the plant. Imagine a water molecule trying to escape from the elderberry pictured above. The thicket of plant hairs on the stem dramatically slows down air movement right at the surface of the plant. This mass of plant hairs is essentially a maze that the escaping water molecules have to slowly wander through before it leaves the plant.
To demonstrate this effect, I set up the following demonstration. I took two small identical plastic tubs and completely filled them with water. The first tub (pictured below) is totally open to the atmosphere.
For the second tub, I inserted a scrub brush, bristle side up, into the top of the tub. The bristles of the brush created a maze similar to that created by plant hairs. The water molecules had to move through this maze in order to escape (evaporate) from the tub of water. And, yes, just like in plants, it did slightly reduce the open surface area of the water.
I weighed the containers as shown before the experiment started. After 36 hours of evaporation, I weighed the containers again. The open container lost 39 grams of water, while the “hairy” container lost only 25 grams of water, 64% as much as the open container.
Get a Grip!
Some of the trichomes found in the plant world are simple, but not straight. A good example found at TCSNA is the seed of the bedstraw (a.k.a. “cleaver”) plant (Galium spp.). The seed, which is shown below, is covered with stiff, curved trichomes.
In case you’re wondering why the seed has these hook-shaped hairs, take a look at the picture below.
The hook-shaped hairs allow the seeds to attach themselves to some innocent passing animal and hitch a ride to a new location. Presumably they will be brushed off, or rub off somewhere after the passing human or other animal has traveled some distance and “voilà” the plant spreads itself to a new growing place.
Plant hairs can defend against insects and other pests that want to eat the plants. Sometimes, the sheer density of the hairs can serve as a deterrent to hungry herbivores. Some insects might not be able to get through the hairs. Some larger herbivores might just experience the hairs as “sharp pokey things” and decide to leave the plant alone. A good example is the ultra-hairy seed of TCSNA’s bigleaf maple (Acer macrophyllum) as seen below.
Sometimes, defensive plant hairs are a little more complicated. One of the premier examples here at TCSNA is our beloved stinging nettle (Urtica dioica) which is pictured below. The stinging nettle uses some of its plant hairs defensively. The larger trichomes have a largely silica (like glass) hollow needle (blue arrow) mounted on a relatively soft green pedestal (red arrow) which contains toxins. When some animal carelessly brushes against it, the tip of the needle breaks off and the movement of the animal against the plant bends the needle over, squeezing the toxin out of the green tissue at the base of the needle. The toxin is thus injected into the offending animal. The toxins in this species include histamine, acetylcholine, serotonin, and formic acid. Typically they cause an itching sensation that for most people lasts no more than 24 hours. Note that the stinging nettle also has much smaller hairs (yellow arrow) that are common on many plants.
While humans tend to be very sensitive to stinging nettle, apparently it isn’t 100% effective against all animals. The banana slug (Ariolimax columbianus) pictured below crawling on stinging nettle seemed to be pretty calm and unconcerned by the trichomes. It may be that the slug is either immune to the chemicals in the stinging nettle, or its famous slime layer is protecting it or it moves so gently that it doesn’t break the nettle’s needles!
Human hairs are all unbranched. This is not necessarily so for plant hairs. The English Ivy (Hedera helix) has hairs that are branched. The pictures below show these branched hairs from two different perspectives.
Finally, there are glandular hairs, which produce chemicals in a small gland at the tip of the hair. One of the most interesting examples at TCSNA is the invasive plant commonly known as “herb Robert” (Geranium robertianum). Another name for this species is “stinky geranium.” If you ever pulled one of these plants, you’ll know why it’s called stinky geranium. On the tips of the hairs (blue arrow) on the stem below, you can see a small red dot. This red dot is actually a gland that produces the stinky chemical. One of the traditional ways humans have used the chemical produced by herb Robert was to rub it on their skin to repel mosquitos. It probably keeps insects away from the plant as well.
A second example of glandular hairs on a plant at TCSNA is hazel (Corylus spp.) Both of the hazel species that we have at the park have a significant number of glandular hairs, as shown in the photo below. The chemicals are located in the brown tips of the hairs. In both pictures below note that in addition to the glandular trichomes, the hazel also sports a large number of very small hairs on the midrib on the leaf.
Many interesting chemicals are produced by glandular hairs on plants. The trichomes of the sweet wormwood plant (Artemisia annua), which is not found at TCSNA, produce the chemical “artemisinin” which is an effective drug against malaria. And the psychoactive chemicals in marijuana (Cannabis sativa) are produced in the tips of the plant hairs of that species.
All in all, the hairs on plants are quite diverse, and have many functions. As has shown here, plant hairs can slow water loss, help spread the seeds to new locations, and protect the plant from herbivores. For the plants, the many functions of these trichomes do indeed make them a big hairy deal!