When
you begin looking closely at plants with your magnifying glass, you'll be amazed at how
hairy many (but not all) plants are! For example, at the right you see the hairy calyx of
a flower of the Common Chickweed, Stellaria media -- a weedy plant found around
nearly everyone's homes. The entire flower is only ¼-inch long (6 mm) and the hairs
themselves only 1/32-inch long (0.8 mm), so most people never see this, despite its
being everywhere!Why is the calyx so hairy? Maybe it's to keep insects from eating the flower (notice how sharp the hairs are), maybe they help keep the flower's sexual parts warm during cold weather, or cool when sunlight hits the flower, maybe the hairs do both things, maybe they do something else, or maybe they accomplish nothing important at all. However, when you see how many plants are hairy, you just have to figure that hairs like these must do something important.
At the right you see the much magnified,stiff hairs,
or trichomes, about 0.04 inch long (1 mm), of the common Eastern US
wildflower called Wild Comfrey, Cynoglossum virginianum. Sometimes it's also
called Hounds Tongue because the trichomes make the leaf feel like the good old rough
tongue of a hound. You can imagine a caterpillar finding itself on this leaf wanting to
begin eating at the leaf's edge, but when it munches down it finds these needlelike hairs
in its way.
At the
left you see a birds-eye view of the very top of a sprouting salad plant in my garden, a
plant called Arugula or Roquette, Eruca sativa. The white part consists of a
dense cluster of white hairs covering immature, soon-to-emerge flower buds, leaves
and shoots. Since Arugula is an early spring plant growing when frosts are still likely
and the hairs look like fuzz on a winter coat, you can imagine that these hairs protect
the delicate growing parts from cold.
Other plant hairs have other special uses. For example, at the right you see
the hairy stem of a small garden weed called Bedstraw, Galium aparine. The stem
is only 1/16-inch across (1.5 mm), and you can see that the hairs are much shorter.
Nonetheless, those hairs are so stiff that when you walk through a tangle of Bedstraw,
some of the stems usually break apart, the hairs tangle in the fibers of your socks or
pants legs, and the broken stems go with you as you walk along. In this way any fruits the
plant may bear are transported along with the stem to a new environment. In other words,
the Bedstraw's hairs help the plant disperse its seeds into new areas. It doesn't need
birds or wind to help it send its family into new territory because it has those hairs!
Above, you see a close-up of yet another very specialized
hair. These are the stinging hairs of a Stinging Nettle, Urtica
chamaedryoides, a common plant in rich woods through the US Southeast. Notice that
each hair consists of two parts -- a very sharp, slender top part and a bulging lower
part. The lower part is like a glass vial filled with chemicals that burn when they come
into contact with an animal's flesh. Therefore, when these hairs come into contact with an
animal's skin, two things happen. First, the hair's sharp top pokes a hole into the skin.
Second, the chemical in the hair's base seeps into the wound, causing much more pain than
the tiny puncture could cause. 
Researchers have implicated the chemicals acetylcholine, histamine
and 5-hydroxytryptamine as causing the irritation when animals brush against
Stinging Nettle hairs.
At the right, on the upper stem of the roadside weed called Spiny-leafed Sow Thistle, Sonchus asper, you see glandular hairs -- hairs with glands atop them. In many species the glands on such hairs are sticky and sometimes you can find small insects such as aphids stuck to them. In such cases you can imagine that the glands protect the plant from organisms who might want to suck the plant's juices.
Sometimes plants use hairs and spines for reasons other than making life difficult for animals. By providing greater surface area, or by creating a sunlight- or wind-deflecting blanket, thick mats of hairs or spines can actually help a plant control its temperature.
Similarly, a carpet of fuzz on a leaf's underside can reduce a plant's water loss through evaporation. Above you see a close-up of the very hairy bottom of leaf of the Black Oak, Quercus velutina. Clearly the Black Oak goes to a lot of trouble to create such an intricate hairiness on the bottom of each of its leaves. However, I'm not sure why it does this, since other very healthy oak species in the same area are hairless!
Not all hairs are simply long and slender. One common kind of hair is shown below:
Here the hairs are branched at their bases forming tiny, spike tufts. Such hairs are said to be stellate, which means "star-shaped." The stellate hairs in the picture grow on a garden eggplant stem. The Black Oak's hairs we spoke of above also are stellate.
Sometimes hairs are hook-shaped so that they can hook onto animal fur, like velcro.
Actually, the world of plant hairs is so diverse that a whole vocabulary has been developed so that those of who talk about plant hairs don't have to keep saying things like "consisting of several slender, almost-microscopic spikes originating from a central base," which I had to say above when referring to the Black Oak's special hairs. If I had been talking to a botanist, I'd have said that the hairs on the Black Oak's lower leaf surface are stellate, and that would have been that. Below is a list of special terms relating to hairs and hairiness, many of which can be used for animals hairs, too.
USEFUL WORDS FOR
TALKING ABOUT HAIRS & HAIRINESS
acicular: needle-shaped
appressed: lying closely and flatly against the plant's surface
arachnoid: cobwebby
canescent: gray-hairy and hoary
capillary: very slender or hairlike
cespitose: matted or growing in little dense clumps
ciliate: fringed with hairs on the margin
clavate: club-shaped (big at one end)
downy: with very short, weak, soft hairs
echinate: with stout, blunt prickles
ensiform: sword-shaped
glabrous: without hairs
hirsute: with rough hairs
hirtellous: like hirsute but with smaller or more diffuse hairs
hispid: with stiff, bristly hairs
hispidulous: like hispid but with smaller or more diffuse hairs
hoary: closely covered with a white or whitish hairiness
indumentum: a heavy covering or hairiness
lanuginose: woolly or cottony
lanulose: wooly with very short hairs
mucro: a short and abruptly sharp or spiny tip
mucronate: ending with a mucro
pilose: shaggy with soft hairs
plumose: feathery, like the pappus hairs of some composites
puberulent: like pubescent but with smaller hairs
pubescent: downy with short, soft hairs
recurved: bent or curved downward or backward
retrorse: bent or turned backward or downward
septate: divided into partitions
sericeous: silky
setose: covered with bristles
stellate: once or twice forked, or arms radiating from base
tomentose: densely wooly or soft-matted hairiness
tomentulose: like tomentose but less so
torulose: twisted or knobby
uncinate: hooked at the tip
velutinous: velvety with erect, straight, moderately firm hairs
villous: shaggy with long, soft, not matted hairs