Nope, we are not talking about the Broadway show (or the movie for that matter), but if you want to read and sing along to
“Aquarius” knock yourself out!.
Hairs in Botany are called Trichomes ( Greek). They come in every size, shape and color imaginable.
Some have glands in their tips and others at their base, or no glands at all. Some are solid, others are hollow – and some of
those have irritating oils or poisons inside the hollow space. They could be singular cells or multi cellular. Its endless!
Their special traits are directly related to the environment in which the plant evolved. The one characteristic that unites
them all is they occur on epidermal tissue (below top) – the soft outer “skin” – and never on secondary tissue like bark.
Keep in mind that hairs may look similar in different species (below bottom) but may have widely distinct functions. The sheer
number of functions they offer plants makes it clear how this is driven by being an environmental advantage.
Non glandular Trichomes
These are simple hairs with no substances in them. In alpine plants (see Cousin It below top) hollow hairs insulate the
central growing centers from the cold. Lavender is very hairy and is a waxy white – it evolved in a windy and sunny
environment. The hairs in African violets (below center) come in 2 sizes (to see the smaller one you need a microscope!!)
they are there to deter insects from eating the leaf as well as to reduce the loss of water through evaporation. Same thing
in tomato (below bottom ) hairs.
Hairs block the intense sun and the UV rays in all those hairy succulents we love (below top)! They slow the effect of the
wind like in Globemallow (below center) or reflect sunlight and break up the wind like in Sagebrush (below bottom). They can
even direct raindrops to roll off the tip of a leaf. (common in tropical rain forests).
Glandular Trichomes – not the same type of glands your grand aunt referred to when complaining about her weight !
These types of trichomes are known to secrete various substances including water, nectar, resins, mucilage, salt, oils,
poisons, irritants, and terpens among others. Glandular trichomes not only vary in the type of substances they secrete, but
also with regards to the mode through which they use these secretions.
The stinging hairs and glandular hairs on Urtica dioica (stinging nettle) is a good example. The hairs have basal bulb that
gives rise to a protruding stiff and slender structure which will break off and penetrate the skin. They secrete a poison that
can burn or irritate.
Marijuana – apparently a very popular plant nowadays – has hairs that function as a defense mechanism. When female
cannabis plants begin to produce flowers they become vulnerable to various insects and animals as well as harmful UV
rays. The hairs are a deterrent for animals because their bitter taste and strong aromas render cannabis flowers
unpalatable. At the same time, they protect the plants from UV rays, damaging winds, and even fungal growth.
One group of trichomes secrete a type of mucilage (think slimy Aloe) that serves to trap insects when they come in contact
with the plant leaves. Mucilage also helps prevent excessive water loss from the leaves. This happens in apples and
sorghum where the unicellular hairs secrete droplets, ensuring the plant does not dry out.
Carnivorous plant leaves like in sundew (below left) forms trichomes with digestive glands at its tip used to eat insects!,
dock and sorrel excrete water, basil (below right) secretes a fragrant oil, brackish or salt water mangrove plants (below
center) excrete excess salt as crystals, and the oils from Cymbopogon hairs (lemongrass) repel insects.
So go seek out some hairy plants and try to figure out why they are that way! You may find the answer most revealing!
Written by Peter Morris – our resident Horticulturalist