Above: a diagram of a section through the disc-like scale hair of Tillandsia, the Air
Plant. The epidermis of plants (epidermis: external covering tissue of young and green
plant parts) is typically adorned with appendages of some description. Stems and
leaves often have one or more type of hair. The ice plant (Mesembryanthemum) has
many water vesicles, swollen epidermal cells storing water and excess salts in their
central vacuoles. The petals of many flowers have papillae, each papilla formed by the
dome-shaped or conical outer surface of an epidermal cell.
Tillandsia is an epiphyte and has to absorb water and minerals from the tree canopy.
Its roots function primarily as holdfasts, fastening it to its host. The leaves have
assumed the role of principal absorptive organs, catching and absorbing rain water
and minerals (which are found in rain, dust and falling plant debris). The leaves can
not simply be permeable to water, since they would desiccate in dry conditions
(mosses are able to do this but their small size means they rehydrate very quickly).
Instead Tillandsia has a remarkable mechanism to open up water-permeable
channels only in wet conditions. The leaf surface is covered with plate-like or
disc-shaped hairs, called peltate trichomes. Each of these trichomes is comprised
of a shield of dead cells with no living contents: central cells form the centre of the
disc, ring cells form a circle around the central disc and wing cells form the main body
of the outer disc.
In dry conditions, as shown above, the wings curl inwards and upwards, so that each
trichome becomes cone-shaped. The shield is mounted on a column of living dome
and foot cells, embedded in the leaf tissue. When it rains, the shield cells absorb
moisture and expand, causing the wing to flatten out as a disc. This creates a small
gap between the shield and the underlying epidermis which wicks in water from the
surface of the leaf by capillary action. Water also moves from this capillary space into
the shield cells and then from shield cell to shield cell, since their walls lack a
waterproof cuticle. This eases the flow of water (capillary action alone is slow). Water
then enters the living dome cells. These cells also probably expend energy in
pumping in salts from the surrounding water, presumably causing water to enter them
by osmosis. Water moves freely from dome cell to dome cell as their cross-walls lack
waterproof cuticles. In this way ion-pumping drives mass flow of water from the
surface of the leaf into the dome and foot cells and hence into the leaf. When the
leaf dries again, the shield curls back into a cone and the central cells deflate they
possibly seal off the water route to prevent water loss by evaporation.
In their dry state, measurements have shown that the pellate trichomes of Tillandsia
fasciculata reflect as much as 25% of the incident light and this apparently serves a
photoprotection function - protecting the photosynthetic machinery from intense
radiation (something which epiphytes with their high position in the canopy may be
The leaves of the Holm Oak (Quercus ilex) have stellate hairs (star-shaped
trichomes) on their adaxial (upper) surface, especially when young. These hairs, and
the scars they leave when they fall from older leaves, aid water absorption and are
situated directly above extensions of the bundle sheath of the leaf vessels, so that
absorbed water is presumably efficiently transported to the vessels. In contrast, the
abaxial leaf surface (undersurface) has a dense covering of stellate hairs containing
water repellent materials, such as waxes and cutin, and which makes the underside
unwettable, in contrast to the wettable upper leaf surface. Accumulation of water on
the lower surface could interfere with carbon dioxide exchange (through the stomata)
needed for photosynthesis.
In many plants the trichomes are thought to create a boundary layer of trapped still
air to limit water loss by advection (air currents) from the stomata. Certainly, in
xerophytes this function is often obvious, with stomata sometimes sunken into
hair-lined pits, as in Marram Grass (Ammophila).
Trichomes come in many shapes and have numerous functions. Sometimes the
definition 'trichome' is used for any epidermal appendage, including papillae and
water vesicles, in its stricter sense it refers to hairs or hairlike structures.
Trichomes have obvious sensory functions in carnivorous plants such as the venus
flytrap (Dionaea muscipula) and the bladderwort (Utricularia). In the flytrap it is well
established that tactile stimulation of the trigger hairs generates electrical potentials
which spread the signal to motor cells throughout the trap.
Trigger hairs are also important in the explosive pollination mechanisms of certain
orchids, such as Catasetum. In the case of Utricularia and Catasetum it is not clear
whether the mechanism is purely mechanical or whether it involves electrical signals.
Similarly in Neottia cordata (Listera cordata, Lesser Twayblade) there are three
mechanosensitive hairs on the rostellum. When these are touched, even very lightly,
the rostellum squirts glue on the pollinating insect's head onto which the pollinia fall
and adhere. Hairs are able to amplify touch stimuli, since a force exerted at the tip
becomes a larger force at the hair base, as the hair shaft acts like a lever. It seems
probable to me that the response involves either action potentials or graded
However, trichomes may have sensitive functions in far more plant species than
generally recognised. Studies have shown that glandular trichomes in the tomato,
Solanum lycopersicum, act as early warning sensors. They respond to touch, for
example as caused by crawling insects, and alert the plant to prepare its anti-insect
Trichomes may also have mechanical functions in trapping insect pollinators, for
example in Aristolochia. These plants trap insect visitors which are potentially
delivering pollen to the receptive stigmas when the flower is in its female stage. When
until the flower enters the male stage the insects are allowed to exit the flower,
possibly taking pollen with them. In Aristolochia, the trichomes are hinged to allow
downward deflection, encouraging insects to fall into the trap, but lock when deflected
upwards, preventing insect escape.
Glandular trichomes secrete chemicals and often have an anti-insect defensive
function, releasing glue to trap an insect which might otherwise feed on the plant or
steal pollen. Trichomes have also been shown to make insect movement difficult,
causing insects to fall off or even to become impaled on a trichome.
The glass-like silica trichomes of the stinging nettle
(Urtica dioica) clearly serve to defend against larger
grazing herbivores. They easily pierce skin with their
sharp tips, breaking the fragile silica structure to release
the formic acid (methanoic acid) contained within into the
Glandular trichomes may be specialised to function as osmophores (flower fragrance
glands) as in the orchid Cyclopogonelatus elatus.
Glandular trichomes may also secrete stigmatic fluid. Stigmas come in tow principle
varieties: wet stigmas are covered in sticky secretion to trap pollen, whereas dry
stigmas only secrete fluid when pollen makes contact with the stigma surface. This is
thought to involve some molecular recognition triggering the sudden opening of
aquaporins (protein channels which form microscopic water channels in cell
membranes). This fluid may aid pollen adhesion, but also has a role in hydrating and
possibly nourishing early growth of the germinating pollen tube. (In some flowers it is
known that initial pollen tube growth can take place using the stored reserves of the
pollen grain, but eventually additional fluid is required. For example, Darwin
commented on pollen tube growth initiating on pollen grains left behind in flowers of
Orchis anthropophora (Man Orchid) without contacting the stigma). Under the light
microscope, the stigma of bluebell (Hyacinthoids non-scripta) for example, is covered
in beautiful flask-like trichomes or stalked papillae, with a translucent glass-like
appearance and which release water immediately on contact with pollen.
31 May 2015
9 June 2015
30 May 2016
Above: stigmatic trichomes in Geranium robertianum (Herb Robert)
Above: trichomes on the sepal-tube (calyx) of Herb Robert (Geranium robertianum).
There are at least three types of trichome reported on this plant. The very long
trichomes here consist of a basal cell, 5 long stalk cells and an elongated terminal cell
which accumulates anthocyanins (visible as a minute red sphere at the end of each
hair). These glands secrete flavonoids (function unknown in this instance).
Above: the stem, and below the leaf, of Herb Robert showing two types of trichome. At
least some of these hairs are clearly glandular (observe the terminal cells full of
coloured liquid) and secrete terpenoids and phenols (function unknown).
Example: Geranium robertianum (Herb Robert)
Above: the leaf, stem and ovary of Geranium robertianum and their trichomes.