Trichomes, from the Greek τρίχωμα meaning "hair", are fine outgrowths or
appendages on plants, algae, lichens, and certain protists. They are of
diverse structure and function. Examples are hairs, glandular hairs, scales, and
papillae. A covering of any kind of hair on a plant is an indumentum, and the
surface bearing them is said to be pubescent.
Algal trichomes Certain, usually filamentous, algae have
the terminal cell produced into an elongate hair-like structure called a
trichome. The same term is applied to such structures in some cyanobacteria,
such as Spirulina and Oscillatoria. Cyanobacteria trichomes may be
unsheathed, as in Oscillatoria, or sheathed, as in Calothrix. These
structures play an important role in preventing soil erosion, particularly in
cold desert climates. The filamentous sheaths form a persistent sticky network
that helps maintain soil structure. Plant trichomes
= Aerial surface hairs= Trichomes on plants are epidermal
outgrowths of various kinds. The terms emergences or prickles refer to
outgrowths that involve more than the epidermis. This distinction is not
always easily applied. Also, there are nontrichomatous epidermal cells that
protrude from the surface. A common type of trichome is a hair.
Plant hairs may be unicellular or multicellular, branched or unbranched.
Multicellular hairs may have one or several layers of cells. Branched hairs
can be dendritic as in kangaroo paw, tufted, or stellate, as in Arabidopsis
thaliana. Another common type of trichome is the
scale or peltate hair, that has a plate or shield-shaped cluster of cells
attached directly to the surface or borne on a stalk of some kind. Common
examples are the leaf scales of bromeliads such as the pineapple,
Rhododendron and sea buckthorn. Any of the various types of hairs may be
glandular, producing some kind of secretion, such as the essential oils
produced by mints and many other members of the family Lamiaceae.
In describing the surface appearance of plant organs, such as stems and leaves,
many terms are used in reference to the presence, form, and appearance of
trichomes. The most basic terms used are glabrous—lacking hairs— and
pubescent—having hairs. Details are provided by:
glabrous, glabrate – lacking hairs or trichomes; surface smooth
hirsute – coarsely hairy hispid – having bristly hairs
articulate – simple pluricellular-uniseriate hairs
downy – having an almost wool-like covering of long hairs
pilose – pubescent with long, straight, soft, spreading or erect hairs
puberulent – minutely pubescent; having fine, short, usually curly, hairs
pubescent – bearing hairs or trichomes of any type
strigillose – minutely strigose strigose – having straight hairs all
pointing in more or less the same direction as along a margin or midrib
tomentellous – minutely tomentose tomentose – covered with dense, matted,
woolly hairs villosulous – minutely villous
villous – having long, soft hairs, often curved, but not matted
Hairs on plants are extremely variable in their presence across species and
even within a species, such as their location on plant organs, size, density,
and therefore functionality. However, several basic functions or advantages of
having surface hairs can be listed. It is likely that in many cases, hairs
interfere with the feeding of at least some small herbivores, and, depending
upon stiffness and irritability to the palate, large herbivores as well. Hairs
on plants growing in areas subject to frost keep the frost away from the
living surface cells. In windy locations, hairs break up the flow of
air across the plant surface, reducing transpiration. Dense coatings of hairs
reflect sunlight, protecting the more delicate tissues underneath in hot, dry,
open habitats. In addition, in locations where much of the available moisture
comes from fog drip, hairs appear to enhance this process.
= Root hairs= Root hairs, the rhizoids of many
vascular plants, are tubular outgrowths of trichoblasts, the hair-forming cells
on the epidermis of a plant root. That is, root hairs are lateral extensions of
a single cell and only rarely branched. Just prior to the root hair development,
there is a point of elevated phosphorylase activity. Root hairs vary
between 5 and 17 micrometres in diameter, and 80 to 1,500 micrometres in
length. Root hairs can survive for two to three weeks and then die off. At the
same time new root hairs are continually being formed at the top of the root.
This way, the root hair coverage stays the same. It is therefore understandable
that repotting must be done with care, because the root hairs are being pulled
off for the most part. This is why planting out may cause plants to wilt.
Significance for taxonomy The type, presence and absence and
location of trichomes are important diagnostic characters in plant
identification and plant taxonomy. In forensic examination, plants such as
Cannabis sativa can be identified by microscopic examination of the
trichomes. Although trichomes are rarely found preserved in fossils, trichome
bases are regularly found and, in some cases, their cellular structure is
important for identification. Uses
Bean leaves have been used historically to trap bedbugs in houses in Eastern
Europe. The trichomes on the bean leaves capture the insects by impaling their
feet. The leaves would then be destroyed.
Trichomes are an essential part of nest building for the European wool carder
bee. This bee species incorporates trichomes into their nests by scraping
them off of plants and using them as a lining for their nest cavities.
Defense Plants may use trichomes in order to
deter herbivore attack via physical and/or chemical means. However, some
organisms have developed mechanisms to resist the effects of trichomes. The
larvae of Heliconius charithonia, for example, are able to physically free
themselves from trichomes, are able to bite off trichomes, and are able to form
silk blankets in order to navigate the leaves better.
See also Seta
Kief References
Esau, K. 1965. Plant Anatomy, 2nd Edition. John Wiley & Sons. 767 pp.