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Contraction
[
Botany ] |
Dictionary of botanic terminology - index of names |
Synonym:
Retraction,
Shrinkage
Transitive and intransitive verb:
To contract (past
and past participle:
contracted, present participle:
contracting, 3rd person present
singular: contracts)
Derived forms:
Contraction,
Contractility, Contractile |
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- TO CONTRACT (verb): To
move, or move something, back inside: to shrink something
backwards from an extended position, to pull inward or towards a
centre; or be drawn in.
- CONTRACTION (Noun): The
act of contracting something, or the condition of being
contracted.
- CONTRACTILITY
(Noun): The capability of something of contracting.
- CONTRACTILE
(Adjective): Able of being contracted.
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Photo 1: Shrunken (contracted)
Cladodes of
Opuntia compressa during
winter rest
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In
botany the contraction
(retraction) is a
phenomenon common among
succulents and cacti
consisting in the reduction of
body volume in response to dry
conditions that helps retard
water loss in the initial
phases of drought.
Analogously retraction due to
dehydration of plant
tissues is a
survival strategy used by
many plant to avoid
cold damages when
temperature slow down
to the freezing-point.
(See: Photo 1)
Many plant species
belonging to several family
have adapted to
xeric conditions by becoming
succulent, and during
their evolution, several
problems had to be solved. First, the
transpirational
surface area could be
reduced either temporarily by leaf
abscission or
permanently by evolutionary
reduction of leaves. Second,
sufficient water storage
capacity had to be available to allow
persistent
organs such as
buds,
roots, and the
stem
axis to survive
droughts. Third,
seasonal
rain/drought
cycles caused the plants'
volume to expand
(increase) and contract (decrease)
cyclically (see:
Dehydration-rehydration cycle)
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Several
succulents during
seasonal
dormancy periods –
often throughout most of the year - retract just below the
soil surface often with
only the summit protruding above making the plants almost
impossible to locate during hot,
dry periods.
Same leaf succulents
like Haworthias have
long, fleshy,
retractile
(contractile) roots. In
the cool wet seasons, these roots serve as a
water store, much
like the
caudiciforms. In the dry hot
seasons, these roots serve two
functions: they return
moisture to the body of the
plant and, while doing so, they shrink, pulling the plant deeper
into the soil to protect the
plant from sun and
heat.
Besides trunks
and stems of
columnar species are often
ribbed (accordion like), and
can expand and retract
with the amount of water they
contain.
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Photo 2:
Contractile roots of
Haworthia emelyae v. comptoniana |
For example the
columnar trunk of a
Saguaro (Carnegia gigantea) has
ribs which enable the plant to
swell and shrink like an accordion depending on
rains. A
mature saguaro may
soak up 50 litres
of water during a
rainstorm, only gradually
transpiring its
supply over long periods of
drought. (See: Table 1)
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Physiology
of Contraction (retraction) of cactus stems.
Cactus cortex is
divided into an inner water-storing
region and an outer
photosynthetic layer. More than
cortical bundles
is needed to move water from the
storage region to the
chlorophyllous region:
water-storing cells should give up water more easily than do
other cells. The
selective advantage of storing water is not just that it
keeps water-storage cells alive
but rather that the water can be made available to cells of the
photosynthetic palisade
cortex, the
apical and
axillary buds, any
flower buds or
developing fruits and so on.
Consequently,
water-storage cells should have thin, flexible
walls that can contract
or shrink readily such that the cell’s volume diminishes as
water is transferred out. On the other hand, the cells that need
the water should be more resistant to shrinkage: if all cell
walls were equally
flexible, all parts of the plant would suffer
water stress equally,
but that is not adaptive.
Instead, water-storage
tissues should give up water so easily that the more
active cells do not
experience water stress
unless drought is extremely
prolonged. In all cacti,
cell walls of the inner
cortex are especially thin
and flexible,
but in many cacti there is
an additional modification: the walls are folded or
undulate, even when young
and recently produced by the
shoot apical meristem.
As the amount of succulent
tissue increases in a stem, so
does the potential for large changes in volume: the plant will
swell greatly after a rain and
shrink during drought The
epidermis and hypodermis
must accommodate this, but whereas
young, growing
dermal tissues are
extremely
extensible, mature
ones are not: the total surface
area of a region of mature
stem tends to be constant. Many succulent stems have contiguous
ribs or
tubercles that can widen
or shrink at the base whenever
the stem swells or contracts When
dry, the stem has lost volume and the ribs are narrow; when
hydrated, the stem is
swollen and its ribs are broad. Thus, volume
cycles, while surface area
remains constant. Ribbed stems occur in
Asclepiadaceae,
Cactaceae,
Euphorbiaceae, and
Vitaceae as well as other families. (See: Table 1) |
Table 1: During a drought,
a ribbed or
tubercled
stem is
dehydrated and has a
small volume (A), but after a
rain it quickly
rehydrate and volume
increase (B) Although the
surface area is unchanged. Typically, ribs or tubercles
touch each other at their base
and the stem axis has no
surface other than rib surface when swollen. |
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