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Retraction
(Noun)
[
Botany ] |
Dictionary of botanic terminology - index of names |
Synonym:
Shrinkage, Contraction
Transitive and intransitive verb:
To retract (past
and past participle:
retracted, present participle:
retracting, 3rd person present
singular: retracts)
Derived forms:
Retraction, Retractility, Retractile |
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- TO RETRACT (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.
- RETRACTION (Noun): The
act of retracting something, or the condition of being retracted.
- RETRACTILITY (Noun):
The capability of something of retracting.
- RETRACTILE (Adjective):
Able of being retracted
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[ From Latin "re" to
revoke, and "tractum" (past participle of
"retrahere" to draw back) = pulled; hence, to pull
something back ]
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Photo 1: Shrunken (retracted)
Cladodes of
Opuntia compressa during
winter rest
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In
botany the 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. Besides trunks
and stems of
columnar species are often
ribbed (accordion like), and
can expand and retract
with the amount of water they
contain. 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)
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 summers, 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.
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Physiology of retraction
(Contraction) 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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