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
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.
Several
succulents during
seasonaldormancy 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.
For example the
columnartrunk 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)
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 palisadecortex, 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
tubercledstem 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.
Some species of climbing
plants develop holdfast roots which help to support the vines on
trees, walls, and rocks. By forcing their way into minute pores and
crevices, they hold the plant firmly in place.
Climbing plants, like the poison ivy (Toxicodendron
radicans), Boston ivy (Parthenocissus
tricuspidata), and trumpet creeper (Campsis
radicans), develop holdfast roots which help to
support the vines on trees, walls, and rocks. By forcing their
way into minute pores and crevices, they hold the plant firmly
in place. Usually the Holdfast roots die at the end of the first
season, but in some species they are perennial. In the tropics
some of the large climbing plants have hold-fast roots by which
they attach themselves, and long, cord-like roots that extend
downward through the air and may lengthen and branch for several
years until they strike the soil and become absorbent roots.
Major references and further lectures:
1) E. N. Transeau “General Botany” Discovery Publishing House,
1994