Retraction

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

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

[ From Latin "re" to revoke, and "tractum" (past participle of "retrahere" to draw back) = pulled; hence, to pull something back ]

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)

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.

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.

Advertising

Holdfast roots [ Botany ]
Dictionary of botanic terminology - index of names

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