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Ribs  [ Biology ]
Adjective: Ribbed

Dictionary of botanic terminology - index of names

  Any rib-like supporting or strengthening part of an animal or plant.  
(1) Ribs (of the stem)   [ Botany ]
  Vertical accordion like structure found on  trunks and stems of columnar species of cacti and succulents, rib can expand and retract with the amount of water they contain.  
Less than 50 % of cacti have ribs, the podaria of some species have become fused into vertical ribs that cover the entire surface of the stem. The barrel cactus provides a good example of this type of ribbing.

The ribs shape allows the plant to expand and increase the volume of water it can hold. The plant may lose up to 80% of the water it stores. As the stem loses water, it retracts (folds inward) at the groves between the ribs, and its volume decreases, minimizing the surface area exposed to direct sunlight.  
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.
Ribbed stems occur in Asclepiadaceae, Cactaceae, Euphorbiaceae, and Vitaceae as well as other families.

(2) Ribs (of the leaves)   [ Botany ]
  Of  vein and venation raised end evident on the leaf lamina  


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