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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. |
