Extending from the holdfast is the stipe, a structure akin to the stem of a terrestrial plant. In some algae, the holdfast is discoid, in others it consists of a collection of fingerlike haptera. Unlike the root of a terrestrial plant, the holdfast generally does not intercalate into the substratum and has no specialized structures for the absorption of water or nutrients. The individual is attached to the substratum by a holdfast. And these forces are not rare waves crash on the shore every few seconds.ĭespite their morphological diversity, wave-swept macroalgae have a consistent basic body plan. Because of the actual compressibility of air at high Mach numbers, this calculation must be taken as a rough estimate, but the message remains the same: to envision the challenge faced by an organism in the surf, one must imagine oneself strapped to the nose of a supersonic airplane. If air were incompressible, a surf-zone water velocity of 25 m s -1 would exert a force equivalent to an air velocity of approximately 1600 miles h -1, somewhat in excess of Mach 2. In other words, the relatively gentle motions of unbroken ocean waves are equivalent to hurricane-force winds. The force imposed on a structure by a water velocity of 2 m s -1 is roughly equivalent to that exerted by a wind of 130 miles h -1. But these dry facts have little impact on psyches accustomed only to the forces imposed by the wind. For example, before waves break they are often accompanied by water velocities of 2 m s -1 ( Denny,1988), while velocities as high as 25 m s -1 have been recorded in the surf zone (M. As large terrestrial organisms, it can be difficult for humans to grasp what it is like to live in the wave-swept environment of a rocky shore.
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