POTASH FROM KELP. 2.7 
WAVE ACTION. 
Waves themselves are necessary to keep kelps alive and growing. 
Macrocystis does not usually grow in the surf. It seems to require 
deeper water in order to make its best growth, as its leaves are too 
fragile to withstand the wrenching and pounding of the breakers. 
San Pedro breakwater furnishes an excellent place in which to study 
the effects of waves on the distribution of kelp. The very tough 
ribbon kelp (regia laevigata) completely fringes the weather side 
of the breakwater, where it receives the full impact of the breakers. 
No Macrocystis grows here, but just beyond the breaker line it is 
abundant. On the lee side of the breakwater, on the other hand, 
there is a rich variety of seaweeds, in which Macrocystis is promi- 
nent. Here streams of water rush through the crevices after a big 
wave strikes the weather side. During the winter and early spring, 
with the aid of the great quantity of driftwood that accumulates, 
the waves wear away the plants in the tidewater zone faster than 
they can grow up, and for considerable areas the rocks are completely 
stripped. In the calm weather of spring, growth begins in crevices, 
pockets in the rocks, and other less exposed places. Egregia is the 
first to appear and Macrocystis comes a little later. When growing 
under some protection Macrocystis makes a more vigorous plant, 
with a longer primary stem or stipe, than when growing in an 
exposed place. Where the water falls in a small cascade on the 
bare face of a rock, kelp plants do not appear until small plants and | 
animals have covered the surface. 
STORMS. 
Kelp beds are sometimes seriously depleted or even completely 
torn out by storms. For example, the Point Loma kelp bed was 
completely torn out by the great storm of the winter of 1888-89 
(Davidson, 3), and the same kelp bed, as well as the La Jolla bed, 
was reduced nearly 40 per cent in area, besides being greatly thinned 
by storms in the late winter and early spring of 1915. Strong north- 
west winds are usually not very destructive, because they have little 
effect on the size of the waves, and because they are very frequently 
accompanied by a strong current that holds most of the kelp down 
out of harm’s way. Southwest winds, on the other hand, usually 
raise a violent swell and usually there is not enough current to keep 
the plants submerged. Under such circumstances, a plant near the 
outer edge of a kelp bed may become detached from its anchorage 
or, if attached to a rather small stone, may “drag its anchor.” No 
matter in what way a plant begins to drift, it always floats with its 
holdfast under water. Drifting through the bed, the fronds of the 
first plant* with which it comes in contact enwrap the holdfast. 
Presently the second plant with its double burden of fronds lets go, 
and the mass starts through the bed, increasing in volume and de- 
structive power as it travels. Thirty or more plants sometimes come 
ashore in one single “drive.” The thicker the kelp beds the greater 
the destruction is likely to be in the more violent storms. 
