56 Ellen Marion Delf 



or twelve inches deep, high up on the shore near Camps Bay. The 

 pool was sheltered from all direct light by the deeply overhanging 

 rock, at one side of which was a narrow vertical cleft. Near the 

 base of the cleft was i plant of Celidium cartilagineum well grown 

 and just submerged. This is a characteristic deep water red 

 Alga, and is rarely, if ever, exposed at low water. It occurs 

 usually at Camp's Bay in deep pools near low water mark, partly 

 hidden by larger thalloid Algae (Gigartina radula and others). 

 Prof. Thoday succeeded in lowering an exposure metre to the 

 depth of the cleft and found that the light intensity just above the 

 level of the Celidium was not more than one thirtieth of the full 

 daylight. This was probably as great an intensity of light as the 

 plant would ever receive, and that only for a short time daily. 

 Another deep water rose red Alga, Plocamium corallorhiza, is 

 characteristic of rock faces on the shores of False Bay where it 

 is sometimes visible at low spring tides. By dredging, this has also 

 been found growing at a depth of 36 feet, in the fertile condition. 

 Red seaweeds of this type are then truly shade plants in their capa- 

 city for utilising light of such feeble intensity. In England a 

 typical shade flowering plant receives from about one half to about 

 one third of the full daylight, in localities where it flowers, though 

 a deeper shade often suffices for the survival of the vegetative 

 organs. 



Other factors besides light intensity determine the distribution 

 of Algae on the coast. One of these is their power of resistance 

 to dessication whilst the tide recedes. In an atmosphere as dry 

 as that of this country, this factor may be of considerable import- 

 ance. Two of the most resistant forms from this point of view 

 are the blue-green Alga, Ly^ngb^a majuscula and the purplish 

 red Alga, Porph^ra laciniata. Both are fairly common forms 

 found at or near high tide levels. L^ngb^a has a blackish 

 appearance and is found in shallow pools or sometimes nearly 

 dried, forming a dense covering on the surface of rocks. In the 



1- This information wtes received from Miss E. L. Stephens 

 of the Cape Town University (Botanical Laboratory), 



