LIGHT AND DEPTH 129 



Some algae may want more, others less ; but without 

 light no vegetation can exist. The depth to which 

 rays of light penetrate the sea is the depth to 

 wh.ch algal life is limited. But this actual limit gives 

 us no idea of the quantity of light each alga needs to 

 thrive well. Each alga has three limits with regard 

 to light : an optimum, a maximum, and a minimum 

 limit. The optimum limit is that degree of light in 

 which a given alga thrives best ; the maximum limit 

 indicates the strongest, and the minimum limit the 

 feeblest, degree of light in which the same plant con- 

 trives to live. The four groups of algae just mentioned 

 have different optima with regard to light. The blue 

 and green algae live in the upper part of the littoral 

 region ; the brown algae love the sunny places of the lower 

 littoral and the upper part of the sublittoral region ; 

 and red algae prefer deep or sheltered localities ; al- 

 though many of the last class live also in places exposed 

 to the glare of the sun, they lose their bright red colours 

 in these exposed localities. Whether it is the quality 

 of the light (blue and violet rays penetrating deeper 

 than red and green ones), or whether it is the quantity 

 of light penetrating the sea that influences the colour 

 of the algae, is a question still open to controversy. 



Depth. The lower littoral and the upper zone of the 

 sublittoral region are the richest in algae ; the upper 

 littoral zone and the lower sublittoral are relatively 

 poor in algae. The greatest depths from which algae 

 have been recorded are near Capri at 66-5 to 71 fathoms; 

 near Minorca, 90 fathoms ; near Spitzbergen, 150 

 fathoms ; in the Barents Sea, 175 fathoms ; in the Ant- 

 arctic, 55-5 to 85-5 fathoms ; and south of the Cape, at 

 40 fathoms. Probably, however, the algae found near 

 Spitzbergen, and in the Barents Sea, will have sunk 

 into these depths from shallower localities. 



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