388 CULOKül'llYLL AND LlUHT IXTENSITV. 



ia (lej)tli, whose widtli varies with the steepness of the slioi-e. Below this naiTow 

 girdle, vegetation is practically extinguished, and the depths of the ocean are in all 

 parts of the globe a plantless waste. This statement is not contradicted by the fact 

 that sea-wracks have been found showing a length of 100, it is alleged even of 200 

 and 300 metres, as, for example, the celebrated 3Iacrocystls pyrifera, between Xew 

 Zealand and Tierra del Fuego. These sea-wracks do not rise perpendicularly from 

 the bottom to the surface of the sea, but proceed from steep declivities, and grow 

 at an angle to the surface, on which account they often take the direction of the 

 curi'ent. One must imagine their position in the water to be almost like that of the 

 Floating Pondweed, or the Water Crowfoot {Poiamogeton fluitans and Ranunculus 

 fluitans), which occur in brooks only a few decimetres deep, and nevertheless maj'' 

 attain a length of more than a metre. 



It is naturally to be expected that plants which grow in the dim light, deep 

 under the water on a rocky reef, would behave exactly like the grotto-inhabiting 

 Luminous Moss; and if the barrel-shaped and spherical cell-structures connected 

 into chains, the cyst-Kke and berry-shaped outgi'owths of the unicellular Caulerpas 

 and Halimedas, as well as the facetted cell-walls of those diatoms living in the 

 abysses of the sea in dim twilight, are accepted as contrivances by which light is 

 collected and focussed on those places within the cells where the chlorophyll-bodies 

 are heaped up, then no mistake will be made. Several of the sea-inhabiting Floridese 

 and sea-wracks belonging to the genera Phylocladiu, Pohjsiphonia, Wrangelia, 

 and Cystosira, even exhibit under the water a peculiar luminosity which may be 

 compared with that of the Luminous Moss, although the optical apparatus is here 

 essentially different. In the superficial cells of the luminous Phylocladias are to be 

 found plates segregated out of the ^^I'otoplasm and closely adhering to the outer 

 walls, which contain a large number of small crowded lenticular bodies. Fi-om these 

 minute lenses the blue and green rays are chiefly reflected, and thus the peculiar 

 iridescence is produced. But, on the other hand, yellow and red rays are refracted 

 on to the chlorophyll-gi-anules, and consequently these plates must be regarded as 

 an apparatus for focussing the light, which, by its passage through the thick layers 

 of water, has undergone a considerable diminution. 



In the depths of the sea, however, another optical phenomenon must be taken 

 account of, by which the illumination of chlorophyll-granules in the plants growing 

 there becomes in the end a favourable one, and that is the fluorescence of erythro- 

 phyll, the fluorescence of that I'ed pigment to which the Floridese owe their charac- 

 teristic colour. In order to make this plienomenon clear, it seems necessary first 

 of all, to rectify a wide-sjjread error with regard to the colour of water generally, 

 and particularly of sea- water. In the very attractively-written work by Schleiden, 

 The Plant and its Life, the seventh chajster, which treats of the sea and its 

 inhabitants, begins witli the followin"- lines: — "O learn to know them, the horrible 

 deeps, which are concealed beneath the shining treacherous surface. You descend, 

 the blue of the sky vanishes, the light of day is gone, a fiery yellow surrounds 

 you, then a flaming red, as if you were plunged into a watery sea-hell, without 



