390 CHLOROPHYLL AND LIGHT INTENSITY. 



plant cells are abstracted from the light which passes through; for the chlorophyll- 

 granules need just the red, yellow, and orange rays if they are to perform their 

 functions; only under the influence of these rays can the decomposition of carbonic 

 acid, the separation of oxygen, and the formation of carbohydrates, take place. 

 The blue rays do not assist at all in this respect; they are even hurtful to these 

 processes, since they assist the oxidation, that is, the decomposition of organic 

 substance. Consequently, phycoerythrin, the red pigment of the Florideae, now 

 appears, and indeed so abundantly, that the chlorophyll-granules in the interior 

 are quite hidden by it. This colouring-matter displays a very marked fluorescence, 

 that is to say, it absorbs a large portion of the light rays falling on it, and gives 

 out other rays of greater wave-length. The blue rays are to some extent changed 

 by it to yellow, orange, and red, and thus the chlorophyll-granules finally receive 

 those rays which act as the propelling force in the decomposition of carbonic acid. 

 But this also affords an explanation of the remarkable phenomenon that sea- 

 plants are only coloured green close to the shore, and only in the most superficial 

 layers of water, while lower down they appear red. Only quite on the surface the 

 emerald-like Ulvaceae and Enteromorphas sway hither and thither, forming thus a 

 light-green belt; these algae are to be sought for in vain in the depths beneath; of 

 the plants which flourish below this region it can no longer be said that they grow 

 green; this mark of vegetation has entirely vanished. Green has given place to 

 red. All the innumerable Floridese are reddened sometimes a delicate carmine, 

 sometimes a deep purple; then again a light brownish-red and a dull, dark crimson, 

 and as we admire in the bush the innumerable gradations of green colour, so is the 

 eye delighted in the manifold shades of red, in which the different variegated 

 species of Floridese, intermixing with one another, display themselves. 



Let us now leave the blue twilight of the sea-depths, and set foot on the strand 

 lapped by the blue waves sparkling with white foam, and climb up one of the rocky 

 crags rising there above the seething waters. Around us is the bright daylight, 

 and broad terraces of rock thickly overgrown with plants, all brilliantly illumined 

 by the unclouded sun. But where is that fresh green which we expect to find up 

 here according to the foregoing definitions in herbs and bushes? Here are not green, 

 but grey foliage and branches, white-haired stems and leaves, and the whole 

 woven and bound together into a carpet, which looks as if it had been strewn with 

 ashes, or as if the wind had for a week brought hither the dust from the neigh- 

 bouring streets and deposited it. The plants here on the sunny rocks have pro- 

 vided themselves with silky, woolly, and felted coverings for the purpose of softening 

 the too glaring light. In the depths of the sea and in the grottoes of the slate 

 rocks, the light was too weak; here, however, it is too strong. The chlorophyll- 

 granules tolerate neither the one nor the other; they require light of a definite 

 intensity. If the limit, which in this matter is exactly defined for each species, is 

 overstepped, the chlorophyll is destroyed. Too much light may be no less injurious 

 to the plants than if the chlorophyll -granules are condemned to inactivity on 

 account of the want of light. 



