such as the blue, which are allowed to pass through. "The rays on the further side of the red, not perceptible to our eyes—the so-called dark heat-rays—are likewise absorbed in their passage through the water, and an object at some depth under water would therefore only be reached by rays of high refrangibility, particularly blue rays. The conditions of illumination for plants growing in the depths of the ocean are consequently in reality quite unfavourable. It is not only that a portion of the light falling on the surface of the water is reflected, and the other portion is weakened by its passage through the water, but besides, those rays which are necessary to the formation of organic matter by the chlorophyll granules in the 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, phycoërythrin, the red pigment of the Floridæ, 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 Ulvacece and Entermorphas sway hither and thither, forming thus a light green belt; these algæ 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 Floridæ are reddened—sometimes a delicate carmine, sometimes a deep purple; then again a light brownish red,
