CHLOROPHYLL AND LIGHT INTENSITY. 387 



Among the mosses which find their home in deep shady places, principally in 

 hollow tree-trunks, and are noticeable there for their glossy green, Hookeria 

 splendens is especially worthy of attention. To be sure, its leaves do not shine as 

 brightly as the protonema of Schistostega, but the appearance is, on the whole, much 

 the same, and here also a similar development is the cause. The leaves of Hookeria 

 are comparatively large, but at the same time very thin and delicate. They are 

 composed of a single layer of rhombic cells, very convex above and below, so that 

 the whole leaf may be compared to some extent to a window with very small 

 so-called "bull's eyes" in the glass. The chlorophyll-granules are here arranged 

 with far less regularity than in the protonema of the Luminous Moss, but they 

 are heaped together just as in that plant on the side of the leaf facing the ground, 

 that is to say, which is turned from the light. The side which is turned in the 

 direction of the scanty incident light has no chlorophyll layer. The hemispherically- 

 convex cells, opposed to this scanty light which falls on one side of the leaf, act like 

 glass lenses; they concentrate the weak light on the chlorophyll-granules heaped 

 up on the other side; but, on the other hand, light is also reflected, and this gives 

 rise to the green lustre with which the Hookeria shines forth from its dim sur- 

 roundings. 



Like those plants which inhabit rocks, grottoes, and stony clefts, and the shady 

 obscurity of hollow trunks, plants whose habitat is at the bottom of the sea, and 

 in the depths of lakes and ponds, are only visited by weakened sunbeams. The 

 illumination becomes the dimmer the deeper the habitat in question lies below the 

 surface of the water, since the intensity of the light penetrating the water dimin- 

 ishes with the increasing length of the distance travelled. At a depth of 200 metres 

 under the sea complete darkness reigns; at 170 metres the intensity of illumination is 

 Hke that observed above the water on a moonlight night; such an illumination is 

 insufiicient to enable chlorophyll-bearing plants to manufacture organic substances 

 from the absorbed raw materials, even although the plants were provided with all 

 possible aids for the collection of this exceedingly weak light. It is only at a depth 

 of not more than 90 metres that light is sufficient for the chlorophyll cells to 

 decompose carbonic acid, and this depth is ascertained to be the lowest limit of 

 chlorophyll-bearing plants. Moreover, these figures are only applicable in the most 

 favourable circumstances in broad daylight, and only when the water is very clear 

 and transparent, which really only seldom occurs, we might even say excep- 

 tionally. The substratum on which the submerged plants are situated, whether 

 sand, mud, or rock, is usually sloping, and is most visited by the oblique rays of the 

 sun. Frequently also small solid particles are suspended in the water, even in water 

 which in the aggregate appears to be quite clear, and so the light is again con- 

 siderably weakened. This happens especially in the neighbourhood of steep coasts, 

 where the seething of the waves works uninterruptedly at the destruction of the 

 solid shore, and consequently at a depth of 60 metres on such steep declivities, 

 plants possessing chlorophyll are seldom met with. 



Generally speaking, the vegetation in the sea is limited to a zone of about 30 metres 



