386 CHLOROPHYLL AND LIGHT INTENSITY. 



cells which are turned towards the dark background of the cave. There they are 

 grouped like a mosaic, and usually so that one green granule forms the centre, 

 while the others surround it very regularly in a circle. Such groups remind one of 

 the arrangement of the floral -leaves in Forget-me-not flowers, and give a very 

 ornamental appearance to the cells. Taken together, these chlorophyll-granules 

 form a layer, which, under a low power of the microscope, appears as a round green 

 spot. With the exception of these chlorophyll-granules the contents of the cell 

 are colourless and transparent, and share these characteristics with the unusually 

 delicate cell- wall. The light which falls on such cells through the opening of a 

 rocky cleft behaves like the light which reaches a glass globe at the further end of 

 a dark room. The parallel incident rays which arrive at the globe are so refracted 

 that they form a cone of light, and since the hinder surface of the globe is within 

 this cone, a bright disc appears on it. If this disc, on which the refracted rays of 

 light fall, is furnished with a lining, this also will be comparatively strongly- 

 illuminated by the light concentrated on it, and will stand out from the darker 

 surroundings as a bright circular patch. This lining has the power of manufac- 

 turing organic substances in the spherical cells of the protonema of the Luminous 

 Moss, and in this way the scanty incident light is turned to the greatest possible 

 advantage; it is refracted and concentrated on those places where the chlorophyll- 

 granules are situated, and consequently these receive in the dark recesses an 

 amount of light which amply suflBces for their special functions. It is well 

 worthy of notice that the patch of green chlorophyll-granules on the hinder side 

 of the spherical cell extends exactly so far as it is illumined by the refracted 

 rays, while beyond this region, where there is no illumination, no chlorophyll- 

 granules are to be seen. The refracted rays which fall on the round green spot are, 

 moreover, only partially absorbed; in part they are reflected back as from a 

 concave mirror, and these reflected rays give the cells of the protonema a luminous 

 appearance. This phenomenon, therefore, has the greatest resemblance to the 

 appearance of light which the eyes of cats and other animals display in half-dark 

 places, only illumined from one side, and so does not depend upon a chemical 

 process, an oxidation, as perhaps does the light of the glow-worm or of the 

 mycelium of fungi which grows on dec:iying wood. Since the reflected light-rays 

 take the same path as the incident rays had taken, it is clear that the gleams of 

 the Schistostega can only be seen when the eye is in the line of the incident rays of 

 light. In consequence of the small extent of the aperture through which the light 

 penetrates into the rock cleft, it is not always easy to get a good view of the 

 phenomenon described. If we hold the head close to the opening, we thereby 

 prevent the entrance of the light, and obviously in that case no light can be 

 reflected. It is, therefore, better when looking into the cave to place one's self so 

 that some light at anyrate may reach its depths. Then the spectacle has indeed 

 an indescribable charm. What has just been said about the isolated cells is 

 exemplified in groups of cells placed behind one another, of which usually 

 many thousands are found in a very small area. ^ 



