382 



CHLOROPHYLL AND LIGHT INTENSITY. 



spherical or spherical bodies, which project towards the centre of the cell-cavity. 

 By this means the area of the chlorophyll-granules attached to the cell-wall is 

 contracted, and consequently the green of the leaf -surface in question is diminished. 

 In the leaves of many flowering plants, also, the chlorophyll-granules which are 

 distributed in the palisade-cells along the elongated side-walls appear, in diffuse 

 light, hemispherical or even conical, and project towards the centre of the cells so 

 that they are illuminated to the greatest possible extent by the light rays passing 

 through. Under the influence of direct sunlight they flatten out, become disc- 

 shaped, and withdraw to some extent from the bright rays passing through the 

 centre of the cells. The significance of all these processes, the changes of shape 

 as well as the displacements of the chlorophyll-granules, is evident when it is 

 considered that an over-abundance as well as a deficiency of fight would be 

 prejudicial, and that for every species the quantity of the sun's rays absorbed 

 by the chlorophyll-granules is definite. Protoplasm, provided with chlorophyll, 



Fig. 97.— Position of the Chlorophyll-granules in the cells of the Ivy-leaved Duckweed (Lemna trisulca). 

 1 In darkness. > In direct sunlight. » In diffuse light. 



tries under all circumstances to obtain this definite amount. When weakly 

 illuminated, chlorophyll-granules maintain a shape and position in consequence of 

 which they present the largest possible surface to the light; when strongly illumi- 

 nated, they assume a shape and position by which the smallest possible surface is 

 so exposed. These processes, especially the displacement of the chlorophyll-granules, 

 obtain a heightened interest from the fact that they can only be brought about by 

 the streaming movements of the irritable protoplasm. It must be borne in mind 

 that it is really living protoplasm which displaces the chlorophyll-granules 

 imbedded in it in order to bring them to the places best suited to the illumination 

 then existing, and to place them in sunlight or shade; so that it always happens 

 that the displaced green bodies are neither too much nor too little illuminated. 



Many unicellular water-plants, especially zoospores, attain the same result not 

 by displacement of the chlorophyll-granules in the interior, but by movements of 

 the entire cells. These green unicellular organisms may be seen swimming towards 

 the light by means of their cilia, and in this way they take up the position always 

 best adapted to the given conditions. If many swarm-spores are collected together 

 in a limited area, it may happen that they all travel to one particular place; there 

 they swarm about in the water and appear to the naked eye like a little green 

 cloud. Or they may settle on the bottom of the pool, there arranging them- 

 selves side by side, so that no one deprives another of light, and they then appear 



