104 THE CELL 



selves through the otherwise transparent cell. The band is able 

 to assume every possible intermediate position in its endeavour to 

 place its surface at right angles to the impinging light. On a 

 warm summer's day this change of position is effected in a very 

 few minutes, being brought about by the active movements which 

 the protoplasm makes inside the cell membrane. 



The effect produced varies in this case also, as with the zoo- 

 spores, according to the intensity of the light. Whilst diffuse 

 daylight has the effect described above, direct sunlight brings 

 about a quite opposite result, for in this case the chlorophyll 

 bands turn one of their edges to the sun. Hence we can educe 

 the following : " Light exerts an influence upon the position 

 of the chlorophyll bands of Mesucarpus. If the light is fairly 

 weak, the bands turn themselves at right angles to the path of the 

 rays ; if, however, it is intense, they place themselves in the same 

 direciion as the rays." Stahl calls the first arrangement surface 

 position, and the second, profile position. 



If illuminated intensely for a considerable period, the whole 

 band contracts to form a dark green vermiform body; it is, 

 however, under favourable conditions capable of resuming its 

 original form. 



The purpose of all these various movements of the protoplasm 

 under the influence of light is, on the one hand, to bring the 

 chlorophyll bands into a favourable position for the exercise of 

 their functions ; and, on the other, to protect them from the in- 

 jurious action of a too powerful illumination. 



Further, the plant-cells which contain chlorophyll granules, and 

 which are connected to form tissues, are also subjected to the 

 influence of light, as is so plainly seen in Hesocarpus. Only in 

 this case the phenomena are somewhat more complex (Fig. 52). 



Sachs was the first to notice that the colour of leaves is lighter 

 when they are exposed to direct sunlight, than when they are in 

 shadow, or when the light is less intense. In consequence of this 

 discovery, Sachs was able to produce light pictures upon leaves, 

 by partially covering them with strips of paper, and exposing 

 them to intense light (IV. 32a) ; after a certain time the strips of 

 paper were removed, and it was then seen that the portions which 

 they covered appeared as dark-green stripes upon a light-green 

 background. 



This phenomenon may be explained by the law which was laid 

 down in the case of Mesocarpus ; this has been proved by the 



