THE METABOLISM OF PLANTS. 299 



in feeble light, they lose their power of movement. On the 

 other hand, exposure to light arrests the spontaneous move- 

 ments of the motile leaves in the majority of plants which 

 possess them. 



A remarkable instance of the influence of light in causing 

 movement is afforded by the change in position of the chloro- 

 phyll-corpuscles in cells exposed to light. Marquard, and 

 after him Sachs, observed that green leaves exposed to sun- 

 light soon assume a brighter green colour than they have 

 when in the shade : this can be made very evident by cover- 

 ing a portion of a leaf exposed to sunlight with some opaque 

 body, a strip of tinfoil for instance ; on the removal of the 

 tinfoil after a few minutes the parts which were covered are 

 seen to have a deeper colour than those which were exposed. 

 The difference of colour is due to the different distribution of 

 the chlorophyll-corpuscles in the cells in the two cases. It 

 appears, from the researches of Famintzin, Borodin, Frank, 

 Stahl, and others, that in diffuse daylight the chlorophyll- 

 corpuscles collect on the free cell-walls, that is, on the walls 

 next the surface in the superficial cells of organs consisting 

 of several layers of cells, and on the upper and lower walls of 

 organs consisting of only one layer of cells : whereas in direct 

 sunlight they collect upon the lateral walls, and in darkness 

 upon the lateral and lower walls. In the former case, which 

 Frank terms Epistropke, the corpuscles lie parallel to the 

 surface of the organ ; in the latter, which is termed Apostrophe, 

 they lie at right angles to the surface (Fig. 36) : hence the 

 green colour of an organ, a leaf for instance, is of a darker 

 hue in epistrophe than in apostrophe. Since the chloro- 

 phyll-corpuscles are themselves incapable of movement, 

 their change of position under the influence of light must be 

 attributed to movements of the protoplasm in which they 

 are imbedded. Frank has in fact observed that the collection 

 of the chlorophyll-corpuscles on any of the cell-walls is ac- 

 companied by an accumulation of protoplasm. Under un- 

 favourable conditions, for instance when the temperature is 

 low, or when the cells are old, the chlorophyll-corpuscles 

 remain in the position of apostrophe. 



