432 PHYSIOLOGY OP GROWTH. 



selves; the changes in position which they undergo are caused by 

 movements of the cell protoplasm. To gain exact information re- 

 specting the movements of the chlorophyll bodies, it is advisable to 

 select for examination moss leaves or fern prothallia, for in the 

 palisade parenchyma of ordinary foliage leaves the chlorophyll 

 grains usually experience only inconsiderable changes in position, 

 and in the spongy parenchyma, in whose cells it is true consider- 

 able changes in the position of the chlorophyll grains may take 

 place, the relations in question are often, for various reasons, not 

 easy to follow. 



If plants of Funaria hygrometrica or fern prothallia (found 

 without much difficulty in greenhouses in which ferns are grown) 

 are kept for some time in darkness, under otherwise normal con- 

 ditions, the chlorophyll bodies pass into the darkness position, i.e. 

 in the case before us they travel to the cell-walls running perpen- 

 dicularly to the surface of the object. In other cases, however, the 

 chlorophyll bodies do not behave in exactly the same manner. If 

 the shading has not been continued too long (only for several hours), 

 so that the contents of the cells have not gone over into a con- 

 dition of darkness-rigor, but are still phototoiiic, it will be found 

 that the chlorophyll bodies are peculiarly sensitive to light. Sods 

 of Funaria or fern prothallia, which have been kept in darkness, 

 are exposed to diffused light in such a way that the plants receive 

 the rays of light from above. After a few hours we place a few 

 Funaria leaves, or a few prothallia, in a drop of water on the slide, 

 lay on a cover-glass and observe. The chlorophyll bodies are no 

 longer disposed 011 the side walls of the cells, as in the darkness 

 position, but have arranged themselves on the front and back 

 walls, and moreover turn themselves broadside to the observer 

 (surface position). We now expose our preparations, on the slide, 

 to direct sunlight, keeping the outside of the cover-glass well 

 moistened with water, so as to avoid overheating. If we examine 

 after a few minutes, we shall perceive that the chlorophyll bodies 

 are still in the same position as before, but their form has changed. 

 The previously polygonal grains have drawn in their angles ; they 

 have become rounded, and the chlorophyll bodies are clearly striv- 

 ing to present as small a surface as possible to the too intense 

 light. This rounding off of the green constituents of the cells is 

 due to movement of the chlorophyll bodies themselves ; but beside 

 this, exposure to very intense light induces a change in the position 

 of the chlorophyll bodies, which is brought about by the proto- 



