170 VEGETABLE PHYSIOLOGY 



occurrence affords us an easy method of demonstrating the 

 activity of the chlorophyll apparatus. If a leaf is partially 

 covered by a piece of opaque material, such as tinfoil, and 

 is then exposed to the light, starch rapidly appears in the 

 illuminated portion. Its presence can be shown by bleach- 

 ing the leaf with boiling alcohol, and then immersing it in 

 iodine, which forms a blue colour with starch. The blue 

 tint only appears where the light has reached the chloro- 

 phyll apparatus. 



These processes are carried out by the chlorophyll 

 apparatus under the conditions set forth. It is evident 

 that such changes as have been described cannot be accom- 

 plished without- the expenditure of a considerable amount 

 of energy. In this need we have the explanation of the 

 composite nature of the chloroplast. The chlorophyll 

 absorbs certain rays of light which fall upon it, and the 

 energy which is liberated by the extinction of their vibrations 

 is taken up by the protoplasm of the plastid and applied 

 by it to effect the decompositions that take place. A 

 very ingenious method of demonstrating that the energy is 

 derived from the rays of light absorbed by the pigment 

 was devised by Engelmann. He observed that certain 

 bacteria were excited to active movement only in the 

 presence of free oxygen. He placed a filament of a green 

 alga upon a glass slide in a fluid containing a number of 

 the bacteria, covered it with a glass cover-slip, and sealed it 

 with wax. He kept it in darkness till the microbes had 

 come to rest, and then by the aid of a microspectroscope he 

 threw a spectrum upon the filament and observed in what 

 parts of it the bacteria accumulated as soon as they began 

 to move. These places corresponded with the positions of the 

 absorption bands which we have seen to be characteristic 

 of the chlorophyll spectrum, the maximum effect being 

 produced by the deep band in the red region. These were 

 evidently the places at which the chlorophyll apparatus 

 of the filament was at work, the movements of the bacteria 

 showing that oxygen was liberated there. Tmiriazeff 



