APPENDIX 347 



dark blue under the action of a solution of iodine. It is by this 

 method that we detect starch in the chloroplast. The formation 

 of starch, however, is not observed in the absence of light or 

 carbonic acid hence we are justified in concluding that the 

 formation of starch is the result of the decomposition of carbonic 

 acid. This is confirmed by the rapidity with which the one 

 phenomenon is followed by the other. The decomposition of 

 carbonic acid is manifested a few seconds after sunlight falls 

 upon the surface of the leaf, and five minutes later starch is 

 already found in the chloroplast. This correlation of the two 

 processes becomes still more apparent if we take into considera- 

 tion the chemical composition of starch. Starch can serve as 

 a representative and type of the group of vegetable substances 

 known as carbohydrates. The carbohydrates contain carbon, 

 hydrogen, and oxygen. Their name is derived from the fact 

 that in them hydrogen and oxygen are in the same ratio as in 

 water, so that they seem to consist of carbon and water. In 

 order to form a carbohydrate out of carbonic acid and water, 

 we have only to remove all the oxygen from carbonic acid, i.e. 

 we must perform exactly what takes place in a plant during 

 the decomposition of carbonic acid. Therefore carbohydrates 

 have precisely the same composition as would be expected of 

 substances formed in the plant from carbonic acid and water. 



In this way the microscope fully confirms the results obtained 

 by means of analysis. Whenever carbonic acid is broken down 

 in a chlorophyll granule carbohydrates form inside it. The 

 following example is a good proof of this correlation of the two 

 processes. A bright spectrum of the sun is thrown in a dark 

 room upon the leaf of a living plant, previously deprived of 

 starch. In an hour's time the leaf is removed, decolourised 

 with spirit, and treated witji a solution of iodine. It appears 

 that starch has formed in those parts of the spectrum alone 

 which are absorbed by the chlorophyll, and the greater the 

 absorption the more abundant the formation of starch; in 

 other words, the leaf obtains an impression of the spectrum of 

 chlorophyll in the form of starch coloured almost black by 

 iodine (fig. 83 IV — compare with fig. 82 and 83, I). 



The group of carbohydrates forms the largest component 

 of our vegetable food. Thus starch forms three-fourths of 

 the weight of a wheat grain and four-fifths of the dry matter 



