14 



TEXTBOOK OF PLANT PHYSIOLOGY 



releasing the oxygen, but retaining the carbon. Therefore, this 

 activity by the plant is very frequently spoken of as decomposi- 

 tion of carbon dioxide instead of assimilation of carbon. 



In reality, however, the process is much more complicated. 

 No direct decomposition of carbon dioxide to carbon and oxygen 

 takes place in the plant, as the free carbon is very inert and, once 

 having formed, would not be able to undergo any further trans- 

 formation. Simultaneously with the assimilation of carbon in 

 the green leaf, the assimilation of the elements of water also takes 

 place. It is not free carbon that accumulates in the leaf, but the 



products of its combination with hy- 

 drogen and oxygen of the water mole- 

 cule, forming various carbohydrates, 

 such as the different sugars and 

 starch. 



Thus by the process of the assimila- 

 tion of carbon and the elements of 

 water, 90 per cent or more of all the dry 

 substance of a plant is formed. The 

 process, therefore, is very aptly called 

 "assimilation" — a term that is justified, 

 since the assimilation of carbon, hy- 

 drogen, and oxygen is closely con- 

 nected with the assimilation {i.e., the 

 formation of organic compounds) of 

 other elements (nitrogen, sulphur, phos- 

 phorus, etc.) constituting the living 

 matter. 

 Starch, the ultimate product of carbon assimilation, may be 

 readily observed in the plant through the microscope in the form 

 of grains. It is easily detected also by means of the well-known 

 iodine reaction, which stains it a deep blue. The formation of 

 starch grains in the green plastids of plants may be observed 

 under the microscope, especially in the thin leaves of water plants 

 (Fig. 5) and mosses or the cells of algae. With sufficient illumina- 

 tion, starch formation may go on even under the microscope, and 

 the growth of the grains in the chloroplasts may be distinctly 

 seen. Under direct sunlight, 5 min. are sufficient for the forma- 

 tion of starch. Even with the comparatively weak light of a 

 kerosene lamp of 100 candlepower, Famintzin, as early as the 



Fig. 5. — Starch grains in the 



chloroplasts of Elodea {after 



Molisch) . 



