ABSORPTION OF CARBON 17 



deposited in the crust of the earth in the form of peat and coal. 

 These reserves are the chief motive power of the world's industry. 



4. The Green Plastids as Special Organs of Carbon Assimila- 

 tion. — The assimilation of carbon dioxide takes place in definite 

 parts of the cell, namely, in the green plastids. An indirect proof 

 of this is the fact that only the green parts of the plant are able to 

 absorb carbon dioxide and to liberate oxygen. A direct proof 

 was obtained by Engelmann by means of a very sensitive bacterial 

 method for detecting oxygen. Many bacteria show mobility only 

 in the presence of oxygen, even in small quantities, and possess 

 the faculty of moving towards the source of oxygen. If a uni- 

 cellular alga is placed in a drop of liquid containing such bacteria 

 and the preparation is exposed to light, all the bacteria will gather 

 around the alga as a source of oxygen. Care must be taken, how- 

 ever, to coat the edges of the cover glass with vaseline in order to 

 prevent the diffusion of oxygen from the surrounding air. If 

 instead of a single-celled alga, a thread of Spirogyra is studied, 

 the chloroplast of which has the form of a spiral band with suffici- 

 ently large portions by hyaline protoplasm between the coils of 

 this spiral, an accumulation of bacteria will be observed only 

 near such parts of the cell wall that are contiguous to the spiral 

 chloroplast. The movements of bacteria will be observed near 

 separate chloroplasts which have been singled out from destroyed 

 cells, though, in this case, the work of chloroplasts continued but 

 a short time, after which they die. 



Plastids without green color, as the colorless leucoplasts and 

 the brightly colored yellow and orange chromoplasts (especially 

 numerous in fruits and petals of flowers) do not eliminate oxygen 

 and, consequently, are not able to assimilate carbon. Leuco- 

 plasts, however, often preserve the faculty of manufacturing starch 

 from sugar. Such, for instance, are the starch formers in the cells 

 of the potato tuber and other underground storage organs. All 

 this serves to confirm the conclusion (Art. 3) that starch formation 

 in the plastids is a secondary process and is not directly connected 

 with carbon assimilation. 



The two principal constituents of chloroplasts are the green 

 pigment, which may be extracted with alcohol and is called chloro- 

 phyll, and the hyaline plasmatic base, the so-called stroma, which 

 preserves the form and dimensions of the chloroplast after the 

 chlorophyll has been extracted. 



