Page:The Scientific Monthly vol. 3.djvu/299

 ORIGIN AND EVOLUTION OF LIFE 293

In the section (Fig. 2, ^1) shown by the arrow, there is a little chain of cells closely similar to those in the existing species of Azoto- bacter, an organism that fixes atmospheric nitrogen and converts it into a form ntilizable by the plant. It is related to the Nitroso coccus, Nitroso monas and Nitrobacter, and lives on simple salts with mannite (CgHj^O,)' as a source of carbon.

���Fia. 2. A. Fossil bacteria frocn the (iewland limestone (Algonklnti) (after Walcott). B. Nitrifying bacteria found In soil. Tbe arrow IndlcBtea a clisiD atruc- tnre similar to that ot Welcott's fossil bacterln. V. Nitrifying bacteria lound in ■oils — a more complex type than the above. D. Mltrogen-Qilng bacteria from the root nodnles o( legumes. Note the granular structure. E. Denitrifying bacteria found In soil and water, P. Bacteria stained to bring out the cbromutln granules.

It was only after the chlorophyllic carbon-storing true plants had evolved that the second great group of nitrifying bacteria arose to develop the power of storing the nitrogen of the atmosphere through life association or symbiosis with plants, and of deriving their carbon, not from inorganic compounds, hut from the carbohydrates of plants. Such Qserg of atmospheric nitrogen and plant carbon include three general types: B. radicicola, associated with the root formation of legumes, Clostridium (anaerobic), and Azotobacter (aerobic)."*

The gradual evolution of cell structure in these organisms can be partly traced despite their excessively minute size. The cell structure of the Algonkian and of the recent Nitroso coccus bacteria (Fig. 2, A, B) is very primitive and uniform in appearance, the protoplasm

■ Maunite is needed by the higher forms of nitrifiera (A^oioba^^ter), but not hy the primitive types, and nas probably not found until plant life fiour-

i» Jordan, Edwin 0,, 1908, pp. 484-491.

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