THE HISTORY OF PLANTS 



405 



meager, we shall have to piece it out with inference and deduction based 

 on our knowledge of the group as it now exists. 



The Bacteria (Fig. A. 4) lack chlorophyll and are classed as plants 

 chiefly because of their similarity in structure to the blue-green algae and 

 the ability of some of them to use chemical energy for the synthesis of 

 organic from inorganic substances. Most of them feed upon organic 

 matter, and in the course of their evolution many have become parasitic, 

 producing disease in plants and animals. We tend, therefore, to think 

 of the bacteria as injurious, whereas in fact a great many are beneficial 

 and essential in maintaining the balance of nature. They are the chief 

 agents in the formation of soils, for without the acids and alkalies they 



Fig. 26.4. Some types of modern green algae. 



produce, the decomposition of rocks would be an exceedingly slow process. 

 Certain bacteria have the ability to take nitrogen from solution in water 

 and to build it into compounds which can be used by higher plants. They 

 are alone among organisms in this ability. Of no less importance is the 

 fact that bacteria are the chief agents of decay, breaking down dead 

 bodies and returning their constituents to soil or water for use by new 

 generations. Without the activities of the soil-forming and nitrogen- 

 fixing bacteria, life could never have flourished on land, and without 

 decay the supply of elements essential for life would have soon been 

 depleted. 



Because pre-Cambrian sediments show the same physical characteris- 

 tics as those of later times, we must infer that bacteria have existed since 

 the earliest periods of earth history. Objects believed to be fossil bacteria 

 have been found in Proterozoic red iron ores of the famous Lake Superior 

 mining district. The formation of these ores (some of which are of Arche- 

 ozoic age) had already been attributed by some geologists to the action 

 of iron-depositing bacteria, and the similarity of the supposed fossils 

 to the modern iron bacteria strongly supports this view. Many supposed 



