The Lower Plants 49 



factors governing the development of plants is to 

 be obtained from such experimental work, the prob- 

 lems are so exceedingly complicated that great cau- 

 tion must be exercised in drawing general con- 

 clusions. Our ignorance of so many of the condi- 

 tions that have governed the evolution of any plant 

 in the past, and the long time that may elapse before 

 the reactions resulting from a stimulus show them- 

 selves, emphasize the necessity for extreme cau- 

 tion in making sweeping generalizations from the 

 results of any experiments. Nevertheless such work 

 is exceedingly valuable in checking and extending 

 the results derived from other sources, such as 

 paleontology and comparative morphology. 



There still exist organisms that may very well 

 be not very different from the first living things 

 that appeared upon the earth. These are the 

 Bacteria, those ubiquitous " germs," cells so minute 

 that under the most powerful microscope many of 

 them appear merely as tiny specks too small to show 

 any definite structure. 



These minute organisms, however, are of the 

 highest importance in the economy of nature, as it 

 is to their activity that most forms of organic de- 

 composition are due, and this decomposition is es- 

 sential in order that inorganic compounds may be 

 reduced to simpler ones that are available as food 

 for the higher plants. In the earth there are myri- 

 ads of other bacteria whose activity results in the 

 fixing of the atmospheric nitrogen, and the produc- 



