Life of Plants and Animals 193 



cell. Bacteria are also representatives of these uni- 

 cellular organisms, but they are usually regarded as 

 plants. 



It is not an easy matter to decide in the case of 

 these microscopic organisms as to whether they are 

 plants or animals. Many forms have been repeatedly 

 transferred from one kingdom to the other, the vital 

 processes in plants and animals being so similar as 

 to make a clear distinction impossible. These diffi- 

 culties disappear as we ascend in the plant and animal 

 series, the plants being defined as those organisms 

 having an outer covering of cellulose, and possessing 

 the power of manufacturing organic compounds by 

 means of the sunlight; while animals, with few excep- 

 tions, are devoid of cellulose, and the coloring matter 

 necessary to utilize the energy of the sun's rays. Hence 

 animals are dependent upon plants for their food. 



THE CELL. Amoebae, paramecia, bacteria, etc., 

 are only a few representatives of the countless myriads 

 of living things which the unaided eye is unable to 

 detect. It is only after the invention of the microscope 

 that we have become aware of these organisms of the 

 microscopic world. Nevertheless, they are interest- 

 ing to us because they are among the very lowest 

 forms of life, and represent the primitive living sub- 

 stance of which our own bodies consist. In fact, 

 our bodies are built up from such microscopic cells, 

 and it is in such a minute cell that our bodies have 

 their origin. Nothing can be more important, there- 

 fore, for the understanding of the higher plant and 

 animal life, including our own, than the lesson which 

 these minute organisms teach us. 



It would, indeed, be a very discouraging matter if 

 it were necessary for us to become acquainted with all 

 these microscopic forms before we could comprehend 

 the vital processes and the laws of life of higher forms. 

 Fortunately the life phenomena in these forms are 



