46 THE SMALLEST LIVING THINGS 



and green regions of the normal spectrum) . These are absorbed 

 by the chlorophyll, and the energy which they represent is utilized 

 in some mysterious way not yet understood in the building up 

 of sugars and starches. This phenomenon, photosynthesis, 

 whereby chlorophyll with the radiant energy of sunlight, creates 

 organic compounds out of carbon dioxide and water, is the fun- 

 damental activity upon which all life is dependent. 



There is an almost infinite variety of the microscopic chloro- 

 phyll-bearing forms — some of them are permanently quiescent, 

 some continually motile, while others are motile at times and qui- 

 escent at other times. Some are always single cells; others are 

 in colony combinations of one kind or another. These are inter- 

 esting philosophically as pointing the way toward multicellular 

 plants and the possibility of specialization by division of labor. 



The power of photosynthesis is the most distinctive feature 

 of plants. Strict construction of this definition of plants, how- 

 ever, would exclude fungi, and many other plant forms which, 

 though colorless, are obviously related to chlorophyll-bearing 

 types. The purpose of biological classification, after all, is to give 

 a common language whereby the subject of observations, experi- 

 ments, and conclusions may be identified and the results confirmed. 



Clarity in Classification Needed 



The filamentous algae, desmids, and diatoms, having no 

 motile organs, for decades have been identified as plants and 

 separated as such from the world of microscopic animals, the 

 smallest forms of which are the protozoa. Until quite recently 

 a great many chlorophyll-bearing forms have been retained in 

 systems of classification of protozoa. As the same forms are 

 included in botanical textbooks under an entirely different termi- 

 nology and classification, a great deal of confusion results. In 

 the interests of clarity it seems wiser for one or the other group 

 of scientists to bow as gracefully as possible to the inevitable and 

 abandon them; and since the questionable forms possess chloro- 

 phyll, it is logical that the zoologists should give them up. All 

 of these intermediate forms are characterized not only by the 

 presence of chlorophyll but also by the possession of flagella as 

 motile organs, either permanently or at some stage of their life; 

 hence the name "Flagellata." 



