PLANTS AND ANIMALS 43 



The Two Criteria 



It is a very simple matter to distinguish an ordinary plant 

 from an ordinary animal; the animal moves, sees, hears, tastes, 

 smells, and touches. The plant is stationary, develops wood 

 instead of bone, and has an entirely different method of getting 

 food. The plant has green-coloring matter termed chlorophyll, 

 which, in some obscure way, is instrumental in the nutritional 

 operations of the plant. The two criteria — movement and 

 method of obtaining food — have been the chief features under- 

 lying the separation of animals from plants. 



It is quite a different matter to determine whether or not a 

 unicellular organism is to be regarded as a plant or as an animal. 

 If it moves, does this fact make it an animal? Obviously not, 

 for there are many organisms which are unquestionably plants 

 (for example, Venus's flytrap and the sensitive plants) which 

 have the power of independent movement, and protoplasmic cur- 

 rents, flow of sap, etc., are characteristic movements of all rec- 

 ognized plants. In the last analysis it is impossible to formulate 

 a definition, or series of definitions, which will enable us clearly 

 to distinguish between all forms of animals and all forms of 

 plants. 



Arbitrary Decisions Necessary 



Under such conditions it becomes necessary to arbitrate and 

 mutually agree that certain types shall be considered more plant- 

 like than animal-like and certain other types the reverse. This, 

 in effect, has been the case in modern biology, and today it is 

 generally agreed that the presence of chlorophyll, together with 

 cellulose and other carbohydrate compounds formed by the or- 

 ganisms in question, are enough to establish a plant-like nature. 



Zoologists, and especially the protozoologists, have been slow 

 to give up the phenomenon of movement as a distinctive animal 

 characteristic, and as a result of this conservatism we find a great 

 many chlorophyll-bearing organisms still retained in our classi- 

 fications as primitive animals, whereas by tacit agreement their 

 dominant characteristics make them plants. Chlorophyll-bearing 

 forms without motile organs (desmids, diatoms, etc.) have been 

 excluded long since from the animal kingdom, but their chloro- 



