EXISTING ORGANISMS— PHYSIOLOGY 91 



green alga, Sphaerella, and the Protozoon, Paramecium, are in 

 many respects the same. Each moves about actively, an evidence 

 of its inherent contractility. Each, if it comes in contact with some 

 object or substance in the water, indicates that it has received a 

 stimulus from that object or substance. The indication may be 

 in the form of a movement involving the end of the body opposite 

 to that which received the stiuuilus, hence we know that some 

 impulse has passed through the organism, an evidence of irrita- 

 bility and conductivity, and in addition of response through con- 

 tractility, which produces motion. Finally, if we watch the organ- 

 isms long enough, and conditions in the environment are favorable, 

 we note that by the acquisition of substances from the environ- 

 ment the individual grows and reproduces itself in some way. 

 Most of these properties can be equally well observed in a complex 

 animal, such as man, but irritability, conductivity, and contrac- 

 tility are so little emphasized in the higher plants as to be totally 

 obscured except in rare cases and in normally inconspicuous 

 phenomena. 



Plant and Animal Metabolism. In these two simple one-celled 

 organisms, however, we see the fundamental manifestation of life 

 processes, but even here there is a strange difference. The animal 

 is practically colorless, but the body of the plant is green. The 

 animal, as it swims about in the water, is constantly occupied in 

 sweeping into its gullet minute organisms which are massed to- 

 gether and passed into the cytoplasm of its body as food vacuoles, 

 which in a few minutes are changed into a part of the animal itself. 

 The plant does none of this. We know through the investigations 

 of scientists that the two differences are closely related. The green 

 substance in the plant is called chlorophyll, meaning literally the 

 green of leaves, and its presence is the basis for an entirely different 

 metabolism, the foundation of all life. 



Photosynthesis. Chlorophyll acts in the plant as a catalytic 

 agent, a substance by means of which a chemical action goes on, 

 although the catalyst is not changed in the process. Plants which 

 possess it, that is green plants, secure from their environment the 

 inorganic substances, carbon dioxide, water and certain salts 

 such as nitrates and phosphates, which may be called their food. 

 In the presence of sunlight as a source of energy, the plant com- 

 bines the first two, carbon dioxide and water, through the agency 

 of its chlorophyll, to form sugars and starches. A different arrange- 



