38 FOUNDATIONS OF BIOLOGY 



of gases between the living matter and its surroundings which 

 is not only characteristic of Sphaerella and all green plants, 

 but of all living things. Plants breathe just as truly as 

 animals, though the active life of most of the latter requires 

 a more or less elaborate respiratory apparatus in order 

 that an adequate gaseous interchange may be effected with 

 the necessary rapidity. 



Thus the green plant may be regarded as a chemical 

 machine for the transformation of energy the radiant 

 energy from the Sun into lifework; the matter and energy 

 which enters, forms, and leaves the organism obeying, to the 

 best of our knowledge, the fundamental laws of matter and 

 energy of the non-living world. 



We have now obtained some idea of one living organism, 

 Sphaerella lacustris, a green plant reduced to the simplest 

 terms a single cell provided with chlorophyll. And we 

 have seen that this chlorophyll is the key to the photo- 

 synthetic activity of the green plant. In other words, the 

 expression ' green plant' does not refer specifically to the 

 color of a plant (in some cases it may appear red, as in 

 Sphaerella under certain conditions), but to the fact that 

 there is present a complex pigment functionally similar to 

 chlorophyll by virtue of which the plant is a constructive 

 agent in nature. It has the power to manufacture its own 

 foodstuffs from relatively simple compounds largely devoid 

 of energy and, in particular, is able to utilize nitrogen in the 

 form of nitrates. 



We pass now from the essentially constructive agents in 

 nature to the chiefly destructive; from the collectors of 

 energy to the energy dissipators; from the green plants to 

 animals and to ' colorless ' plants. 



