34 ANIMAL BIOLOGY 



energy that chlorophyll has secured for the plant and stored away 

 in its food. Therefore the food must be oxidized — burned — in 

 order to release the energy, and for this the plant must have 

 available a supply of free oxygen. Protococcus obtains this 

 oxygen dissolved in water and also, in sunlight, from that liberated 

 through photosynthesis. The process involved, for the sake of 

 simplicity, may be represented by the equation: 



C 6 H 12 6 + 6 2 = 6 C0 2 + 6 H 2 



which, it will be noted, is the reverse of the equation for photo- 

 synthesis. This intake of free oxygen by the cell and outgo of 

 carbon dioxide and water, the chief products of combustion, is 

 known as respiration. It is essentially the securing of energy from 

 food, involving the exchange of carbon dioxide for oxygen by 

 protoplasm. And this interchange of gases between the living 

 matter and its surroundings is not only characteristic of Proto- 

 coccus and all green plants, but of all living things. Plants respire 

 just as truly as animals, though the more active life and complex 

 bodies of most of the latter require an 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, Proto- 

 coccus vulgaris, 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 photosynthetic 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 or brown) , but to the fact that there is present a complex pig- 

 ment functionally similar to chlorophyll by virtue of which the 

 plant is a constructive agent in nature. It has the power to manu- 

 facture 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 essential constructive agents in nature 



