CHAPTER V 



PLANTS, THE FOOD OF ANIMALS AND THE SOURCES 

 OF ANIMAL ENERGY 



HELMHOLZ, when he outlined the great theory of the con- 

 servation of energy, is said to have been widely criticized for 

 his "play of fancy" in believing that all living things get their 

 energy from the sun, transforming it into vital activity. Today 

 this "fancy,' 7 refined through thousands of experiments on the 

 physiology of animals and plants, is an accepted fact. It is 

 known that not all living things can use the solar energy di- 

 rectly, plants alone having this power, while animals obtain 

 it indirectly through the vegetable world. The importance, 

 therefore, of the plant organism in general biology cannot be 

 overestimated. In the present chapter we will trace back this 

 energy through the food of animals to its ultimate source. 



A. THE FOOD OF ANIMALS 



Hydra, as we have seen, like Paramecium or Amoeba, takes 

 in solid food in the living state, digests and assimilates it. The 

 protoplasmic molecules throughout select from the dissolved 

 proteins the elements needed for their reconstruction. This 

 food, rich in potential energy, consists mainly of minute crus- 

 tacea, rotifers, protozoa, unicellular plants and bacteria. The 

 potential energy carried over by these organisms must in turn 

 be traced back to their food. The Crustacea, rotifers, protozoa, 

 etc., are animals, and live upon solid living materials as Hydra 

 does, but being minute, their food must be correspondingly 

 small. Could Hydra be cut up in small enough pieces, minute 

 Crustacea, rotifers and protozoa might equally well get their 

 nourishment from its protein, a type of retaliation with which 

 Huxley has made us familiar in the possible mutual gastric 

 relations of man and lobsters. 



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