102 PLANTS, .THE FOOD OF ANIMALS 



We have seen that Paramecium and perhaps the majority 

 of protozoa live on bacteria, extracting from these minute cells 

 the elements necessary for their protoplasmic reconstruction 

 and growth. Rotifers and Crustacea feed upon the larvae 

 of animals, on smaller Crustacea and rotifers, and upon uni- 

 cellular animals and plants such as protozoa, diatoms desmids 

 or protococcacea, etc. These Crustacea, rotifers and larvae 

 live on unicellular animals and plants. The food of Hydra 

 therefore traced back upon any line finally brings us to the 

 minute chlorophyll-bearing plants and bacteria, the former 

 getting their energy directly from the sun the latter from 

 decomposing proteid matter and intermediate compounds 

 less complex than proteids. These ultimate food materials of 

 Hydra therefore are living things some of which have the power 

 to manufacture their nutriment from simple elements with the 

 aid of the sun while the others live upon dissolved proteid 

 matters from decomposing animal and plant tissues or else 

 utilize waste matters like urea and relatively simple chemical 

 compounds for their sources of energy. 



The work which Hydra does is done at the expense of energy 

 which is transformed from the stored-up energy in proteids, 

 which in turn is derived from the energy of foods obtained 

 from other animals or plants where in turn it is obtained from 

 other foods until finally the green plant gets its energy from 

 the sun. A necessary step therefore in the biology of animals 

 is to trace out as far as possible the transformation of solar 

 energy into that of protoplasm. The initial steps in the process 

 are connected with the structures and functions of plants and 

 enough of these will be given to pave the way for a clear 

 understanding of the work which the plants do in nature. 



Plants like animals, are divided into two great groups proto- 

 phyta and metaphyta, although these 'designations are not 

 always used in classification. The metaphyta bear the same 

 relation to the protophyta that the metazoa bear to the pro- 

 tozoa, viz., many-celled as contrasted with single-celled organ- 

 isms. For our purposes a glance at each type will suffice to 

 give a clear understanding of the essential relationships of 



