Modes of Nutrition 279 



is utilized by some species in almost every taxonomic group of plants 

 and animals. Parasites live on or in other living organisms and obtain 

 organic food directly from the bodies of their hosts. 



Mixotrophic organisms are capable of both autotrophic and hetero- 

 trophic modes of nutrition of one sort or another. Insectivorous 

 plants possess special anatomical adaptations for entrapping insects 

 and digesting them (holozoic nutrition) and at the same time carry 

 on photosynthesis (holophytic nutrition). Many green flagellates 

 similarly display the capability of utilizing either of these types of 

 nutrition, and, since they are also motile, it is thus a constant source 

 of discussion whether to classify these forms as plants or as animals. 

 When light is available these flagellates can photosynthesize like 

 typical green plants, but at other times they live saprophytically. 

 Euglena, for example, is found living in this manner in the decompos- 

 ing organic matter on the bottom of a pond. Other green flagellates, 

 are provided with a functional mouth and are capable of the in- 

 gestion of solid food particles like a typical animal. The green alga 

 Chlorella is another Dr. Jekyll and Mr. Hyde, since it is capable of 

 either phototrophic or saprophytic nutrition, and other members of 

 the phytoplankton may be found to lead this double life. 



The fact that most animals and colorless plants require for their 

 nutrition the organic matter of other organisms means that the food 

 of these heterotrophic forms has an ecology of its own. The food 

 organisms themselves grow, change, move around, and die. Further- 

 more, many of these prey species not only serve as a source of nutri- 

 ment for the predators but may also influence them in other ways as 

 part of their environment. For animals, then, our consideration of 

 nutrients as an ecological factor overlaps the general consideration 

 of the presence of other organisms as environmental influences which 

 will be taken up in subsequent chapters. 



But with green plants the situation is quite different. Here the 

 nutritional needs are substances that are relatively simple in the 

 chemical sense. The absorption of nutrients from the environment 

 by plants is a relatively direct and much less complicated process than 

 the food getting of animals. This fact is no doubt one reason why 

 certain aspects of ecology were first developed by botanists. With 

 the holophytic organisms we can often see more clearly the limitation 

 of growth and distribution by the influence of nutrients. We shall 

 accordingly consider first the ecological aspects of the nutritional 

 requirements of green plants since these are by far the most important 

 of the autotrophic forms. Then we shall discuss the significance of 

 the nutrient factor for animals and other heterotrophic forms, includ- 



