ECOLOGY 269 



living things which are capable of combining gaseous nitrogen with hydro- 

 gen and carbon are certain bacteria, especially those often associated with 

 the roots of leguminous plants, such as peas and beans. 



Chlorophyll is not generally present in animals, but there are certain 

 of the simplest of Hving things which can live either as plants or animals. 

 For example, one microscopic organism, Chlamy domonas, when in light 

 manufactures its own food by using the chlorophyll in its own body, but, if 

 the same organism is kept continuously in the dark, it will lose its green 

 color and die if it cannot devour organic food, such as bacteria or other 

 minute things. Chlorophyll is not only essential as the machinery with 

 which organic foods may be manufactured, but it is of great importance 

 to animals in other ways. Some animals, such as certain protozoans, 

 sponges, corals, and flatworms, contain small green plants within their 

 bodies. These serve as little slaves which work to make food for their 

 captors, and in turn receive protection and the essential materials for such 

 synthetic activities. Chlorophyll is also known to be the source of pigments 

 which serve important functions in animals. Even haemoglobin, which 

 in blood is so essential for carrying oxygen to the tissues in man and other 

 animals, is believed to be derived from the chlorophyll which is taken into 

 the bodies of animals as food. 



Plants, then, manufacture practically all the organic food in the world 

 and animals must obtain it from them. They are also the chief factors, as 

 bacteria and other fungi, in reconverting the bodies of dead plants and 

 animals into simple substances, such as water, carbon dioxide, and ammonia, 

 which may again be used in the synthesis of foods. Somewhere in their 

 substance plants also contain vitamins, which though they occur in minute 

 Quantities, have lately been shown to be of vast importance for the proper 

 nutrition of animals. 



The dependence of animals on plants for organic foods brings about 

 certain general relations between the two. Animals that eat plant food 

 usually do not restrict their diet to particular species or parts of plants. 

 However some of them are very specific in their food preferences. For 

 example, the boll weevil eats only cotton and the pronuba moth feeds on 

 certain parts of the yucca flower. Animals that are non-specific in their 

 food habits act more or less as regulators in nature. They tend to keep 

 down the most abundant plants, which might otherwise become pestifer- 

 ous. Vegetarian animals are of course always much more numerous in any 

 district than the carnivorous species which prey upon them. There is a 

 series of dependencies in regard to food. Plants must be present in sufficient 

 numbers to support vegetarian animals without being seriously depleted; 

 carnivores can never be so abundant as to eat all the herbivores on which 

 they depend for food. In any locality the plants and animals are so adjusted 

 that their rates of growth and reproduction keep conditions fairly stable 



