[Chop. XLI NON-GREEN PLANTS 507 



We mav applv at once all that we know about the physiology and 

 heredity of green plants toward an understanding of non-green plants, 

 for the living part of a non-green plant is protoplasm and there are 

 definite species of them just as there are of green plants. The principal 

 differences between these two types of plants lie in the processes by 

 which thev obtain food and in the consequences of these processes. 

 Some of these consequences are beneficial to other plants and to animals, 

 and some of them are harmful and destructive. 



Parasitic plants. When a plant grows within or on, and also subsists 

 upon food from the living parts of another organism, it is a parasite. The 

 parasitized organism is the host. It may or mav not be injured by the 

 parasite. If it is injured, it is said to be diseased. Some parasitic plants 

 are beneficial to the host. The nitrogen-fixing bacteria in the roots of 

 clover, for example, are beneficial to 

 their host, since upon their death 

 their chemically bound nitrogen be- 

 comes available to the clover plant. 



All sorts of gradations may be 

 found from extreme parasitism to a 

 total lack of it. Some parasites grow 

 only within certain organs of one 

 kind of host. Others grow within or 

 on numerous kinds of hosts. Still 

 others are but partial parasites. Some- 

 times both host and parasite are 

 benefited rather than harmed by 

 their relationship. 



Some plants are merelv perched 

 upon or attached to others, but se- 

 cure no food from them (Fig. 222). 

 These plants are known as epiphytes, 

 in contrast to parasites. Examples of 

 epiphytes are Spanish moss hanging on the branches of a tree (Fig. 

 223 ) , and the mosses and lichens attached to the bark of trees wherever 

 trees grow. In deciding whether a plant is a parasite, it is necessary to 

 consider both its position and its source of food. It may be even more 

 difficult to decide when an animal is a parasite. A protozoan may live 

 within the cells of a plant. Roundworms burrow into roots and survive 

 on the food within the root. Aphids obtain food by sucking the "juice" 



Fig. 222. An epiphytic orchid the 

 roots of which merely hold the plant 

 on the branch of the tree. After Kerner. 



