CHAPTER XV. 



NUTRITION OF PARASITES AND SAPROPHYTES. 



207. A parasite is an organism, plant or animal, which 

 lives on or in another living organism at its expense, deriving all 



or a part of its nourishment from it. 

 The plant or animal on which the parasite 

 lives is called the " host." The parasite 

 derives a part or all of its food from its 

 host, usually inflicting more or less injury 

 upon the host or even causing its death. 

 Parasitic plants are represented in nearly 

 all the great branches of the plant king- 

 dom. There are some among the flower- 

 ing plants, some among the algae, the fungi 

 and bacteria, but by far the greater num- 

 ber are found among the fungi and bac- 

 teria. One reason for this is that none 

 of the fungi or true bacteria have chloro- 

 phyll; therefore, they cannot fix carbon, 

 that is, cannot make their own carbo- 

 hydrate foods, but are dependent on 

 chlorophyll-bearing plants for it. The 

 fungi and bacteria which are parasitic on 

 green plants then obtain their carbohy- 

 drates directly from their hosts. Those 

 which are parasitic on animals derive 

 their carbon food from animals, but 

 animals get their carbohydrates directly from green plants, which 

 they eat, or in the final analysis from plants or animals which do 

 feed on green plants either dead or alive. 



208. The fungi and bacteria which are not parasites 

 derive their carbohydrate food by growing on dead plants or 



126 



Fig. 95. 



A saprophytic tungus (Cre- 

 pidotus) growing on a rotten 

 limb in the iorest. 



