358 ECOLOGY 



Parasites. Not only do dead and decaying leaves 

 provide a habitat for many plants, but commonly living 

 plants (and even animals) provide habitats for many 

 species, especially Fungi. Familiar examples of this are 

 the rusts, mildews, and blights, which often destroy valuable 

 crops. A plant which lives at the expense of another 

 organism is called a parasite ; and the organism upon which 

 the parasite preys is known as the ' host '. The guest, how- 

 ever, is uninvited and takes advantage of the ' hospitality ' 

 to the injury of the ' host ', and sometimes causes its death. 



Living habitats are often selected by plants other than 

 Fungi, e. g. many flowering plants prey to a greater or less 

 extent on their neighbours. The most familiar example 

 is the Mistletoe, which grows perched on the branches of 

 the Apple, Poplar, and other trees. Birds eating the berries 

 are unable to swallow the seeds because of the sticky 

 material around them, so they scrape them off on to the 

 branch, where they germinate. Suckers enter the branch 

 and form a union with the wood of the ' host ', from which 

 they draw the mineral food for the mistletoe. The influence 

 of this mode of nutrition is seen in its yellow-green leaves, 

 which contain chlorophyll, and the products of photo- 

 synthesis enable the plant partly to maintain itself. The 

 leaves, too, are evergreen, and are able to form organic 

 substances at favourable periods throughout the year. 

 These may be passed downwards and contribute somewhat 

 to the nourishment of the host when the latter is not in leaf. 

 The Mistletoe, therefore, is not entirely dependent on its 

 ' host ' for food, and may be regarded as a partial parasite. 



Many flowering plants are partially parasitic, and attach 

 themselves by means of suckers to the roots of neighbouring 

 plants, especially grasses. They produce green leaves and 

 closely resemble plants which obtain their food in the 

 normal manner. Examples are Cow-wheat, Eyebright, 

 Yellow Rattle, and Louseworts. The leaves of the Yellow 



