EPIPHYTES AND PARASITES. 141 



Plants are thus employed to form from the atmosphere and soil 

 those organic products which^ are requisite for the nourishment of 

 man and animals. Nutrition derivable from the atmosphere being 

 generally diffused, is accessible to all plants, and is perpetually re- 

 newed. Nutrition derivable from the soil being fixed to certain 

 localities, requires that those elements contributing to it be mechani- 

 cally supplied as they become exhausted. While an animal consumes 

 carbon so as to form carbonic acid, gives off ammonia in various 

 excretions, transforms organised into mineral matters, and restores 

 its elements to air and earth ; a plant, on the other hand, fixes carbon 

 in its substance, and gives off oxygen, forms from ammonia solid 

 compounds, transforms mineral into organised matters, and derives its 

 elements from the air and earth. Thus, says Dumas, what the 

 atmosphere and soil yield to plants, plants yield to animals, and 

 animals return to the air and earth, a constant round, in which matter 

 merely changes its place and form. 



Epiphytic and Paeasitio Plants. 



Some, plants grow without any attachment to the soU, and are able 

 to derive in a great measure, from the atmosphere, all the materials 

 required for their growth. Such plants are called Epiphytes {sit}, upon, 

 and purov, a plant), or air-plants, and may be illustrated by the Til- 

 landsias, Bromelias, and Orchids of warm climates. Such plants, 

 when attached to the surface of trees, may perhaps derive some 

 nourishment from the inorganic matter in thfe decaying bark ; but they 

 do not become incorporated with, nor do they send prolongations into, 

 the trees. Orchidaceous plants, which are always perennial, are found 

 in the greatest variety and profusion in those regions where heat and 

 moisture abound. Extremes of cold or dryness are the least favour- 

 able to their growth. Tillandsias and Bromelias flourish in dry hot 

 air without any contact with the earth. 



There are other plants, however, which are true Parasites (iragoi, 

 beside, and alrog, food, deriving food from another), sending prolonga- 

 tion's of their tissue into other plants, and preying upon them. Many 

 Fungi, for instance, develop their spores (seeds) and spawn (mycelium) 

 in the interior of living or dead plants, and thus cause rapid decay. 

 The diseases of corn, called smut and rust, and the dry rot in wood, 

 are due to the attacks of these parasitic Eungi. The minute dust or 

 powder produced by these plants consists of millions of germs which 

 are easily carried alaout in the atmosphere, ready to fix themselves on 

 any spot where they can find a nidus. There are also flowering plants 

 which grow parasitically, and they may be divided into two classes : 

 1 . Those which are of a pale or brownish colour, and have scales in 

 place of leaves ; and 2. Those which are of a green colour, and have 



