74 THE AMERICAN BOTANIST 



tropics and there one species attains a. length of more than 

 thirty feet! Their color is perhaps most frequently gray- 

 green, but it may be l)rilliant red or yellow, dead black or 

 pure white. They live on rocks, earth, the trunks of trees or 

 even on the leaAes^ and one species is reported to be truly 

 aquatic. Several kinds are of economic importance, among 

 them the reindeer moss, the chief food plant of grazing 

 animals in the far north. Several others have been used as 

 food for man in an emergency. The manna of the Israelites 

 is supposed to have been a lichen. Still other species have 

 been known from hoary antiquity as a source of dyes.. The 

 familiar litmus so commonly used as an indicator of acids 

 and alkalies is obtained from a lichen. Last but not least of 

 the lichen's good qualities is the efifect it has upon the rocks, 

 steadily breaking them down into soil in which other plants 

 can grow. 



Origin of Life. — The only living things on our planet 

 that can change carbon dioxide and water into foods are the 

 plants. They are able to do this by means of minute green 

 bodies in the leases and other parts which turn the energy in 

 sunlight into a form which they can use. Recently it has been 

 discovered that the ultra-violet rays of "light" can change car- 

 bon dioxide and water into sugar just as the green bodies or 

 chloroplasts do. Here we have an instance of the formation 

 of food upon which beginning plants might live without the 

 assistance of the plant green. It has always been assumed 

 that the first plants were green, but with a source of food pres- 

 ent, this would not be necessary. Plant food, however, is not 

 protoplasm. This latter subsUmce, which is the only part of 

 a plant that may be said to be alive, is far more complex than 

 simple sugars, but it is reported that even this may be formed 

 if the necessary mineral salts are present under conditions of- 



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