LESSONS IN BOTANY. 197 



mere accidental changes may be eliminated, so that at the end 

 of the season a clear and complete history can be given of the 

 particular species studied. 



Lichens, a large and well-known group of plants, belong in this 

 division. These plants are common everywhere and have been 

 carefully studied, yet there has been much difference of opinion 

 about their true nature. The idea seems to prevail that they are 

 fungi which live in association with algse, which may be proto- 

 phytes or zygophytes, plants of a higher order as parasites on 

 those of lower orders. A lichen of Asia Minor is used for food, 

 and the so-called Reindeer moss is a lichen. They grow on the 

 bark of living trees, on rocks and decaying wood, and on the 

 ground ; they vary greatly in form and color, and are interesting 

 subjects for study. The ergot of rye, the rust of wheat, and the 

 smut of wheat, oats and barley and their allies, belong to this 

 division of plants. 



The most valuable and best known group of this division are 

 the mushrooms. They are saprophytes, and the largest of the 

 Fungi, growing with great rapidity and decaying rapidly after 

 maturity is reached. Many species are poisonous, but many are 

 edible, and in France they are extensively cultivated for food ; in 

 one cave there are over six miles of mushroom beds. 



The only chlorophyll-bearing plants in this division are the 

 Characese. They are submerged water plants, slim and delicate 

 in structure, sometimes attaining the height of two or three feet. 

 They grow in close patches at the bottom of fresh-water lakes, 

 ditches and streams ; in deep or shallow, stagnant or running 

 water. They are easily grown in an open dish, and associated 

 with spirogyra, or some other of the scums, make an attractive 

 and entertaining addition to a group of house plants. 



The divisions already considered constitute the Thallophytes 

 of some authors, who generally divided them as follows : Algse, 

 aquatic chlorophyll-bearing plants; Fungi, terrestrial, sapro- 

 phytic, or parasitic plants destitute of chlorophyll; and Lichens, 

 terrestrial plants containing green gonidia. But the classifi- 

 cation already given, based mainly on the character of the 



