214 THE PLANT WORLD 



III. Useful bacteria (11) cause decay of dead organic matter; 

 (12) give flavors to foods ; (13) cause diseases ; (14) take nitrogen from 

 the air ; (15) make fertilizers. 



IV. Bacteria (16) are killed by sterilization; (17) in milk cause it 

 to sour ; (18) fix the nitrogen in the air ; (19) will grow readily in hot, 

 dry places, provided they have food ; (20) may be killed by exposure 

 to sunlight. 



V. Fungi (21), such as wheat rust, are responsible for great losses to 

 crops each year ; (22) are able to make their own food ; (23) like molds, 

 give flavor to cheese ; (24) are more useful than harmful ; (25) may be 

 used as foods. 



VI. Yeast plants (26) are a kind of fungi ; (27) cause fermentation by 

 means of their growth in fluids ; (28) cause bread to rise because they 

 give off carbon dioxide gas in their growth; (29) produce carbon 

 dioxide and alcohol in food substances; (30) reproduce by a process 

 called budding. 



VII. Algae (31) are chlorophyll-bearing plants ; (32) cannot make car- 

 bohydrate food; (33) have thread-like roots; (34) may reproduce by 

 division of cells; (35) are one source of contamination of our drinking 

 water. 



VIII. The main groups of the plant kingdom are (36) the angio- 

 sperms, dicotyledons, and algae; (37) algae and fungi; (38) the 

 mosses, lilies, palms, and trees; (39) the thallophytes, bryophytes, 

 pteridophytes, and spermatophytes ; (40) phyla. 



IX. Alternation of generations (41) occurs when a plant gives rise to 

 other plants just like itself; (42) is best seen in the algae; (43) in a 

 plant takes place in two stages, one producing sexual cells, the other 

 asexual spores ; (44) occur in the moss and fern, when the life history 

 shows an alternating of the part of the plant bearing asexual spores 

 with that part which bears sexual cells ; (45) occur in flowering 

 plants, when the pollen grain and ovules represent the sexual genera- 

 tion, while the seed and the plant that grows from it is the asexual 

 generation. 



X. Spermatophytes (46) are seed-producing plants ; (47) are flower- 

 ing plants only; (48) include both gymnosperms and angiosperms; 

 (49) grow by means of asexual spores ; (50) include all of our common 

 flowers, shrubs, and trees. 



