166 PLANT BIOLOGY 



184. Alternation of generations in the moss. The protonema 

 and the leafy shoots with their antheridia and archegonia are known 

 as the sexual generation because it is this plant that produces eggs 

 and sperm- cells which must unite before the egg can develop into 

 the spore-bearing plant. The slender stalk with the capsule at the 

 end which is produced by the fertilized egg-cell is called the asexual 

 generation, since the spore-bearing plant can reproduce without the 

 union of two kinds of cells. The spore-bearing plant is dependent 

 on the leafy plant for all its food. In the fern, on the other hand, it 

 is evident that the spore-bearing plant and the plant producing 

 eggs and sperms are entirely independent plants. In both of these 

 groups of seedless plants, however, there is an alternation of genera- 

 tions. 



186. Suggestions for the study of mosses. No. 66. The teacher 

 should secure plenty of material for the demonstration both of 

 the plants with spore cases and if possible the plants with arche- 

 gonia and antheridia. On account of its size the pigeon wheat 

 moss is desirable. The material may be collected and dried, since 

 both generations are not likely to be obtained at the same time of 

 year. If spore cases are on hand, the work might then be done when 

 the sexual plants can be secured in a fresh condition. The pupil 

 should describe and draw the leafy moss plant. The location of 

 archegonia and antheridia should be stated. Then the spore-bear- 

 ing plant should be described, together with the relation to the sexual 

 plant which produced it, and the spore case opened to show the spores. 

 The two plants should then be drawn and labeled. 



C. Algce 



186. Spirogyra. Any one who has ever been in parts of the 

 country where ponds or very slowly moving bodies of water abound 

 must have noticed either at the bottom or on the surface of the water 

 a green, slimy mass. It is so frequently found on the surface that it 

 is called " pond scum." If one examines a small portion of this mass 

 even with the naked eye, one will see that it consists of a great num- 

 ber of interlacing threads. When looked at with the compound 



