AN EXTRA GENERATION 329 



protective cells and a mass of inner cells which form the 

 sperms. Each sperm is a small motile cell swimming by 

 means of cilia. The eggs are formed in organs called 

 archegonia, shaped like long slender flasks. The accom- 

 panying illustrations will give an idea of their shape and 

 structure. Essentially they consist of three parts : stalk, 

 venter, and neck. The neck is long and slender with an 

 outside layer of protective cells and a single axial row of 

 cells known as the neck-canal cells. The enlarged middle 

 part called the venter has one or more layers of protec- 

 tive cells on the outside. The row of neck-canal cells is 

 continued down into the venter by two cells known re- 

 spectively as the egrgscell (at the bottom) and the ventral 

 canal cell. When the egg is ripe the ventral and neck- 

 canal cells die and turn into a sort of mucilage or jelly, 

 which swells and forces the top off the neck. This sub- 

 stance oozing out into the water directs the sperm to the 

 open neck. When one finds its way down the neck and 

 fuses with the egg a fertilization membrane is formed 

 about the egg and the mucilage hardens and blocks the 

 passage. 



An Extra Generation. — It is proper at this point 

 to call attention to the striking peculiarity of these higher 

 plants. In the plants we have been discussing heretofore 

 a fertilized egg generally develops again into the sort of 

 plant which produced it. But here there is a regular alter- 

 nation of two sorts of plants in the life cycle. The ferti- 

 lized egg remains in the venter of the archegonium and 

 begins to grow and divide. Eventually this young plant, 

 which we call an embryo, ruptures the wall of the enclos- 

 ing structure and pushes up into the air. When it is 

 mature it is found to consist of three parts. At the 

 bottom is a sort of foot embedded in the end of the 

 branch (Fig. 84.) which bore the archegonium. Above 

 that is a slender stalk an inch or so long and bearing at its 

 top an enlarged body known as the capsule. The foot is 

 an organ which serves as a means of attachment and at 



