THE 1TEKIDOPHYTES. 185 



The alternation of generations is clear and distinct. The 

 ordinary form of the plant is the sporophyte. When the 

 spores germinate, there is first formed a protonema which 

 develops into a small thalloid leaf called the prothallium. 

 This is usually about an eighth of an inch in diameter. This 

 is the gametophyte ; it is inconspicuous and is short lived. 

 The prothallia may be obtained for study from the damp 

 flower pots, walls, or soil of greenhouses in which Ferns 

 are cultivated, or they may be gfowri by sowing the Fern 

 spores on moist soil, kept protected by a bell glass, and 

 exposed to suitable conditions of light, heat, and moisture. 

 It requires from six to ten weeks to grow the prothallia 

 from the spores. (See Fig. 110.) 



If the under side of one of these heart-shaped prothallia 

 be examined under the microscope, special differentiations 

 of the cells will be found near the sinus of the heart ; 

 these are the archegonia or pistillidia; they are rounded 

 aggregations of cells, with a large, centrally situated cell 

 that divides into two. Of these two cells, the lower 

 develops into the egg cell, or oosphere ; the upper de- 

 velops into a tube which becomes filled with a mucilagi- 

 nous substance that is afterwards discharged, leaving the 

 passage to the oosphere open. (See Fig. 110, A.) 



More distant from the sinus of the prothallium are the 

 antheridia, situated among the root hairs. In these, spi- 

 rally coiled antherozoids are developed, being finally dis- 

 charged by the rupture of the cell wall. Accompanying 

 each antherozoid is a small cell, the use of which is not 

 understood. Each antherozoid is provided with cilia, by 

 means of which it swims about when the prothallium is 

 wet. (See Fig. 110, .#.) It may in this way pass to other 

 prothallia, where, by fertilizing the oosphere, a hybrid 

 variety is produced. If the antherozoids come in con- 

 tact with the mucilage around the mouth of an archego- 

 nium, the mucilage aids in directing them into the tubu- 



