118 STUDY OF COMMON PLANTS. 



made out if younger specimens of different ages are com- 

 pared. 



After the points named have been observed, drawings of 

 the mature prothallium should be made and compared with 

 those of earlier stages. If the material is suitable for the 

 purpose, intermediate stages of development also may be 

 studied. 



III. On the lower side of the mature prothallium arche- 

 gonia and antheridia are produced. These are organs of 

 reproduction, corresponding in function to the "essential 

 organs " of flowering plants. The archegonia are usually 

 situated near the sinus. They are flask-shaped bodies, the 

 lower portion of which is sunk in the tissue of the 

 prothallium, while the neck projects above the surface. 

 The neck consists of a wall made up of four longitudinal 

 rows of cells, surrounding a single row of canal-cells which 

 leacT down to the obsphere. The latter is the cell from 

 which, after fertilization, the embryo, i.e. the young frond, 

 arises. 



The antheridia are, as a rule, more remote from the 

 sinus, and present the appearance of small, hemispherical 

 protuberances, consisting of a wall one layer of cells thick, 

 which encloses the mother-cells of the anther ozoids. The 

 latter are minute, ciliated, protoplasmic bodies, and are the 

 active agents of fertilization. They are best observed by 

 placing in water on a slide prothallia that have been kept 

 rather dry for some time. After the water has been 

 absorbed by the antheridium the latter ruptures, and the 

 antherozoids in great numbers are seen in active motion, 

 swarming in the field of the microscope like so many 

 animalcules. Under favorable circumstances they have 

 been seen to move towards an archegoniuni and 'enter 

 it, passing down through the canal-cells which have now 



