20 OUR NATIVE FERNS AND THEIR ALLIES. 



liverworts called the prothallium. Different ferns vary in the 

 method of forming this prothallium, some producing it im- 

 mediately at the spore and others 

 after the formation of a thread- 

 like growth known as the pro- 

 embryo. The prothallium is en- 

 tirely composed of cellular tissue, 

 and in the true ferns (PoLYPO- 

 DIACE^E) is broadly cordate or reni- 

 form in shape, and bears large 

 numbers of root-hairs from the 

 under part of its posterior portion 

 (Figs. 12, 13). 



The prothallium varies in size 

 from less than one tenth of an 



FIGS. 12, 13. Prothallium of Pteris , ., , ( , 



serrulata Linn, f., showing two mCD up tO One third Of an inch 



stages of growth. (After Moore.) in its wides t part. On the under 

 surface of the prothallium two sorts of organs are produced 

 analogous to the stamens and pistils of the SPERMAPHYTA, 

 respectively known as antheridia and archegonia. The position 

 of these organs on the prothallium varies in different sub orders. 

 In some species, notably the ostrich-fern, the two kinds of sex- 

 ual organs are produced on separate prothallia, so that the plant 

 becomes dioecious instead of moncecious. Analogous cases are 

 familiar to all among flowering plants like the willows, poplars, 

 and box elders (dioecious), and begonias and melons (monce- 

 cious). 



63. Antheridia. These are small masses of tissue developed 

 in the same manner as the root-hairs, consisting of a single 

 layer of cells forming the wall, and containing a number of 

 spirally coiled threads, usually with a number of cilia on their 

 anterior coils. At maturity the antberidium swells by the ab- 

 sorption of water and finally bursts its wall, discharging these 

 coiled filaments, which possess the power of locomotion, and for 

 this reason are called anther ozoids. These antherozoids often 

 drag with them a little vesicle which seems to play no part in 

 the process of reproduction (Fig. 14). 



64. Archegonia. The archegonium (sometimes called 

 pistillidium) is also a rounded mass of tissue usually less prom- 



