244 THE PTERIDOPHYTA 



development of the structural characters of the vege- 

 tative body described in the last section, which adapt 

 them for terrestrial life. 



In the Ferns we find a thalloid structure called the 

 prothallus (Fig. 38, A), resembling in a general way the 

 thallus of Pellia, which can only live in damp places, 

 which, like Pellia, forms rhizoids (rh.) on its lower 

 surface, and which bears sexual organs (Fig. 38, A, an, 

 a, B and D) very much like those of Pellia (but only 

 on its lower surface). The spefms are formed and 

 liberated in miich the same way, though they differ 

 in having numerous flagella (Fig. 38, C) instead of 

 only two, and fertilisation takes place in a film of 

 water on the surface of the prothallus. The fertilised 

 egg (zygote), germinates in situ, producing an embryo 

 within the archegonium wall. The embryo soon sends 

 a sucker (the foot) into the tissue of the prothallus 

 (Fig. 38, E). But now comes a wide divergence. Instead 

 of developing into a sporogonium which remains 

 attached to the thallus during its whole life, the 

 embryo of the Fern soon develops a root which pene- 

 trates the soil, and a leaf which, rises above the soil and 

 begins to carry on photosynthesis (Fig. 38, F, r and /). 

 The development of the stem is slow, but a second 

 larger leaf and a second larger root are shortly pro- 

 duced. During this time the young plant has been 

 drawing food from the prothallus, but after a while it 

 becomes quite independent (the prothallus ultimately 

 dying off), and grows constantly bigger, successive 



Fig. 38. — Life history of Fern. A, prothallus from below ; rh., 

 rhizoids; an., antheridia; a, archegonia. B, antheridium con- 

 taining coiled sperms. C, single sperm with numerous flagella. 

 D, archegonium; e, egg; n, neck. E, section through embryo 

 drawn in outline, showing prothallus, foot, beginnings of first 

 root, first leaf, and position of growing point of stem, not yet 

 developed ; rh., rhizoids of prothallus. F, young plant stiU 



