DEVELOPMENT OF PLANTS 



271 



plant. The antheridia and archegonia are developed, as in Ric- 

 ciocarpus, upon the upper surface of the thallus, but they are 

 confined to special portions of it which appear at first as mush- 

 room-shaped outgrowths (Fig. 190, a). These outgrowths, how- 

 ever, are but modified branches of the thallus and bear the 

 sexual organs upon their upper surfaces in lines radiating from 



Fig. 191. 



Fig. 192. 



Fig. 191. Section of an archegonial branch similar to a, Fig. 190 — ar, 

 archegonia; in, involucre or curtain that hangs down on either side of the 

 rows of archegonia; ac, air chambers; th, thallus. 



Fig. 192. Section of a young antheridial branch: an, antheridia sunken 

 in cavities of the branch, which is also provided with air chambers similar 

 to those of the normal thallus. Some of the antheridia have discharged 

 their gametes, as at x. 



the center of the branches. Fertilization is effected, as in all 

 forms, by means of dews and rains which flood these organs with 

 water. An examination of a young archegonial branch will show 

 how admirably it is constructed to ensure fertilization. The 

 archegonia radiating in lines have their necks strongly curved 

 upwards and away from the center of the branch. These lines 

 of archegonia are separated by ridges which thus act as a water- 

 shed, deflecting into the upturned necks any water containing 

 gametes that may chance to fall upon them. After fertilization 

 these mushroom-shaped branches elongate, raising the sexual 

 organs up into the air, where the antheridial branch assumes the 

 form of a lobed disc, while the archegonial branch terminates in 



