THE PTERIDOPHYTA : FILICALES, THE FERNS 531 



Development of the Archegonium 



The archegonia develop on the under side of the central rib or thickened 

 cushion of cells. They appear only on the sides of this rib and stand 

 horizontally, that is to say, with the necks roughly parallel to the lower surface 

 of the prothallus. The details of their development are essentially similar 

 to those of the archegonia in Dryopteris. There is, indeed, a marked con- 

 stancy in the development of the archegonia all through the Ferns. In 

 Osmunda the chief differences are that the neck is eight tiers long and is 

 straight, not bent. 



Development of the Embryo 



Only one embryo comes to maturity on each prothallus, though several 

 may begin development. The first two divisions of the oospore are parallel 

 to the axis of the archegonium and at right angles to each other, while the 

 third is transverse and cuts across the first four cells to form an octant. The 

 further development is exceptional in that the stem, root and first leaf are 

 all derived from one-half of the octant, while the other half only forms the 

 foot. The latter organ is much larger than is usual in the Leptosporangiatae 

 and penetrates deeply into the prothallus, forming haustorial cells over its 

 surface, like the foot of the sporophyte in Anthoceros. The embryo develops 

 its parts slowly and remains for a considerable time a mere globular mass, 

 and the cotyledon does not break out of the archegonial wall until compara- 

 tively late in development (Fig. 529). All these are relatively primitive 



Prothallus 



Foot 



Calyptra 



Primary root 



Fig. 529. — Osmunda regalis. Vertical 

 section of the prothallus and attached 

 embrvo. 



