26 



VASCULAR CRYPTOGAMS 



chlorophyll, but never loses its connection with the megaspore, even 

 after the commencement of the germination of the sporophyte. The 

 first archegone makes its appearance on the elevated dorsal ridge of the 

 prothallium, and subsequently two others are formed, one on each side 

 of the first. If one of these is fertilised no more archegones are pro- 

 duced, and the prothallium ceases growing. But if no impregnation 

 has taken place the prothallium continues to grow, and produces from 

 one to three additional rows, each consisting of from three to seven 

 archegones. In Azolla the prothallium has the form of a slightly convex 



disc, consisting, in its central part, 

 of several layers of cells, at the 

 margin of only one. A single 

 archegone is first formed, near the 

 centre of the prothallium. If this 



Fig. 9. — Sah'biia nataiis. Longitudinal section 

 through megaspore and prothallium. a, wall 

 of sporange ; b, epispore, formed of hardened 

 mucilage ; c, coat of spore ; d, diaphragm 

 separating prothallium from spore-cavity ; pr, 

 prothallium ; ;;/, neck of archegone ; /, //, 

 first two leaves of embryo ; s, scutiform leaf 

 or cotyledon. (After Pringsheim, x 70.) 



Fig. 10. — Archegone of Salvinia fiaians 

 in three stages, a, b, c, divisions in the 

 neck-cells , d, neck-canal-cell ; e, oosphere ; 

 /i, neck-cells. (After Pringsheim, x 150.) 



is fertilised, no others are formed ; if not, it is followed by a few others. 

 The archegone of Salvinia is a nearly globular cavity, its venter being 

 completely buried in the tissue of the prothallium. The central cell is 

 at first somewhat elliptical, its axis lying obliquely to the surface of the 

 prothallium. Its apex is at first covered by four cells belonging to the 

 epiderm, arranged in the form of a cross. Each of these four neck-cells 

 divides by transverse septa into a row of three cells, the four rows thus 

 forming a short neck. The large central cell now elongates upwards 

 and forces its way between the lowermost four cells of the neck, and its 



