MARCHANTIALES 



261 



may be taken as a fair example (Fig. 121), is more complex than that 

 of the Ricciaceae in having polarity of structure, and differentiation of 

 the internal mass of sporogenous cells. Both these characteristics suggest 

 further steps in sterilisation of potentially fertile cells. The polarity is 

 marked in the more or less oblong external form, but more definitely 

 by the fact that the basal tissue is sterile, and develops as the foot 



I. 



a.c. 



w. 



FIG. 121. 



Sporogonium of Fcgatclla (Conocephalus). I., venter of fertilised nrchegonium with 

 eight-celled embryo. II., transverse section of a similar embryo. III., IV., older em- 

 bryos in longitudinal section. V., part of a longitudinal section of a developing capsule, 

 showing differentiation of archesporial tissue into elater-forming cells (el), and sporogenous 

 cells (sp). VI., part of longitudinal section of receptacle with embryo. VII., similar 

 section showing two mature sporogonia in outline. ac = air-chambers ; ca/=calyptni ; 

 caps = capsule ; /= foot ; = neck of archegonium ; / = pore ; rcc. st. = stalk of receptacle ; 

 rkiz. =rhizoids; s = setaof sporogonium. I.-V. Xs6o. VI. X?6. VII. X 20. (After Cavers. ) 



which supports the fertile capsule (Fig. 121, vn.). It is stated that the 

 whole hypobasal half of the zygote is thus sterile, though this limitation 

 may not apply for all cases. This state as compared with that of Riccia 

 might be expressed as a consequence of sterilisation of the whole product 

 of the hypobasal half of the zygote ; but it is still a matter for debate 

 whether the Ricciaceae really represent the progenitors of the Mar- 

 chantiaceae. The presence of this polarity may be held as a biological 

 concomitant of the position of these larger sporogonia relatively to the 

 thallus which bears them : the spherical form of Riccia is suitable to its 



