jrNGERMANNIALES 



267 



clue to the steps of evolution of the elaterophore : that it arose by partial 

 sterilisation of the archesporium, with the consequence that the fertile 

 zone is relegated to the exterior. The development in Pellia is essentially 

 the same, but the attachment of the elaterophore is to the base of the 

 capsular cavity. It is worthy of note that these modifications occur in 

 the Aqakrogynous Jungermanniaceae, which, as we have already seen, are 

 more advanced than the Akrogynous as regards the differentiation of the 

 seta and capsule. 



FIG. 129. 



A, median section of young sporogonium of Aneura ambrosioides. The internal mass 

 of cells of the sporogonial head ("archesporium") is already differentiated so as to 

 indicate the sterile elaterophore, and the outer fertile region. , the same, older : the 

 indications of sterilisation have extended outwards, and it is only the peripheral fringe of 

 cells (shaded) which will be sporogenous. C, transverse section of the same. X 150. 



It thus appears that in the Jungermanniales partial sterilisation of the 

 sporogenous tissue has occurred analogous to that in the Marchantiales ; 

 but in addition there is reason to believe that the elongated seta has 

 here originated also by relegation of the spore-producing function from 

 the lower segments to the upper, or even to that tier of them which is 

 apical. 



'C. THE ANTHOCEROTALES. 



The Anthocerotales stand clearly apart in the mature characters of the 

 sporogonium. Its large size and early freedom from the calyptra, the 

 continued intercalary growth, and the specialisation for self-nourishment 

 are external characters which dissociate the family from other Liverworts : 

 while internally, the presence of a columella, at least in the larger forms, 

 and the origin of the archesporium from the outer cells of the young 

 capsule, also point in the same direction. There are, however, differences 

 of detail within the family, which indicate with some degree of probability 

 a line of derivation from a Jungermanniaceous type, the nearest approach 

 being in the genus Notothylas. 



