26o BRYOPHYTA 



that the event of reduction was deferred in the course of its descent. The 

 cell-mass thus produced in Riccia is at first homogeneous, as was probably 

 the case definitively in certain of its ancestry. Differentiation comes 

 later in the sporogonium of Riccia, as it probably did also in the race : 

 in place of every cell being equally liable to the tetrad-division, this is 

 carried out only by those which lie internally : those forming the 

 superficial wall are sterile, and form only somatic tissue. There is ample 

 evidence of such sterilisation of fertile cells occurring elsewhere, both in 

 plants related to Riccia, and in other phyla (Chapter VII.), so that no 

 a priori objection can be taken to its place in the theory : there is, 

 however, no direct proof that this was actually the case. The remaining 

 cells which lie centrally then all undergo the tetrad-division, which on 

 the above theory was the primitive condition for all the cells of the 

 sporophyte. 



Till recently it was thought that the fruit-body of Coleochaete supplied 

 a prototype of an undifferentiated mass of cells, all fertile, such as this 

 theory contemplates; but it has now been shown that in Coleochaete 

 reduction occurs at the first segmentation of the zygote, and accordingly 

 the old comparison is no longer permissible. There is, however, a growing 

 body of evidence, from several distinct phyla of Thallophytes, that the 

 event of chromosome-reduction consequent on sexuality may be deferred 

 in the individual life : that a sterile, or vegetative phase of the nature 

 of a sporophyte, varying in structure and in mode of origin, but similar 

 in being partly somatic, partly fertile, may be thus intercalated between 

 the two events. The Florideae, the Ascomycetous Fungi, and the Uredineae 

 provide examples of such intercalation of a sporophytic phase : these point 

 an analogy in this respect with the simplest Archegoniatae, though along 

 phyletic lines almost certainly apart from the latter (Chapter V.). Thus 

 the view now stated of the phyletic origin of the simple sporogonium of 

 Riccia by continued segmentation of the zygote, and deferred tetrad-division, 

 with sterilisation of the superficial cells, is in the main hypothetical, it 

 is true \ but it has a reasonable basis, partly on the facts of the individual 

 development, partly on analogy. In the absence of still simpler sporo- 

 phytes affording comparisons within the series of the Archegoniatae 

 themselves, this analogy, together with the facts of the individual develop- 

 ment in Riccia itself, make the view thus stated appear more probable 

 than any alternative hitherto proposed. 



Riccia being the simplest type of sporogonium in the Archegoniatae, 

 the basis of the antithetic theory has been fully stated, as applied to the 

 facts of its development. The same theory may be extended from it to 

 other forms also, in which the sporophyte, though more complicated, 

 arises from the zygote by similar though more extended segmentation. 

 Steps in advance are illustrated in other Marchantiales, which will now 

 be described. 



The sporogonium of the Marchantiaceae, of which Fegatella (Conocephalus) 



