CHAPTER XXI. 

 BRYOPHYTA. 



I. HEPATICAE. 



THE Liverworts include three main series, which differ, not only in the 

 structure of the gametophyte but also in the details of the sporophyte ; 

 the differences . are sufficient to require their separate treatment : the 

 three series are the Marchantiales, the Jungermanniales, and the Antho- 

 cerotales. The results acquired from any one of these series may be 

 brought into comparison with those from any other, and suggestive 

 side-lights may thus be gained on the methods of advance of the 

 sporophyte which they illustrate ; but the extent of their differences 

 shows that they cannot readily be held to constitute one consecutive 

 evolutionary sequence. 



A. THE MARCHANTIALES. 



These include those Hepatics which show the simplest of all Archegoniate 

 sporophytes ; and the fruit-body of Riccia is the extreme example of 

 simplicity of construction. As in all other Archegoniatae the sporophyte 

 of Riccia originates from the ovum, contained in the venter of the 

 archegonium (Fig. 118), which is here deeply sunk in the tissue of 

 the thallus. The naked ovum at fertilisation is contracted away from the 

 wall of the archegonium, but after fertilisation it forms a cell-wall, and 

 expands till it completely fills the cavity. It then segments, the first 

 plane of segmentation being inclined to the axis of the archegonium : this 

 is followed by other cell-divisions resulting in cleavage of the sphere into 

 octants, after which the segmentation becomes less regular. It is only at 

 a comparatively late period that walls parallel to the outer surface separate 

 a superficial series of cells forming the wall of the fruit-body, from the 

 mass of cells which lie within (Fig. 119). All these latter cells are fertile, 

 while the superficial cells composing the wall are sterile and subsequently 



they become disorganised, their substance being absorbed by the developing 



R 



