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 /ticcia is the extreme example of 
simplicity of construction. As in all other Archegoniatae the sporophyte 
of Ricca 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 
