MARCHANTIALES 



259 



primitive type, though the suggestion has also been made that it may 

 really be reduced ; but in view of the fact that the gametophyte in the 

 Ricciaceae is a well-developed structure, amply capable of nourishing not 

 only one but many such sporogonia, there appears no immediate reason to 

 hold that this sporophyte is other than primitive in its simplicity. The 

 points of special interest in it for the purposes of comparison with the 

 more complex Archegoniatae are these : that it shows no distinction of 

 apex and base : that the whole central mass of cells is fertile, each cell 

 producing spores, while none are diverted to purposes of nutrition or ot 



' !B$9$ 



FIG. IIQ. 

 Ricciocarpiis natans. Young sporogonia in longitudinal section, 



FIG. 120. 



Ricciocarpus natans. The upper figure 

 shows tin- spherical .spore-mother-cells 

 surrounded by nutritive material. The 

 lower sin us the tetrads formed from 



surrounded l>y the archegonial wall. The younger (X666) shows them : the sporogonia] wall (shaded) is 

 the amphithecium (shaded) surrounding the sporogenous cells : in still seen surrounding them, and covered 

 the older (Xs6o) these are separated, as the free, and rounded externally by the archegonial wall of two 



spore-mother-cells. (After Garber.) 



cell-layers. x666. (After Garber.) 



dispersal : and that the superficial cells forming the wall are segmented off 

 by periclinal walls of relatively late origin, indicating some relatively 

 recent differentiation of them from the cells which lie within. 



A reasonable theory of the phyletic origin of a simple sporogonium, 

 such as that of Riccia founded on these facts, would then be, that it 

 sprang from the simple zygote, as in point of fact all normal sporophytes 

 do. The simplest possible case of a sporophyte would be that the 

 chromosome-reduction which follows on fertilisation should take place on 

 the first segmentation of the zygote, and in certain Algae this appears 

 actually to occur (Chapter V.). But in the sporogonium of Riccia the 

 reduction which accompanies tetrad-division is held over till a limited 

 number of segmentations of the zygote have been completed : this suggests 



