66 EMBRYOGENESIS IN PLANTS 



The bryophyte ovum is a spherical, elhpsoidal or pear-shaped body 

 which is free within the venter, i.e. it does not fill the whole of the venter 

 cavity. The indications are that the protoplasm of the ovum is probably 

 not homogeneous. Thus Cavers (1904) has depicted the egg in 

 Couocephalum conicum as being ovoid with a clear receptive spot at the 

 upper end, while Meyer (1929) has described and illustrated it as an 

 inverted pear-shaped structure with dense protoplasm at the base and 

 more vacuolated protoplasm above. Very soon after fertilisation, if not 

 before, the polarity of the embryo is established. All bryophytes have 

 an exoscopic embryogeny, the first partition wall of the zygote being 

 typically, though not invariably {see the Anthocerotae), at right angles 

 to the archegonial axis, Figs. 5a, 13. As conspicuous differences in the 

 growth of the upper (epibasal) and lower (hypobasal) segments soon 

 become evident, it could be argued that regional metabolic differences 

 were already present in the egg at the time of fertilisation. Factors 

 determining this heterogeneous, polarised distribution of metabolites 

 might include physiological gradients in the gametophyte tissue, an 

 oxygen gradient having its source at the open neck of the archegonium, 

 or disturbances caused by the entry of the spermatozoid into the distal 

 region of the ovum. In Anthoceros, the first partition wall is parallel to 

 the axis of the archegonium as in leptosporangiate ferns, but the 

 embryogeny is nevertheless typically exoscopic. 



PARTHENOGENESIS AND APOGAMY 



According to Steil (1939) parthenogenesis is unknown in bryophytes 

 but apospory has been described by a number of observers (cited by 

 Steil). Apogamy has been recorded for one moss, Phascum cuspidatiim 

 by Springer (1935); the apogamous sporophytes originated from 

 aposporously produced diploid gametophytes. 



THE HEPATICAE — MARCHANTIALES 



The initial embryonic developments are closely comparable in all 

 liverworts but some diversity is characteristic of the ensuing phases in 

 the diff'erent groups. Fig. 13. After fertilisation, the ovum in Marchantia 

 and related forms enlarges until it fills the venter, when it secretes an 

 enveloping wall. The zygote, now an ellipsoidal or pear-shaped body 

 extended in the axis of the archegonium, is divided by a transverse wall. 

 Fig. 14. In its further development, the embryo may show considerable 

 diff'erences in difl'erent genera: in some, the second wall is at right- 

 angles to the first and a quadrant stage results; in others, the next 

 partition walls are parallel to the first and a short filamentous stage 

 results. But in some species, e.g. Conocephalum conicum, both the 



