70 EMBRYOGENESIS IN PLANTS 



metabolites. In some genera, the axial growth of the embryo con- 

 siderably exceeds its transverse growth, whereas in others such differen- 

 tial growth is considerably less evident. In the former the young 

 embryo will almost certainly divide by a sequence of transverse walls; 

 in the latter it is likely to divide into quadrants; both being in con- 

 formity with Errera's law. Since differential growth is probably gene- 

 controlled, (though it may also be affected by environment factors), it 

 follows that the initial embryonic segmentation pattern may have 

 phylogenetic significance. But in that relatively small genetical 

 differences may apparently have quite considerable effects on the 

 distribution of growth, as in Conocephalwu conicum, it would appear 

 that the extent of differentiation and of specialised development, rather 

 than the nature of the initial segmentation, are the criteria which should 

 be used as evidence of evolutionary status. 



In the Marchantiales, the more elaborate members, e.g. Marchantia, 

 have an adult sporophyte which consists of a foot, a seta and a spore- 

 containing capsule, whereas in the simpler members, e.g. Riccia, Fig. 

 14h, j, the adult sporophyte consists of a mass of spores, with a sur- 

 rounding wall, one cell thick. Nevertheless, the early stages in the 

 embryogeny of Riccia are closely comparable with those in Marchantia, 

 i.e. there is an octant pattern of cell division. The division of the 

 octant cells by curved walls is also common to both genera but there- 

 after the close resemblance between them ceases. In Riccia the next 

 cell divisions are periclinal, an outer capsule wall and an inner arche- 

 sporial tissue being thus formed, i.e. the early embryogeny is followed 

 immediately by the sporogenous phase. The extremes of development 

 thus exemplified by Marchantia and Riccia are of special phylogenetic 

 interest, a number of intermediate conditions being also known, e.g. 

 in Dumortiera the sporophyte consists of a large sporogenous region, a 

 very short seta and a parenchymatous basal foot region. 



From the evidence presented above, the Marchantiales seem likely 

 to afford a fruitful field for further studies of embryogenesis. Already 

 we have seen that, within a single species, e.g. Conocephalum conicum, 

 the early embryogeny may show considerable variability, under the 

 impact of either genetical or environmental factors. A somewhat 

 similar conclusion emerges from comparative studies of the embryogeny 

 in different species of Marchantia. In M. polymorpha, the embryo 

 passes through a characteristic quadrant stage (Durand, 1908). In 

 M. domingensis Anderson (1929) has shown that the spherical embryo 

 does not invariably divide by a transverse wall or pass through a typical 

 quadrant stage: other segmentation patterns are possible, e.g. the 

 epibasal cell may divide by oblique walls, yielding a two-sided apical 

 cell. In M. chenopoda the first dividing wall of the zygote is obliquely 



