122 EMBRYOGENESIS IN PLANTS 



DISCUSSION AND CONCLUSIONS 



Any attempt at a causal analysis of the embryonic developments in 

 Selaginella and Isoetes will necessarily be tentative and the conclusions 

 reached are likely to be incomplete and they may be erroneous. Yet, 

 if we are to advance in our knowledge of embryogenesis, it is essential 

 that the morphological evidence should not simply be left as so much 

 descriptive data and that explanations of the observed phenomena 

 should be attempted. In this connection the discovery by Wetmore 

 and Morel (1951) that the prothallus of Selaginella pallescens can be 

 cultured aseptically in vitro should prepare the way for new experi- 

 mental investigations. 



Bower (1922, 1935) has pointed out that although the orientation of 

 the embryo in Selaginella is usually related to the axis of the arche- 

 gonium, attention must also be paid to the effect of gravity. As a 

 convention, the archegonium in Selaginella is usually illustrated with 

 the neck directed vertically upwards ; but, in Nature, the oval spore is 

 most likely to lie on its side, and therefore both the axis of the arche- 

 gonium and of the young embryo are horizontal. The dispositions of 

 the shoot and root in the young sporophyte bear out this view. The 

 illustrations show that the effects of gravity become evident at an 

 early stage in the embryogeny. 



If the archegonial axis in Selaginella is horizontal, the elongation of 

 the zygote and the first transverse wall seem likely to be determined by 

 factors within the zygote or in the surrounding gametophyte tissue. 

 The axis of the archegonium coincides with the direction of physio- 

 logical gradients from the centre to the surface of the prothallus. 

 Accordingly, either before or after fertilisation, the ovum may be 

 affected by these gradients, and a differential distribution of its meta- 

 bolic substances may result. The segmentation pattern in any species 

 of Selaginella develops in a regular and characteristic manner, and this 

 may be attributed partly to the distribution of gene-determined meta- 

 bolites in the embryonic reaction system, and partly to physiological 

 conditions in the surrounding gametophyte tissue. 



As in Lycopodium, the distal pole of the Selaginella embryo is 

 almost immediately established as the region of active protein synthesis, 

 meristematic activity and organ formation, the basal pole being a 

 region of accumulation of osmotically active substances, active cell 

 enlargement and vacuolation. Thus, from an early stage, the embryo 

 has an active apical meristem, and the organisation of a miniature 

 shoot. As in procumbent shoots, the horizontal embryo may be 

 characterised by a differential distribution of growth-regulating 

 substances as between its upper and lower sides. In conjunction with 



