124 EMBRYOGENESIS IN PLANTS 



important. In S. denticulata, for example, the foot, in whole or in 

 part, is formed from the suspensor. In Isoetes, however, there is no 

 suspensor, either because this organ was never present in the Isoetales 

 or because it has been lost, or eliminated, in the course of descent. 

 Although the embryo of Isoetes is not thrust into the prothallus as it is 

 in Selaginella, its organogenic development is nevertheless broadly 

 comparable : a basal foot makes intimate contact with the gametophyte 

 tissue and transmits nutrients to the apical region. Bower has referred 

 to the zygote of Isoetes as having become 'inverted' as a consequence 

 of the loss, or absence, of a suspensor. This conception raises problems 

 of some difficulty; for, as we have seen, there is support for the view 

 that the development of the zygote is characterised by a heterogeneous 

 distribution of metabolites at the two poles. Now, where a suspensor is 

 eliminated in the course of descent, presumably as a result of mutation, 

 the morphological modification will be referable to a change in the 

 previous biochemical differentiation at the two poles. The suspensor 

 segment of a zygote, as we have seen, has limited potentiality for growth 

 but is a locus for the accumulation of osmotically active substances. 

 Even where the suspensor is small or absent, the basal cells of the foot 

 have these same characteristics. In some species the suspensor may 

 have little functional importance and in general it is an organ of limited 

 growth. Whether the genetical change determining the elimination of 

 the suspensor is of a major or minor character is not known (see 

 Chapters IX, XVI). If the ancestors of Isoetes possessed a suspensor, it 

 is difficult to explain how the biochemical differentiation at the poles of 

 the two-celled embryo could have become completely reversed. Con- 

 siderations such as these suggest that the taxonomic relationship of the 

 two genera cannot be a close one. 



Bower (1935) has remarked that it is perhaps surprising that the em- 

 bryonic developments in Lycopodium and Selaginella are so much alike, 

 considering how very different are their respective prothalli : L. selago 

 and S. spimdosa, for example, are closely comparable in a number of 

 features. We do not, of course, know the magnitude of the genetical 

 difference that separates the heterosporous and the homosporous con- 

 ditions : it may perhaps be considerably less than is usually supposed by 

 morphologists. Certainly, it would appear that the main factors deter- 

 mining the embryogenic development in the two genera were already 

 present in the common ancestor and that subsequently the two lines of 

 descent have afforded some remarkable examples of parallel evolution. 



Lastly, it may be noted that the early embryogeny of Selaginella is 

 not at all unlike that of some flowering plants, in which the proembryo 

 consists of a multicellular suspensor with an approximately spherical 

 embryonic region, the latter giving rise to the shoot, leaves and roots. 



