EMBRYOGENESIS IN THE LYCOPODINEAE 123 



Other factors, this may account for the development of a more or less 

 conspicuously swollen foot and for the upward curvature of the 

 embryo. As auxin production at the shoot apex and the response of 

 tissues to auxin are probably gene-controlled, the size of the foot may 

 be expected to vary from species to species. 



The swollen foot is generally held to be a suctorial or haustorial 

 organ, i.e. takes up nutrients from the prothallus and passes them on to 

 the growing apical region. To what extent, in the early embryonic 

 development, the absorption of nutrients takes place over the whole 

 embryo is not known, but from quite an early stage the embryo apex 

 behaves very much like an adult shoot apex, i.e. as if it received all its 

 nutrients from the tissues below. 



The data for Selaginella support the view that the characteristic 

 embryonic developments, i.e. the 'stages' illustrated in morphological 

 studies, are fundamentally due to the patternised distribution of 

 specific metabolites (see Chapter 111). That biochemical and bio- 

 physical conceptions have an essential place in interpretations of the 

 embryogeny of Selaginella is evident from the anatomical data recorded 

 by Bruchmann. In S. poiilteh, for example, the slender suspensor does 

 not simply thrust the embryo down into the dense tissue of the gameto- 

 phyte ; there is evidence of an active dissolution of the tissue by enzymes 

 secreted by the distal region of the embryo, very much like the digestion 

 of the endosperm by the embryos of flowering plants. Examples of this 

 are seen in those species in which the archegonium elongates into a 

 long tubular or sac-like structure which Bruchmann and Goebel have 

 described as an Emhryosclilauch (embryo-tube). In S. kraussiana this 

 tube penetrates deeply into the prothallial tissue, just like a stout 

 fungal hypha, with the small, two-celled embryo lying at the inner 

 extremity ; while in S. galeottei the long and sinuous tube, with a small 

 four-celled embryo at its inner extremity, is surrounded by prothallial 

 tissue in a state of dissolution or partial digestion. The characteristic 

 size of the foot and the position of the first rhizophore in different 

 species afford further evidence that a specific and orderly distribution 

 of metabolites underlies the observed developments. 



The suspensor in Selaginella is of interest on both phylogenetic and 

 morphogenetic grounds. There can be little doubt that the factors 

 which determine its presence are transmitted in heredity. But precisely 

 how these factors, or genes, produce the observed effect, i.e. the 

 differentiation of the zygote into two cells of different constitution and 

 reaction, is not known. Selaginella also shows a trend in the direction 

 of the loss of the suspensor: in various species the suspensor is replaced, 

 as it were, by a deeply penetrating embryo-tube. In the genus as a 

 whole, the suspensor may be more or less well developed and functionally 



