102 EMBRYOGENESIS IN PLANTS 



be due to a greater diffusion from the apical meristem of the substance 

 (or substances) which determines the inception of vascular tissue. 

 Holloway examined plantlings of different species in some detail and 

 noted that the development of vascular tissue in the foot of the young 

 plant varies in extent in different individuals of the same species. He 

 thought that this is possibly related to the size of the parent prothallus. 



PHYLLOGLOSSUM DRUMMONDII 



The embryonic development in this species has been briefly 

 described by Thomas (1901) and Sampson (1916). The prothallus and 

 embryo are generally like those in Lycopodium cemuum. The embryo 

 is attached laterally, the first organ to emerge being the first leaf which 

 is green and photosynthetic, A tuberous swelhng is soon formed below 

 this leaf. The first root may or may not have developed at this stage. 

 Bower (1935) has shown how, in its annual growth, the tuberous adult 

 plant closely follows the embryonic development. 



DISCUSSION 



The Suspensor. Much importance has been attached to the presence 

 of a suspensor in pteridophytes. It has been regarded as a primitive 

 feature that has been 'retained' in some modern survivors. Jeffrey 

 suggested that it is useful in pushing the developing embryo deeper 

 down into the prothallial tissue (Campbell, 1911). Here we may note 

 that the suspensor enlarges like the adjacent parenchymatous cells of 

 the foot. The formation of a suspensor shows that the physiological 

 conditions at the outer or neck end of the zygote are very different 

 from those at its inner end. Whether the distinctive developments of 

 the suspensor and the embryonic cell are due to a gene-controlled 

 differentiation in the zygote, to metabolic gradients in the prothallus, 

 to gradients of gaseous concentration, or to some residual effect of 

 fertilisation, is not known. Certainly there would appear to be some 

 quite fundamental difference between those pteridophytes with the 

 embryonic cell adjacent to the neck of the archegonium, as in Psilotiim, 

 and those in which it is directed inwards, as in Lycopodium. But, as 

 already mentioned (p. 25), it may be that a relatively small difference in 

 the reaction system may determine whether the embryogeny is exoscopic 

 or endoscopic {see also p. 328). (An endoscopic embryogeny is not always 

 associated with the presence of a suspensor. In the Marattiaceae, 

 where the archegonium points downwards, the embryogeny is endo- 

 scopic whether a suspensor is present or not : see Chapter IX.) 



The Foot and the Protocorm. Treub regarded the protocorm as a 

 primitive organ — an ancestral feature which has been retained in some 

 contemporary species. He even held that it might have been of general 



