S>ostel5ia 71 



independent organism, the axial rela- 

 tion of the stem and root would be- 

 come of more importance to the plant 

 than the axial relation of the stem and 

 foot, and as the stem and root must 

 both become free from the gameto- 

 phyte, the foot would necessarily 

 come to occupy a lateral position 

 with respect to the vegetative axis 

 of the plant. The embryos of the 

 Bryophyta are bi-polar, the foot occu- 

 pying a basal position, but with the 

 advent of the root in the Pteridophyta 

 their embryos become tri-polar. When 

 a main vegetative axis is established, 

 however, it is between the two members 

 which will persist in the bi-polar veg- 

 etative body. 



The establishment of this axial 

 polarity at earlier or later periods in 

 the embryogeny of different plants 

 accounts for the apparent displacement 



