EMBRYOGENESIS IN GYMNOSPERMS 



175 



Cycadophytes and the Coniferophytes. For these groups Sahni (1921) 

 introduced the terms Phyllospermae and Stachyospermae respectively. 



Recently Eames (1952) has indicated that Ephedra, Welwitschia and 

 Gnetum, hitherto included in the Gnetaies, do not constitute a coherent 

 and related group. Johansen (1950), who has given a valuable survey 

 of gymnosperm embryology, follows Hagerup (1933) and others in the 

 view that the group is not a natural one and describes his materials under 

 the headings: Cycadophyta, Ginkgophyta, Coniferophyta and Ephe- 

 drophyta. 



THE ENVIRONMENT OF THE OVUM 



The gymnosperm ovum differs markedly from that of any of the 

 pteridophytes in that its encasing archegonium is deeply-seated within 

 a massive ovule. The developing ovum and embryo thus participate 

 in the nutritional resources of the adult, sporophytic plant. As one 

 example, it will be convenient to consider here the ovum in the cycads. 

 As the megaspore enlarges to form the prothallus it becomes still 

 more deeply embedded in the nucellar tissue. When eventually the 

 several archegonia are formed, they are surrounded on all sides, except 

 for the small region of the neck, by cells densely stored with reserve 

 materials. The egg is of very large size and at maturity contains a 

 conspicuous nucleus. During the phase of free nuclear division in the 

 formation of the gametophyte (prothallus or 'endosperm'), a surround- 

 ing, tapetum-like, nutritive layer, one or two cells thick, and known as 

 the endosperm jacket, is differentiated. Evidence of the existence of 

 physiological gradients is afforded by the formation of walls in the 

 gametophyte from the external nutritive wall progressively inwards 

 towards the centre. The inception of the archegonium initials at the 

 micropylar end suggests that there is a polarised distribution of meta- 

 bohtes within the prothallus. In this development the possibility of a 



