190 EMBRYOGENESIS IN PLANTS 



account already given (p. 172). As the ovum in Piiius and other genera 

 enlarges to its mature, pre-fertilisation state, it becomes highly vacuo- 

 lated, some of the vacuoles being said to contain protein. It accumulates 

 abundant food reserves and is surrounded by a conspicuous tapetal-like 

 sheath or 'jacket.' The process of fertilisation, and the first and second 

 mitoses of the fusion nucleus, have not only important biological 

 aspects: they also present a range of biochemical and biophysical 

 problems of great interest but on these we have practically no informa- 

 tion whatsoever. The details of the discharge of the contents of the 

 pollen tube into the ovum vary. Sooner or later a male nucleus moves 

 down into the ovum and fuses with the female nucleus. The other 

 nuclei near the archegonial neck degenerate. The four free nuclei 

 formed by the first two divisions of the zygote at first occupy a central 

 position in the proembryo. Soon fine fibrils are formed in the surround- 

 ing cytoplasm; these increase in diameter and length, become aligned 

 along the axis of the proembryo and eventually extend to its base. 

 Some of the fibrils then apparently become adpressed and attached to 

 the archegonial membrane and 'soon the nuclei become pulled to the 

 base of the archegonium by means of the tractive force exerted by the 

 fibrils' (Johansen, 1950). This curious phenomenon is undoubtedly 

 related to the polarity of the proembryo, for it has been observed that 

 these fibrils have the same basipetal alignment, irrespective of the 

 orientation of the cone or ovuhferous scale. 



The eggs in the Coniferales, as compared with those of the Cycadales, 

 are comparatively small and the number of free nuclear divisions is 

 greatly reduced. An extreme case is found in Sequoia where there is no 

 free nuclear stage, the egg being little larger than that of a pteridophyte 

 and a cell wall being laid down at the first nuclear division. Fig. 44. 

 In general, however, a free nuclear proembryo is characteristic of the 

 Coniferales. 



PINACEAE 



In Pinus, which has been fully investigated, the zygote nucleus 

 undergoes mitosis; this is immediately followed by a second mitosis, 

 the four free nuclei thus formed being situated about the middle of the 

 proembryo. This is the full extent of the free nuclear phase. The four 

 nuclei now move to the base of the archegonium where a third division 

 takes place, this last terminating with wall formation, Fig. 37 {see also 

 p. 174). However this basipetal nuclear movement may be explained, 

 it suggests the existence of polar gradients due to a biochemical 

 differentiation within the proembryo. The nuclei, in fact, have moved 

 to what will become the primary morphogenetic region of the embryo. 

 The eight nuclei at the base of the proembryo are now disposed in two 

 tiers each of four nuclei, with walls between. The cells of the upper tier 



