178 EMBRYOGENESIS IN PLANTS 



include Cycas, Stangeria, Bowenia, Dioon, Zamia, Encephalartos, 

 Macrozamia, Microcycas and Ceratozamia. 



The large mature ovum is as described in the previous Section (p. 1 75). 

 In these plants we encounter the very unusual and interesting pheno- 

 menon of fertilisation of the ovum being effected by motile sperma- 

 tozoids. Moreover, the pollen tube is essentially a haustorial structure 

 which penetrates the nucellus and supplies nutrients to the sperma- 

 tozoids: it is not a carrier of the male gametes as in angiosperms. The 

 spermatozoids are thrust into the ovum with such force that their 

 ciliate region is stripped off, apparently because of the resistance 

 offered by the dense cytoplasm of the ovum. The whole situation is 

 of great physiological interest. Some observations which shed light on 

 the constitution of the ovum may be noted here. At the time of 

 fertilisation, the archegonial cavity, Fig. 38, is moist, but not filled with 

 liquid. At the beginning the spermatozoids swim in the pollen-tube 

 liquid in which they are discharged from the greatly swollen and highly 

 turgid tubes. This pollen-tube liquid probably contains high concen- 

 trations of sugar solution; at any rate, it is known that the sperma- 

 tozoids can move about freely in a 30 per cent solution of cane sugar. 

 It is held that when this pollen-tube liquid of high osmotic pressure 

 comes into contact with the turgid neck cells of the archegonium, it 

 withdraws water from them, reduces their turgor pressure and permits 

 some of the contents of the egg to pass into the archegonial chamber, 

 'leaving large vacuoles at the top of the egg' (Chamberlain). The 

 spermatozoid is then drawn violently into the egg cytoplasm. In 

 Bowenia semilata, according to Lawson (1926), each archegonium 

 secretes a drop of liquid through the neck cells, and this, together with 

 the liquid from the pollen-tube, enables the spermatozoids to swim in 

 the archegonial cavity. The neck cells are also said to swing open with 

 a hinge-hke movement, whereby the spermatozoids gain access to the 

 ovum. These reactions may induce changes in the constitution of the 

 upper region of the egg which are important in the subsequent 

 embryonic development. Having gained access, the male nucleus 

 moves into the centre of the ovum, and fuses with the female nucleus. 



The Proembryo. The fertilisation of the ovum nucleus is followed 

 by free nuclear division. This initial phase of development leads to the 

 formation of the proembryo. At the beginning of this phase, all the 

 nuclei are seen to be dividing simultaneously; their numbers thus 

 increase in an approximately geometrical manner, 7, 8, 9 and 10 

 simuhaneous divisions yielding 128, 256, 512 and 1024 nuclei respec- 

 tively. At about the seventh division, not all the nuclei divide and 

 departures from the theoretical numbers have been observed. There is 

 evidence of polarity in the behaviour of the groups of nuclei associated 



