204 EMBRYOGENESIS IN PLANTS 



the base of the archegonium. The nuclear dispositions seem to be 

 determined by the shape and size of the archegonium. Cleavage poly- 

 embryony is general and is evident at an early stage. The mature 

 embryo has two cotyledons (Buchholz, 1932; Doak, 1937; Sugihara, 

 1938, 1939). 



The section of the Cupressaceae represented by Actinostrobus and 

 CaUitris shows some curious and interesting variants of the general 

 pattern of gymnosperm embryology. In Actinostrobus (Saxton, 1913), 

 the archegonia, in groups of twenty-five to thirty, are formed laterally 

 in the gametophyte, i.e. the transverse plane of the archegonium is 

 aligned with the axis of the gametophyte. The microgametes traverse 

 the nucellus laterally, two adjacent archegonia being fertilised by the 

 contents of one pollen tube. Many of the archegonia remain unfertilised 

 and soon degenerate. When the zygote nucleus divides, the two 

 daughter nuclei are aligned on the long axis of the archegonium. The 

 further nuclear divisions are such as to yield two cells next the arche- 

 gonial neck (which undergo no further development and soon dis- 

 appear), and four embryonic initials. Fig. 46d-g. These four cells 

 appear to be polarised in the transverse plane for each of them forms a 

 small distal initial cell and four very large suspensor cells. Fig. 46f, 

 four embryos being usual in this genus. The details obtained by Looby 

 and Doyle (1940) for the proembryo of CaUitris are somewhat similar. 

 The further development of Actinostrobus is characterised by the very 

 great elongation of the suspensor, by the absence of a rosette, by the 

 division of the embryo initial cell by two vertical walls, so forming a 

 tier of four cells (no apical cell has been recorded), and by the elabora- 

 tion of the secondary suspensor. The cotyledon number is usually two 

 and the shoot apex is broad and massive. The peculiar features of the 

 embryonic development, then, appear to be largely consequential on 

 the position of the archegonia. From the developments outlined above, 

 the physiological gradients in the axis of the prothallus appear to be 

 more potent in the embryonic development than are gradients in the 

 long axis of the archegonium. In a recent comprehensive account of 

 CaUitris Baird (1953) has noted that the ovule is pollinated long before 

 the appearance of the megaspore mother cell, that the mature pro- 

 thallus is long and narrow, that the archegonia are formed laterally 

 and aligned along the pollen tube and that the embryogeny is essentially 

 the same in Actinostrobus and CaUitris. 



In Juniperus communis there are no unusual features in the pro- 

 embryo development: it consists typically of three tiers with four 

 distal initial cells. Some curious and unique developments, however, 

 have been observed in the four cleavage embryos. The terminal and 

 subterminal cells, which in other gymnosperms would comprise the 



