202 EMBRYOGENESIS IN PLANTS 



from 2-3, with 90-4 per cent with two cotyledons, whereas in the latter 

 the range is from 2-6, with 33-3 per cent with three cotyledons, and 

 62-8 per cent with four cotyledons. Differences in genetical constitution 

 are thus reflected in the embryonic development. While the general 

 developmental pattern is comparable in the two species, there are 

 differences in the proembryo cytology, in the rosette development, in 

 the number of cotyledons formed, and so on. These differences are 

 indicative of both quantitative and qualitative differences in the growth 

 processes of the two species. A consideration of the gametophytic and 

 embryological differences in Sequoia sempervirens (Redwood) and 

 Sequoiadendron giganteum (the Big Tree) has led Buchholz (1939) to 

 the view that they should not only be regarded as belonging to distinct 

 genera, but that they are probably descendants of widely separated 

 genera — a view that is said to be supported by the fossil evidence. 



In the other sub-family of the Taxodiaceae, including the genera 

 Taxodium, Cryptomeria and Cwininghamia, the embryogeny, with 

 various differences of detail, is generally like that in the Pinaceae. 

 Cryptomeria japonica has two to four cotyledons, Taxodium distichum 

 has two to nine, T. mucronatum has three to five, Cuuninghamia 

 lanceolata has two to four, and Taiwania cryptomerioides has two or 

 three, (for details, see Johansen, 1950; Buchholz, 1932, 1940). 



CUPRESSACEAE 



The several genera included in this family show marked differences 

 in the details of their embryonic development. In Thuja, which stands 

 apart from other members of the family, there is no cleavage poly- 

 embryony and only one embryonal initial undergoes further develop- 

 ment — a marked simplification of the general embryonic process in 

 gymnosperms. Fig. 46a-c. The early embryogeny is characterised by 

 the presence of a distinctive apical cell, but this disappears before organ 

 formation begins. 



Libocedrus, Biota and Chamaecyparis form a coherent group, very 

 different from Thuja, and with several distinctive features including the 

 variable disposition of the free nuclei and the initial wall formation at 



Fig. 46. Embryos of Cupressaceae, etc. 



A-C, Thuja occidentalis. A, Four-celled proembryo; the ventral nucleus is seen 

 above. B, Older embryo; the subapical tiers are elongating into the suspensors. 

 C, Suspensors and young embryo; the distinctive apical cell in B is no longer in 

 evidence (redrawn from Land). D-G, Actinostrobiis pyramidalis. D, Walls have 

 been formed in the four-nucleate proembryo. E, After some further nuclear divi- 

 sions, the embryonic initials have been delimited. F, The suspensors have now 

 elongated. G, A four-celled embryo ( x 250; D-F, after Saxton; G, after Looby 

 and Doyle). H-M, Jimiperus communis. Stages in the development of the cleavage 

 proembryos {see Text) (redrawn from Cook). N, Sa.\egothaea conspicua. Walled 

 proembryo (x 215, after Looby and Doyle). 



