188 EMBRYOGENESIS IN PLANTS 



has five leaves, the first two being decussate with the cotyledons, the 

 other three being irregularly disposed. On germination, the cotyledons 

 have a haustorial function; the first leaves are deeply lobed; and the 

 strong first root grows down into the soil. 



Experimental Investigations. Steward and Caplin (1952) have 

 shown that a growth-regulating substance, analogous to the 'coconut- 

 milk factor' present in the watery endosperm of coconuts, is present 

 in the developing gametophyte in the ovule of Ginkgo biloba. 



Tsi-Tung Li (1934) and Radforth (1936) have reported on the 

 culture of Ginkgo proembryos in various sterile synthetic media. 

 Radforth found that there is a noticeable difference between embryos 

 grown in vivo and in vitro, the latter having a diameter nearly twice that 

 of equivalent proembryos grown in vivo. The in vitro embryos were 

 symmetrical, whereas those grown in vivo were asymmetrical, i.e. the 

 formation of the suspensor can be artificially delayed. The cultured 

 embryos also gave other indications of responses to nutritional factors. 

 Having thus obtained evidence that extrinsic factors, i.e. substances in 

 the culture solution, can modify the form and development of the 

 embryo, Radforth concludes that, under natural conditions, the 

 suspensor 'is also the result of reaction to external influence,' i.e. to the 

 surrounding prothallial tissue, and should therefore be regarded as 'a 

 secondary feature imposed upon the symmetrical three-dimensional 

 growth which characterises the earlier stages of proembryo develop- 

 ment. As such it cannot have the significance of primitiveness ascribed 

 to it by both Bower and Lang, who regard the axial single filament 

 type of suspensor as even more primitive than the conical.' Nor can 

 the embryo of Ginkgo be regarded as a 'primitive spindle.' 



PhyJogeny. Ginkgo biloba, like the living cycads, has been described 

 as a 'living fossil.' Ginkgo and related genera are recognisable in 

 Liassic (Lower Jurassic) strata, while other evidently ancestral forms 

 flourished in Permian times. The embryo is unspecialised and of the 

 bulky eusporangiate type. The simple leaves, with their parallel open 

 venation, are very fern-like. Most members of the order have been 

 extinct since the end of the Mesozoic : Ginkgo biloba is the only species 

 that has persisted to the present day. Chamberlain (1935) considers 

 that Ginkgo may have come from the Cordaitales, a purely fossil order, 

 or that both may have originated from the same ancient pteridophyte 

 stock; but Florin (1949) takes the view that the Ginkgoinae, Cordai- 

 tinae, Coniferae and Taxinae belong to the same natural group, the 

 Stachyospermae or Coniferophyta, and that they constitute parallel 

 evolutionary lines which probably had already separated in Upper 

 Devonian or Carboniferous times. 



