XL] GINKGO 7 



germination of Ginkgo seeds after 45 days' immersion in sea- 

 water^. The embryo has normally two hypogean cotyledons 

 though three are not uncommon. Velenovsky^ mentions a pecu- 

 Uarity^ another indication of the isolated position of the genus, 

 in which seedUngs of Ginkgo differ from those of other- Phanero- 

 gams i the cotyledons are succeeded by two elongated scales with 

 a forked apex; the next higher leaves^ in. which a small bilobed 

 lamina is a characteristic feature, show at the base two divergent 

 prongs representing the fork of the lower scales. The lamina of 

 the fohage-leaf thus arises in the angle of the V-shaped distal 

 end of the earher scale-leaf. 



A microspore on germination developes 2 — 3 prothallus-cells 

 and the generative cell forms two large (110|Lt x 80 /la) spirally 

 ciliated antherozoids. After fertihsation the egg-nucleus divides^ 

 as in some Cycads, until 256 free nuclei are formed^, but in Ginkgo 

 the subsequent production of walls results in a tissue, called by 

 Lyon* the protocorm, which completely fills the egg; whereas in 

 Cycas this tissue is massed at the base and in Zamia wall-formation 

 is also restricted. In Conifers the number of nuclei is much less 

 and the proembryo still further reduced. It is probably legiti- 

 mate to deduce from these facts that Ginkgo is in respect of its 

 embryogeny the most primitive of the Gymnosperms : in this and 

 other characters it is alhed more closely to the Cycads than to 

 the Conifers. Saxton^ who has described the later stages in the 

 embryogeny of Encephaldrtos draws attention to certain features 

 shared by that genus and Ginkge. 



The leaf-traces arise from the stele as a pair of collateral bundles, 

 as in the Palaeozoic genus Mesoxylon, which pass up the petiole. 

 Annual rings are fairly distinct though, as NicoP recognised, less 

 obvious than in Conifers. The walls of the late-summer tracheids 

 are hardly thicker than those of the spring-elements and the diffe- 

 rence between the early and late wood is often slight'. Circular 

 bordered pits occur either in a single or double row on the radial 

 walls of the tracheids and are fairly common on the tangential 

 walls. The pits may be separate or in contact, occasioneilly 



1 Ewart (08) p. 78. ' ^ Velenovsky (07) p. 457, fig. 291ff.. 



' Coulter and Chamberlain (03). « Lj'on (04). » Saxton (10«). 



8 Niool (34) A. p. 147. ' ' Nakamura (83) p. 25; Fujioka (13) PI. xvra. 



