SJ 



. 



XL] GINKGO 7 



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

 water 1 . The embryo has normally two hypogean cotyledons 

 though three are not uncommon. Velenovsky 2 mentions a pecu- 

 liarity, another indication of the isolated position of the genus, 

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

 gams ; 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 foliage-leaf thus arises in the angle of the V-shaped distal 

 end of the earlier scale-leaf. 



A microspore on germination developes 2 3 prothallus-cells 

 and the generative cell forms two large (110/z x 80 /z) spirally 

 ciliated antherozoids. After fertilisation the egg-nucleus divides, 

 as in some Cycads, until 256 free nuclei are formed 3 , but in Ginkgo 

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

 Lyon 4 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 allied more closely to the Cycads than to 

 the Conifers. Saxton 5 who has described the later stages in the 

 embryogeny of Encephalartos draws attention to certain features 

 shared by that genus and Ginkge. 



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

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

 Annual rings are fairly distinct though, as Nicol 6 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 7 . 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, occasionally 



1 Ewart (08) p. 78. 2 Velenovsky (07) p. 457, fig. 29 la, 



3 Coulter and Chamberlain (03). 4 Lyon (04). * Saxton (10 4 ). 



6 Nicol (34) A. p. 147. 7 Xakamura (83) p. 25; Fujioka (13) PI. xvm. 



