GINKGOALES 



[CH. 



and reproduced in fig. 631, D: tlie apex of the shoot is seen at a. 

 FertiUsation is said to occur after the fall of the ovule, but Hirase 

 states that some seeds contain an embryo while still attached to 

 the tree. The seeds are comparable in size with large cherries; 

 the broad integument consists externally oi a thick sarcotesta 

 rich in secretory tissue but without a vascular supply, and an 

 inner sclerotesta which is usually two-angled (platyspermic) but 

 occasionally three-angled and radiospermic (fig. 631, C). An 

 account has recently been pubhshed^ of some remarkable examples 

 of Ginkgo seeds gathered from one tree : the stony coat showed 2, 

 3 and 4 ribs and many transitional forms. The sclerotesta is hned 

 by a few layers of loose cells which form a papery membrane in 

 ripe seeds. The absence of vascular tissue in the sarcotesta is a 

 character in which the seeds differ from those of Cycads and 

 Taxads. At the base of the ovule is a single concentric strand 

 which spUts into two branches and these pass through the shell 

 and divide into several bundles on the inner face of the integument 

 forming a continuous mantle^ of short reticulate tracheids as in 

 the Palaeozoic seed Stephanospermum^. The base of the ovule 

 is enclosed in a shallow cup or collar, a structure that is probably 

 homologous with the lamina of a fohage-leaf but which has re- 

 ceived various interpretations. A suggestion has been made that 

 the collar may be homologous with the cupule of Lagenostoma'^. 

 The nucellus is joined to the integument except at the apex Where 

 it forms a prominent cone in which a pollen-chamber is developed : 

 this chamber becomes roofed over by nucellar tissue and at a 

 later stage a blunt outgrowth is produced from the summit of the 

 prothallus, servirig as a 'tent-pole' to support the roof of the 

 pollen-chamber. There are two or more archegonia at the apex 

 of the prothallus differing from those of Conifers and Cycads in 

 the shorter and more spherical form of the egg-cell and similar to 

 those in some Palaeozoic seeds. Fujii^ draws attention to -the 

 remarkable capacity for pollination exhibited by Ginkgo and 

 speaks of the conveyance of microspores over a distance of 500 

 to 1000 metres. Another fact worthy of remark in view of the 

 wide distribution of the Ginkgoales in the Mesozoic era, i.? the 



' Affourtit and La Riviere (15). Sprecher (07) figs. 120, 147 148 



' See Vol. m. p. 326. ^ Shaw, "P. J. P. (08). 5 Fujii^O) p. 216. 



