54 



J. WATSON, S.J. LYDON & N.A. HARRISON 



Fig. 21 Ovule attributed to Ginkgoites weatherwaxiae sp. nov., V.64552. Wessex Formation. Worbarrow Bay. Dorset. Apex showing vesicles and distinct 

 longitudinal wall in megaspore membrane cuticle, cell walls of nucellar cuticle and two pollen grains lodged within pollen chamber; x 375. 



fragment of Ginkgoites weatherwaxiae that its presence was not 

 detected until revealed by maceration. We have decided to attribute 

 it to this species as no features of the ovule are in disagreement with 

 this interpretation. Unfortunately, there are no cuticular characters 

 that can be used to link this structure to this, or indeed to any other, 

 leaf species. If it does not belong to G. weatherwaxiae, it could 

 potentially belong to any of the ginkgoaleans, cycads (Watson & 

 Cusack Drury, in preparation) or numerous unidentified gymno- 

 sperms known from leaf cuticles from the English Wealden. 



Fig. 20A shows the remaining two layers of cuticle present in this 

 ovule in the light microscope. The ovule is orthotropous, and broken 

 at the chalaza with no evidence of the funicle. The absence of 

 integument cuticles means that there is no direct evidence of the 

 micropyle, and we have no way of knowing if thick flesh or a stony 

 layer existed within the integument. Unfortunately, evidence of the 

 structure of the ovule-bearing organ is also lacking. 



Following light microscopy, the ovule was first mounted whole on 

 a stub, displaying the damaged nucellar cuticle (Fig. 20B). It was 

 then removed and remounted to allow SEM study of the reverse 

 surface (Fig. 20C). This process revealed the thicker inner cuticle 

 over most of this surface, the delicate nucellar cuticle having been 

 lost. 



The nucellar cuticle completely envelops the inner cuticle and 

 consists of cells with a smooth outer surface and straight anticlinal 

 walls which are longitudinally aligned and decrease in size towards 

 the micropylar end. They are best seen at the edges of the specimen 

 (Fig. 20A) and near the apex, particularly in the SEM which shows 

 anticlinal walls on the inner surface (Figs 20C, 21 ). The mucronate 

 apex or 'beak' which extends above the gametophyte and forms the 

 pollen chamber is typical of most gymnosperm orders (Batten & 

 Zavattieri 1996:710). 



The inner cuticle is very thick and contains conspicuous vesicles 

 which are often aggregated in the centre of ill-defined polygonal 

 cells (Figs 20D, 2 1 ). In the SEM, the outer surface shows these cells 

 to be separated by fine cracks (Fig. 20C, E) and the vesicles can also 

 be seen to form raised impressions on the cuticle surface (Fig. 20E). 



This cuticle has a distinctive wall, associated with a crack in the 

 nucellar cuticle, running longitudinally down the middle from the 

 micropylar end. This can be seen both in the light microscope (Figs 

 20F; 21 ) and in the original SEM view of the surface where a break 

 in the outer cuticle exists (Fig. 20B). In this view it can be seen to 

 meet with a similar wall on the right-hand side a little further down. 



These distinctive features of the inner cuticle are best interpreted 

 by comparison with the development of the female gametophyte of 

 Ginkgo biloba. as described by Soma ( 1 997 ). During the free nuclear 

 division phase, the gametophyte increases in size and the megaspore ■ 

 membrane thickens. Cell wall formation follows, gradually proceed- 

 ing in files from the periphery towards the centre. The innermost 

 cells of the gametophyte fail to join and the opposite files of cells 

 form two distinct abutting walls, so that a mature female gametophyte 

 can be split easily in two. The tissues of the mature female 

 gametophyte show a high degree of cell differentiation. The very 

 outermost cells are filled with lipid droplets and surround cells 

 bearing protein lipid and starch reserves (Rohr 1997: Soma 1997). 



In most ginkgoaleans the megaspore membrane is granular and 

 non-cellular (Archangelsky 1965; Zhou & Zhang 1989,1992; Zhou 

 1993). It seems to us that in this ovule the outer layer of the 

 gametophyte is also cutinized, preserving its polygonal cell struc- _ 

 ture, and that this cutinized layer cannot be distinguished from the :fl 

 megaspore membrane. The globular vesicles seen throughout these 

 gametophytic cells almost certainly represent lipid droplets, and the 

 longitudinal split of the gametophytic tissue, defined by a distinct 

 wall, may well be evidence of centripetal development in two 

 distinct parts as in G. biloba. Although cutinization of the gameto- 

 phyte has not been previously described in fossil ginkgoaleans, 

 impressions of gametophytic tissue on the megaspore membrane 

 have been recorded in the ovules of Yimaia hallei (Sze) Zhou et 

 Zhang from the Middle Jurassic of Henan, China (Zhou & Zhang 

 1992). Ginkgo yimaiensis Zhou et Zhang, of the same age and 

 provenance, also has a megaspore membrane which may bear obscure 

 outlines of 'prothallial cells' (Zhou & Zhang, 1989: 122). A similar 

 phenomenon was described by Harris ( 1 943 ) in the Yorkshire Jurassic 



