REVISION OF THE ENGLISH WEALDEN FLORA 



water with Thymol crystals added. Surprisingly large specimens, 

 such as long thin needles, escape through the middle sieve but these 

 can be retrieved before discarding or storing the contents of the 

 bottom sieve. With a mesh size less than 1 mm it is often laden with 

 sediment and usually far too comminuted and voluminous to con- 

 template long term storage and study. These fractions have in some 

 cases been dried and searched for megaspores etc. with varying 

 degrees of success. 



Searching of the concentrates for plant parts is done under water 

 (with thymol) in petri dishes, with low power stereo microscopes, 

 using very fine paint brushes (10 noughts if possible) or sharpened 

 split bamboo sticks. The sorted leaves, shoots, cones, seeds etc. are 

 stored in water/thymol in small, corked glass tubes. For long term 

 storage the cork can be sealed with paraffin wax. Samples prepared 

 in this way by Watson have been successfully stored without fungal 

 growth since 1964. 



SYSTEMATIC CONSIDERATIONS 



With the first clearly recognisable members of the orders Ginkgoales 

 and Czekanowskiales appearing in the Triassic, their origins and 

 systematic affinities have been long debated. Mesozoic floras from 

 the base of the Jurassic to the Lower Cretaceous span the zenith of 

 both the Ginkgoales and the Czekanowskiales and by the Middle 

 Cretaceous both orders were in decline with the Czekanowskiales 

 becoming extinct before the end of the Cretaceous (Batten 1984). 

 The Ginkgoales continued, but on entering the Tertiary were re- 

 stricted to a single leaf-genus and soon to a single species which was 

 forced, by the rapid spread of the vigorous angiosperms, into the less 

 hospitable habitats in the northern latitudes (Tralau 1967, 1968). The 

 onset of the Pleistocene ice-age contracted all floristic zones towards 

 the equator; the higher latitude species being forced to retreat to high 

 altitude habitats. The distribution of Ginkgo biloba L., the sole 

 surviving member of the Ginkgoales, was contracted to a high 

 altitude refugium in China and it is still debated whether G. biloba 

 survived truly in the wild in China or was saved from extinction by 

 cultivation in temple gardens (He et al. 1997). In modern times the 

 species has been re-introduced in other parts of the world and shows 

 exceptional resistance to extremes of temperature, infection and 

 polludon (Kim et al. 1997). 



Early studies on the systematic affinities of the Ginkgoales relied 

 upon evidence from Ginkgo biloba which has long been recognised 

 as a gymnosperm from the possession of naked ovules. However, its 

 dense wood and habit of bearing long and short shoots initially 

 prompted its inclusion in the conifers (Smith 1797) and the form of 

 the ovules with a fleshy coat and a collar-like structure at the base 

 which was reminiscent of an aril, led to its attribution to the 'Taxineae' 

 (Richard 1826: 135). The discovery by Hirase (1896) of ciliated 

 antherozoids, prompted Engler & Prantl (1897: 19) to remove G. 

 biloba from the Coniferales and to erect a new order, the Ginkgoales, 

 to accommodate both the living species and the Mesozoic leaf- 

 genera Ginkgoites, Baiera, Phoenicopsis and Czekanowskia. 

 Following the discovery of distinctly non-ginkgoalean ovuliferous 

 structures in Czekanowskia (Harris 1951), Pant (1959) established 

 the order Czekanowskiales to which Czekanowskia, Phoenicopsis 

 and others were assigned. Despite this, the Ginkgoales {s.s. ) and the 

 Czekanowskiales have continued to be treated, in the main, as 

 closely related orders. 



Most workers (Seward &Gowan 1900; Chamberlain 1935;Arnold 

 1947, 1948; Florin 1949, 1951; Meyen 1982; Stewart 1983) have 

 agreed that the Ginkgoales (sensu lato) evolved, if not from the 



31 



Cordaitales, then from the same ancestral stock. Opinion has varied 

 as to whether the ancestral stock was in the pteridosperms or the 

 group which gave rise to them. Chamberlain ( 1 935: 432) entertained 

 the possibility that the Ginkgoales and the Cordaitales had their 

 origins in the Pteridospermales. Palaeobotanical evidence presented 

 by Arnold ( 1 948) rendered the pteridosperm-origin of the Ginkgoales 

 unlikely, with pteridosperms evoked as the ancestors of the cycads 

 and bennettites, all being linked by the possession of manoxylic 

 wood, frond-like leaves and, as proposed later (e.g. Sporne 1965: 

 30), radially symmetrical (radiospermic) seeds. This was the core of 

 the cycadophyte line (Arnold 1948) which remained distinct from 

 the other main group of gymnosperms, the coniferophytes, as far 

 back as could be traced. The coniferophytes, consisting of the 

 cordaites, conifers, ginkgoes (and taxads), was characterised by 

 pycnoxylic wood, simple leaves and bilaterally symmetrical 

 (platyspermic) seeds, with origins unknown (Arnold 1948). This 

 fundamental concept of two main gymnosperm clades, the 

 Cycadopsida and the Coniferopsida required the assumption of a di- 

 (or even poly-) phyletic origin of seed plants, thus rendering the 

 gymnosperms as a group of plants with a common level of organisa- 

 tion (naked ovules) rather than the natural taxon Gymnospermae. 

 The discovery by Beck (1960) of the Devonian Progymno- 

 spermophyta, however, provided a possible common basal stock for 

 the cycadopsids and coniferopsids and led some (e.g. Beck 1976; 

 Rothwell 1981) to postulate a monophyletic origin of the gymno- 

 sperms. However, two groups have also been recognised in the 

 progymnosperms, the Archaeopteridales and the Aneurophytales. 

 The possibility that each led independently to the evolution of the 

 seed habit was supported by Beck (1981) who suggested that the 

 Archaeopteridales gave ri.se to the Coniferopsida including the 

 Ginkgoales and Czekanowskiales and the Aneurophytales to the 

 Cycadopsida. Stewart (1983, text-figs 23.1, 26.1) and Stewart & 

 Rothwell ( 1 993, charts 26. 1 , 29. 1 ) basically supported Beck ( 1 98 1 ) 

 in these derivations, but questioned the traditional assumption of 

 common ancestry for the Ginkgoales and Czekanowskiales. They 

 proposed the retention of the Ginkgoales (s.s.) in the Coniferopsida 

 with probable origins in the Cordaitales and the transfer of the 

 Czekanowskiales to the Cycadopsida, possibly having arisen from 

 the Glossopteridales. 



Recent progress in the field of molecular phylogenetic analyses, 

 as reviewed by Hasebe (1997), shows that all extant gymnosperms 

 constitute a monophyletic group, and that Ginkgo may well have 

 closer affinities to cycads than to conifers, thus challenging the view 

 of Ginkgoales as a member of the clade Coniferopsida. 



All the schemes of gymnosperm phylogeny mentioned hitherto 

 envisage a pre-Permian origin of the Ginkgoales {s.s.). either from 

 the cordaites in the Carboniferous or directly from the cordaitalean 

 stock, now considered to be the progymnosperms, in the Devonian. 

 However, since no clearly recognisable Ginkgoales are known before 

 the Triassic, this leaves a gap in the scheme which can only be filled 

 by vaguely Ginkgo-Wkt Permian leaf-genera of unsubstantiated 

 affinity. 



It is also possible to evoke an ancestor for the Czekanowskiales 

 from amongst these Permian genera when only the leaves are consid- 

 ered, although there is no evidence for short shoots such as are 

 characteristic of Czekanowskia. However, Leprostrobiis. the bivalved 

 ovuliferous structure of Czekanowskia. shows a remarkable likeness 

 to some pteridosperm fructifications and short shoots covered with 

 bud scales are also known in the pteridosperms (Meyen 1982; text- 

 fig. 2C, D). 



From the evidence available it appears that there is a rather more 

 remote relationship between the Czekanowskiales and Ginkgoales 

 than has traditionally been assumed. Nevertheless, it remains con- 



