带状叶属一新种——Desmiophyllum speciosum sp. nov.

A NEW SPECIES OF DESMIOPHYLLUM

前言Desmiophyllum一名系Lesquereux(1878)根据美国石炭纪一种与狭型科达树(Poacordaites)相类似的叶部化石创立的,模式种D. gracile的叶是单个或成组地接触在一保存不完全的轴上,被认为是一个不清楚的种,可能与Cordaites同物异名,因而Desmiophyllum长期未被采用,后来Solm-Laubach将这个属名的含义加以厘定,用于代表中生代叶枝形态不明、单独保存的狭形叶但不能归入已知属的叶部化石。此后,这个属名方被古植物学者广泛采用。从国内外定为Desmiophyllum的各个种的叶角质层构造来看,内容复杂,特征各异。

This paper suggests that Desmiophyllum Lesquereux, a non-committal genus, should represent the detached leaves of Mesozoic gymnosperms, as their structures are not so well-known and their affinities are uncertain. It is more appropriate to use the form genus Harrisiophyllum for the fossil deciduous strap-shaped leaves or leaflets with coniferous features, represented by H. indicum (Sahni) P. P. Pant, Srivastava, R. Pant, etc., the cuticle of which has been studied in greater detail.The present paper gives a description of Desmiophyllum speciosum sp. nov. and its comparison with other similar genera and species. The specimen of the present species was collected from the Dameigou Formation (Middle Jurassic), Dameigou area, Qinghai.Description and comparisonDiagnosis: Leaf linear strap shaped, exceeding 13 cm in length, tapering off to a .point near apex, typically 3—4 mm in middle part; lower part gradually tapering to under 1 mm in width and extending into petiole-like base. Veins obscure, but sometimes vaguely seen, parallel, probably 4—8 in number.Cuticle very delicate, hypostomatic; upper cuticle about 5 μm in thickness, with isodiametric, rectangular or slightly elongated polygonal cells, usually not arranged in longitudinal rows, except occasionally for narrow strips of one or two rows of cells. Lateral and end walls fairly conspicuous, often irregular in width (1.2—2.5 μm), slightly inuous; cell surface smooth or finely mottled, sometimes marked with longitudinal striae. Margins of lamina about 500 μm in width, with nearly rectangular or elongated polygonal cells arranged in longitudinal rows. Stomata absent or poorly developed. Papillae and trichomes absent.Lower cuticle consisting of both upper and lower layers, with elongated polygonal or quadrate cells arranged in longitudinal rows. Stomatal straps about 103.5—480 μm wide; nonstomatal strips about 61.5—283 μm wide, but sometimes indistinguishable or discontinuous. Stomata generally arranged in rows with various lengths or scattered, closed or separated by 1—6 ordinary epidermal cells, hypocheilic or partly amphicyclic, and mostiy elliptic, 61.5—82.3 gin×49.2—67.6 μm in size, usually orientated longitudinally but with a few placed obliquely. Guard cells sunken, with sharpened ends; inner margin thickened. Subsidiary cells 4—7, mostly 5—6 in number, more strongly cutinized than those common cells, with surface bearing conspicuous dark papilla and projecting upwards or over stomatal pit. Upper layer with much more obvious papilla, rounded or elongated, 2.3 μm in diameter or 12.3—18.48 μ3×7.38—12.3 μm in size. Lower layer rather thin; cells outline clearly marked, with small papillae either present or absent, sometimes rare or obscure.Remarks: D. speciosum sp. nov. is well distinguished from all species with cuticular structure of Desmiophyllum by its strongly papillate epidermal and subsidiary cells on the lower side. In the external pattern, the present species is more or less similar to Phoenicopsis, Pseudotorellia and Vittifoliolum, but can be all distinguished by their cuticles. Ertmophyllum pubescens Thomas comes very close to the present species in having equally thick cuticles, similar shape of epidermal ceils and conspicuous papillae, but differs in the shape of leaves and the detailed structure of stomatal apparatus. In general cuticular structures, Sphenobaiera spectabilis (Nath.) and Baiera boeggildiana Harris also can be compared with our species, but they are quite different in the shape of the bifurcating leaves.The new species is temporarily included in Desmiophyltum because its apices, base and veins of leave are not fairly clear, but the writers consider it possible to erect a new genus for the fossil leaves with ginkgoalean affinity.