THE ULTRASTRUCTURE OF SPORES OF COOKSONIA-PERTONI

The ultrastructure of spores isolated from sporangia of Cooksonia pert oni from Pridoli and Lochkovian rocks of the Welsh Borderland has been elucidated using both scanning and transmission electron microscopy. The Silurian material, attributable to C. pertoni subsp. synorispora, contains Synorisporites verrucatus while the Devonian C. pertoni subsp . apiculispora yields either Streelispora newportensis or Aneurospora sp. All three spore taxa show an exospore (= exine) composed of two la yers, both of which extend into the murornate and verrucate sculpture in Synorisporites verrucatus, into the papillae and proximal folds of Streelispora newportensis, and into apertural folds (where observed) i n all three taxa. In contrast, only the outer layer occurs in the apic ulate sculpture of the Devonian spores. The homology of the layers is briefly discussed in relation to extant vascular cryptogams. Such ultr astructural observations compare favourably with published description s of the same taxa recorded from dispersed assemblages using light mic roscopy. Similar ultrastructure has already been recorded in cf. Ambit isporites isolated from Pridoli Cooksonia pertoni from Long Mountain, Shropshire, thus providing support for the hypothesis that, while the gross morphology and spore structure of C. pertoni remained unchanged from the Silurian into the Devonian, spore sculpture evolved from smoo th to verrucate to apiculate.