Vegetative Characters, Growth Habit and Microsporangiate Strobilus of Lycopsid Minostrobus chaohuensis
Late Devonian Minostrobus chaohuensis is one of the earliest monosporangiate-strobilate isoetaleans. Based on new material of this plant, the vegetative axis and microsporangiate strobilus are studied in detail, and the whole plant knowledge is summarized. The vegetative axis is isotomously branched. The stem is up to 55 mm in diameter with helically arranged leaf cushions. Stems and thick branches bear long fusiform leaf cushions and interareas with vertical linear ornamentations. A ligule pit, oblanceolate leaf scar, and vascular bundle scar appear on the leaf cushion. Distal axes have persistent lanceolate leaves and rhombic leaf bases. The microsporangiate strobilus is cylindrical in shape, possesses sporophyll with alate pedicel and long triangular lamina, uniseriate sporangial wall, subarchesporial pad inside the sporangium, and microspore with cingulum. Based on comparisons with other isoetaleans, the usage of the terms “leaf cushion” and “leaf base” is discussed, and Minostrobus chaohuensis is considered as a tree-like lycopsid. It suggests that arborescent isoetaleans with monosporangiate strobili had appeared and diversified in the Late Devonian. The multi-dichotomous branching system of Minostrobus provides new data on the evolution of growth architecture in rhizomorphic lycopsids.
Growth architecture and microsporangiate strobilus of Sublepidodendron grabaui (Lycopsida) from the Late Devonian of South China.
Abstract The arborescent lycopsid Sublepidodendron grabaui was widely distributed in the Late Devonian of South China. Its morphology and anatomy have been described, but the microsporangiate strobilus remains unclear. New material collected from the Guanshan Member of the Wutong Formation in Zhejiang Province allowed us to provide more details of this plant. The pseudomonopodial axis without secondary xylem bears medullated stele and branch traces arranged in irregular helices. The lateral branch possesses solid exarch primary xylem. This pseudomonopodial branching system possibly represents the crown or cauline lateral branch. The microsporangiate strobilus contains microspores belonging to the Lycospora granulata Group. The monosporangiate nature of S. grabaui is confirmed, and the classification of Sublepidodendraceae into the suborder Dichostrobiles is supported. The L. granulata Group microspores are suggested to occur not only in bisporangiate strobili but also in monosporangiate strobili.
Stem Growth and Epicormic Branch Formation in Understorey Beech Trees (Fagus sylvatica L.)
Abstract A study of the formation of epicormic branches on suppressed beech trees revealed developmental sequences that had not hitherto been studied in depth. By combining architectural analysis and anatomy, it was possible to trace the history of the plant and thus fully understand the ontogenesis of the individuals studied. The main sequences observed were: (1) the formation of moribund trees that were gradually covered in epicormic branches from the base of the stem to the crown following gradual forest closure; (2) the formation of this new crown of epicormic branches was combined with a very sharp reduction in stem cambial activity (partial rings, if any) and modifications to the ligneous sections; and (3) resumed girth increment following the establishment of epicormic branches. After describing in detail the methods and individual variation, we highlight the spatial and temporal patterns of the phenomena observed. Lastly, we put forward functional hypotheses with a view to setting these results against a background of forest tree ontogenesis in general.