Morphological Development And Diversity Of Stamens Of Five Species Of Clematis In The Ranunculaceae Family

As the male reproductive organ of angiosperms,androecium play an important role in plant reproduction.The stamens of angiosperms are diverse in number,color and structure.The morphological and structural changes of stamens show important evolutionary significance for improving pollination efficiency.In Clematis(Ranunculaceae),androecium is numerous and some groups have different numbers and types of staminodes.There are nectaries in the filaments of stamens.In this study,the stamen morphology,ontogeny,anther and pollen structure,epidermis micromorphology and filament nectary structure of five species in Clematis:C.macropetala,C.rehderianawere,C.tangutica,C.glauca,C.lasiandra studied by scanning electron microscope,light microscope and transmission electron microscope.The results indicate that:(1)stamen morphology and type:among the five species,C.macropetala has staminodes,and stamens can be divided into four types according to shape and anther size:petaloid staminode(St1),spatulate staminode(St2),spatulate fertile stamen(St3)and linear fertile stamen(St4).(2)Stamen development:in five Clematis species,the morphological development of normal fertile stamens includes five stages:hemispherical stamen primordia initiation,primordium enlargement,the top of primordium expanded and differentiated the filaments and anthers,stamen elongation,at maturity.In C.macropetala,the characteristics of stamens and staminodes development are similar in the early stage but gradually different in the later stage.Petaloid staminode(St1)had delayed development and no anther differentiation.Spatulate staminode(St2)degenerated at early stage of anther differentiation.St3 and St4 are fertile,but their size of anther are different.The most outer fertile stamens of C.rehderiana also experienced delayed development.The pollen sacs of 5 species all split longitudinally at maturity,and the anther wall was composed of a layer of epidermis and a layer of fibrous endothecium or part of the middle layer remains.The mature pollen grains are spheroidal,long spherical and oblate spherical with three germination furrows and granular ornamentation on the exine,and are all 2-celled with a large vegetative cell and a generative cell.(3)Stamen epidermis micromorphology:14 epidermal cell types were observed in stamens of Clematis,including 3 secretory epidermal cells and 11 non-secretory cells.The epidermal cells of filament/petal are similar,the cells in the secretory area of the ventral surface of the filaments are all fusiform,the cells at the base of filaments are all rectangular,the anther wall cells of fertile stamens are long or quadrilateral,the connective epidermal cells are nearly round papillary or quadrilateral,and the anther wall cells of rudimentary anther are polygonal.Petaloid staminode(Sti)of C.macropetala degenerated into a simple flat structure with simplified cell types,only has 4 types and no anther epidermal cells.5-7 types exist in spatulate staminode(St2),spatulate fertile stamen(St3),linear fertile stamen(St4)and C.rehderiana,C.tangutica,C.glauca,C.lasiandra.(4)Filament nectary structure:C.rehderiana,C.tangutica,C.glauca,C.lasiandra have nectary near the base of filaments.In C.macropetala,except for the outermost petaloid staminode(St1),nectary was found at the near base of filaments in spatulate staminode(St2),spatulate fertile stamen(St3)and linear fertile stamen(St4).The secretory epidermal cells of C.macropetala and C.lasiandra are fusiform with curved furrows or globular protuberances on the surface.The nectary is composed of only one layer of secretory epidermis which has a large nucleus,dense cytoplasm,and well-developed wall-ingrowth.Nectar is released through micro-channels in the cuticle of the outer wall.The staminodes and fertile stamens in Clematis have similar developmental characteristics in early stage and different morphological characteristics in late stage.The staminal nectary located on the base of filament of fertile stamens or staminodes.The position,structure and micromorphology of staminal nectary in Ranunculaceae show obvious diversity,which provide evidence for further study on the pattern of stamen differentiation and its possible new functions in Ranunculaceae.