Morphological Study Of The Petals Of Lardizabalaceae

Angiosperms are the closest group of plants in the evolutionary process.Compared with other groups,the most striking feature of them is the reproductive organs-flowers.As the latest round of floral organs,Petals are believed to have evolved from other floral organs.Petals originate from bracts and stamens,and thus the origin and evolution of petals become a key issue in exploring the origin of angiosperms.In terms of morphology structure,location of nectaries and nectar production,petals shapes show variety in the long process of evolution,especailly in the Ranunculales core groups.The morphology structure of these groups are rich and varied,with the secretion functioncalled "honey leaves".Therefore,Ranunculales becomes an ideal material for studying the origin of petals.Lardizabalaceae is one of the core families in Ranunculales whose flowers are mostly unisexual,dioecious or monoecious and hybrid.Perlianth is consist of calyx and corolla.The petals are usually small,greenish,spathulate,bilobed.In this study,weused light microscopy,scanning electron microscopy and transmission electron microscopy to investigate petal development,structure and ultra-structure of mature petals,morphology of staminodes in 3 genera of Lardizabalaceae.The purpose of this paper is to observe the morphological and structural characteristics of petals by means of morphological methods,and can provide evidence for the origin and evolution of petals in Ranunculales.The main result are as follows:Mature petals are diverse:mature petal of Lardizabalaceae was flat and the petals of Sinofranchetia were inverted trapezoid and bilobed apex in the early development.The mature petals were spathulate,bilobed apex and a stalk in the base.The petals of Sargentodoxa were pointed apex in the early development whereas mature petals were obovate or diamond,a stalk in the base.The petals of Holboellia were oval while mature petals were ovate,and the stalk of petals were not obvious.Petals experienced four key stages in the development process.(?)Stage 1,Petal initiation and the primordium is rectangle in shape;(?)Stage 2,temporarily retarded development.The petal stops developing when carpels initiated;(?)Stage 3,petal enlargement and differentiation;(iv)Stage 4,mature petal during blooming.The morphological differences of mature petals are mainly derived from the differentiation of the period of development(Stage 3).Petals in Siofranchelia and Sargentodoxa have stalks in the base whereas the petals in Holboellia without obvious stalk.Petal structure and epidermal cells show diversity:petal consists of epidermal layers,parenchyma cells and vascular bundle.There are five to nine layers parenchyma cells in nectary tissue.Parenchyma volume larger,arranged loose,have a larger cell intercellular space.Petals have a vascular bundle.Epidermal cells include secretory and non-secreting cells.There are 3 types secreting cells according to cell shape.Type 1:irregular polygonal cells in Siofranchetia and Sargentodoxa.Type 4:cylindrical cells in Holboellia.Type 5:papillary cells in Siofranchetia,There are 5 types non-secreting cells.Type 2:irregular polygonal cells in Siofranchetia and Sargentodoxa.Type 3:long rectangular cells in base of petal.Type 6:polygonal papillary cells in Siofranchetia.Type 7:Irregular cells in Holboellia.Type 8:polygonal ridge cells in Holboellia.Secretory tissue present on the top of petal which consists of epidermis,parenchyma cells,and vascular bundle in Siofranchetia or epidermis,parenchyma cells in Sargentodoxa and Holboellia secretory epidermis is small with thicker cuticle,the nuclei is bigger and numerous micro-channels occur on the outer wall.There are abundant plastids,mitochondria and plasmodesmata in the parenchyma cells.A few modified stomata in abaxial surface in Sargentodoxa.The nectry released to the outside through the cell wall rupture in papillate cells in Siofranchetia.Staminode:the anther wall consists of 5-6 layers of cells.Staminode is petaloid,only in Sargentodoxa.The epidermal cells of anthers are secetory epidermis.Maturation of the pollen capsule is flat and not cracked.Staminodes are tiny in Sinofranchetia and Holboellia.Filaments are unobvious or shorter in Holboellia.Pollen sac is long oval.Anthers disdehiscence during bloom.Filaments are obvious or longer in Sinofranchetia,and ther are few dehiscent pollen sac.In the process of development,petals and stamen occur continuously.The petals primordia are crescent-shaped,the stamen primordia is hemispherical.When the stamen primordia appears,the petals have the delayed developmental phenominenon.The number of vascular bundle in the petal is 1.Sterile stamens have secreting function.On the other hand,petals and sterile stamens are similiar in morphology and secretion area.