The Structure, Development And Evolutionary Analysis Of The Seedlings Of Arabidopsis Thaliana And The Other Two Angiosperms

Seedling is an important stage in the individual development of plant. There are a lot of detailed reports for the morphology and classification of seedling, but few for its structure, especially for the initiation, differentiation and connection of primary vascular system, and the occurrence and formation of the organs. Therefore three typical plants including Arabidopsis ihaliana, Platycodon grandiflorus and Ligusticum jeholense were chosen to study the spatial and temporal characterictic in the primary vascular system. What's more, the developmental mode of shoot apical meristem(SAM) in Platycodon was observed. The main conclusions were the following.1. The spatial and temporal characteristics of vessel elements in the axial organs of Arabidopsis, Platycodon and Ligusticum seedlings were that vessel elements first differentiated in the lower part of cotyledon node zone (CNZ), then extended downwardly to hypocotyl and root and upwardly to the middle and upper part of CNZ, and lastly finished the connection with the ones in epicotyl-shoot zone.2. The initiating sites were first disclosed in Arabidopsis and Platycodon. Vessel elements were initiated firstly in the lower part of CNZ in Arabidopsis, then in the lower part of cotyledon; while in cotyledon in Platycodon. The difference of vessel initiating sites between the two plants was perhaps due to the difference in the concentration and flow of auxin.3. The connecting of CNZ and epicotyl-shoot zone in the three seedlings occurred in two ways. One was was by de- and re- differentiation of parenchyous cells of the middle and upper part of CNZ in Arabidopsis and Platycodon, the other was by the differentiation of meristematic tissues in the middle and upper parts of CNZ in Ligusticum.4. The haplostele styles in the lower part of CNZ of three objective plants were found, namely , it was diarch-mesarch, singular and round in Arabidopsis; diarch-exarch and line-shaped in Platycodon, and diarch-mesarch and concavo-concave in Ligusticum.5. The vein styles in cotyledon indicated that Arabidopsis's was simple pinate-margin vein, and Ligusticum jeholense's was ternate-forked.6. The dynamic development of SAM in Platycodon is described. At the early stage, SAM had a few cells, and no subarea could be seen. From the appearance of the first and the seventh leaf , SAM was cup-shaped, and the base of SAM was divided into specific compartments. These compartments had the same features as found in cryptomeria-spruce type of SAM of gymnosperm, whose central cell zone has fewer and slowly divided cells. Therefore the zone supplied cells slowly for the periphery meristem and ribbed meristem, and, too, the leaf 1 and the stem grew slowly later. At the appearance of the eighth leaf primordia, SAM formed a bulge and had more cells.The results not only showed the variety of the structures of seedlings in their individual development, but also provided the new evidence for evolutionary analysis. At the same time it can assist the studying of other seedlings when using physiological, biochemical and molecular methods.