Histological Study On Vegetative Organs Of Thalassia Hemprichii And Cymodocea Rotundata

It is the first time in China that the vegetative organs (roots, stems, and leaves) of Thalassia hemprichii and Cymodocea rotundata have been studied in histology. By means of optical microscope and scanning electron microscopy observation systems, the microstructure of seagrass vegetative organs and the ultra-structure of the leaves surface were observed systematically, and the morphological and anatomical features of the two kinds of seagrasses were described in detail, the differences between the two seagrass species and the terrestrial monocotyledons and the differences between the two seagrass species themselves were analyzed in details. Furthermore, the thesis summarized seagrass's structure characteristics of its adaptation to submerged life in seawater, provided theoretical basis for the biology, ecology and physiology study of seagrasses. The results indicated as follows:From the transverse section of the two seagrasses'roots, they presented a suborbicular surface, and can be divided into three parts called epidermis, cortex and vascular from outside to inside. Epidermal cells become glabrescent gradually with the root hair's death. The cortex has developed aerenchyma and it account for a large area of roots. The ratio between the radius of Thalassia hemprichii's stele and cortex is about 1:40 and the ratio of Cymodocea rotundata's is about 1:27. It is clear that the steles of both seagrasses are highly degraded. Endodermis have Casparian strip or six-thickening structure.From the transverse section of the two seagrasses' stem, they presented a suborbicular surface and can be divided into the epidermis, basic organization and vascular tissue. The epidermis cells were covered with cutin, had not been seen stomatal apparatus. It has developed basic organization, especially its ventilating tissues witch account for about half of the basic organization. One difference between the two species of seagrasses was the arrangement of vascular bundles. There was only one beam of central vascular bundle and several small vascular bundles around it in the stem of the Thalassia hemprichii, and there was one beam of central vascular bundle and some outer vascular bundles in the stem of the Cymodocea rotundata, its central vascular bundle is large and in the central parts of the stem, but the outer vascular bundles were in the outer surrounding of the basic organization and distributing in about two rounds. From the transverse sections of the two seagrasses'leaves, they can be divided into three parts:the epidermis, the mesophylls and the veins (vascular bundles).Their upper and lower epidermis cells are both made up of the uniseriate epidermis cells, marched in rows along the long axis. The epidermis cells were in the column regular shape with no intercellular spaces. Their outer walls were thickened and the outside of the outer walls was covered with continuous cutin. The chloroplasts in the epidermal cells were numerous and extensive and the epidermal cells are the main place of photosynthesis. There was no stoma in epidermis and no differentiation of palisade and spongy structures in the mesophyll cells. But its aerenchyma is developed, and has special structure to connect and support the pneumatophore. The structure is composed of many parenchyma cells, witch has some chloroplasts in it,has large intercellular spaces, thus the whole structure is good for gas storage and proper exchange.From the leaf's water slices and stereoscanphotographs, we find that the surface of Thalassia hemprichii's leaves have no special structure while Cymodocea rotundata's leaves have something special named bulliform-like cells with no regular arrangement and have big vacuoles in these cells. At the same time, we found that there are also some bulliform-like cells in both upper and lower epidermis cells from the cross-sections photos, which will be stained red when stained by safranine. Their cellwalls were thinner than epidermis'. Therefore, we can suppose this structure was related to the exchange with outside and Cymodocea rotundata's salt-secreting function.To sum up, this study suggests that Thalassia hemprichii's and Cymodocea rotundata's main structure characteristics of its adaptation to submerged life in seawater include:(1) The vegetative organs have developed ventilating tissues. (2) The root's endodermis has Casparian strip or six-thickening structure. (3) The roots, stems and leaves have less vascular tissue and mechanical tissue. (4) Leaf epidermis cells have numerous and extensive chloroplasts. (5) Stems and leaves have no stomatal apparatus. (6) Cymodocea rotundata have some bulliform-like cells in the leaves' surface.