| Taxodium ascendens Brongn.,belonging to Taxodiaceae,is the eminent tree specie to resist the floodwater,which possesses the characters of fast-growing and straight grain. Now it is the fast-growing specie which was introduced into the alluvial plain in the south of the Yangtze River.In the paper,using the cytological and anatomical methods, observations were made on the occurrence and development of vascular cambium in Taxodium ascendens.The anatomical properties of cambial cells and differentiation xylem cells or phloem cells were explored at different phases in this paper,and the histological relationships between cambial cells and xylem cells were discussed.Also seasonal changes in width of the functional phloem thickness and polysaccharide grains substances were observed.At the end,some wood properties were compared among three different environments,which provide theory foundation for the plantations oriented cultivation and regulation,also for the wood rational utilization.The three phases were found in development process of vegetative shoot apex:the extended swelling in apical meristem,the growth of the leaf primordium and obvious cytological zonation,the leaf primordium kept on differentiating into primary structure of the leaf.The apical meristem was a Cryptomeria -Abies type of organization.Four meristematic zones could be clearly distinguished,which were the apical initials,the peripheral meristem,the central mother cells and the pith meristem.The differentiation of the vascular tissue could not be observed before the leaf primordium.The cortex and pith cells were vacuolated first,that is,apical meristems translated into residual meristem,then into the procambial strand.The differentiation of the protoxylem was later than the protophloem.The transition from procambium to cambium was a gradual process in the stem.The development of primary vascular bundle had a close relationship with the leaf trace.At the same time,the procambium cells from the similar cells differentiating into the long or the short cells,then into the long fusiform initials and short ray cells.The cambium is non-storied and the cambial rays are uniseriate.In Wuhan,the buds in Taxodium ascendens germinate generally in the last ten-day of March.The beginning of the cambium activity in 2007 first was discovered in the middle of April.In last third of July,the activity had reached the fastigium.The cambium had fewer layers from the last ten-day of the July because of the greater speed in differentiation than the division. Until the beginning of October,only a few immature elements could be found,which indicated the translation from the activity to the dormancy.In the middle third of November,the cambium contained 3-4 cells in the radial rows,which made clear that the cambial came into the dormancy until next spring.Some approaches of quantitative anatomy,micrographic image analysis were applied to investigate the histological relationships between cambial cells and xylem cells. Significant differences(at 0.05 level) in tangential and radial width of cambial fusiform cells,lumen diameter,length-width ratio,double wall thickness-diameter ratio, diameter-width ratio,while no differences in ratio of tracheids and ray tissue were found among different stages;Significant differences(at 0.01 level) in tracheid length,width, double wall thickness,microfibrillar angle and cell wall percentage were also detected. Correlation analysis indicated that cambial cell numbers had positive correlation with tracheid lumen diameter(at 0.01 level),with tracheid width(at 0.05 level);The tangential width and radial width of cambial fusiform cell had positive correlations with tracheid length,width,double wall thickness,ratio oftracheids and ray tissue,cell wall percentage (at 0.01 level),with length-width ratio,double wall thickness-diameter ratio, diameter-width ratio,ratio of tracheids(at 0.05 level);Meanwhile,a positive correlation existed between radial width of cambial fusiform cell and tracheid lumen diameter(at 0.05 level).Some approaches of quantitative anatomy and micrographic image analysis were also applied to investigate the relationships between cambial cells and phloem cells.No significant differences in tangential width of phloem parenchyma cells,phloem fiber. Significant differences in tangential width of sieve cells(at 0.05 level),while significant differences in radial width of phloem sieve cells,parenchyma cells,phloem fiber(at 0.01 level).Correlation analysis indicated that cambial cell numbers had positive correlation with tangential width and radial width of sieve cells(at 0.05 level),with in radial width of phloem fiber(at 0.01 level);At the same time,a positive correlation existed between tangential width of FI and tangential width,radial width of sieve cells(at 0.05 level), while between radial width of FI and tangential width,radial width of sieve cells(at 0.01 level).Seasonal change in width of the functional phloem thickness varied with the cambium activity.In the early fall,when the cambium activity reached the fastigium,the width of the functional phloem thickness also had the maximum value.Significant differences(at 0.01 level) in width of the functional phloem thickness among diferent stages were found.A strong correlation existed between dynamic changes of cell polysaccharide grains substances and cambium activity.The four phases were obviously distinguished in dynamic changes of cell polysaccharide grains substances of secondary vascular tissues: polysaccharide grains disappeared gradually in the growing seasons,accumulated gradually at later stages,vanished again in the dormancy stage,and then piled up until the next spring in the quiescence.Multiple comparisons indicated significant differences(at 0.05 level) in basic density between drought and wetness environments were not found.There were no differences among five trees in the same environment,while significant differences in three different ones(except between trees 5 and 15,trees 10 and 2).There were some differences among tracheid morphologies among three different environments.Multiple comparisons indicated there were no differences between wetness environment and drought environment in the tracheid length and double wall thickness of early wood,weighted value,while some differences had found in double wall thickness of late wood;There were significant differences in the tracheid length of late wood among three different environments;There were significant differences in tracheid width(at 0.05 level);In the length-width ratio of early wood,there were differences between wetness environment and hydrophily environment,there were significant differences in late wood between drought environment and hydrophily one,in which no significant differences were detected;No significant differences in diameter-width ratio,double wall thickness-diameter ratio of early wood,weighted value were observed among three different environments;In late wood no significant differences were detected between drought environment and wetness environment. |
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