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rows. During this period, there were 0 to 2 immature xylem cells. These results are summarized in Table 1. 0 2 4 6 8 10 12 April May June July August September October Sampling time Raidal diameter/μm first stage second stage third stage forth stage *A month was divided into four periods, the first stage was 7th of each month, the second one was 15th of each 110 month, the third one was 22nd of each month, and the fourth one was 30th of each month. The following figures are the same. Fig. 2 The changes of radial diameter of cambium cells 0 5 10 15 20 25 April May June July August September October Sampling time Tangential diameter/μm first stage second stage third stage forth stage Fig. 3 The changes of tangential diameter of cambium cells 115 The radial and tangential diameters of cambial cells in different stages during the active phase were measured and analyzed. The results of mean and standard deviation are shown in Figs. 2 and 3. According to the figures, the radial diameter of cambial cells reached the maximum of 8.68 μm on April 15th. The cambial cell division was in the active period in April, during the time that the radial extension of cambial cells was more remarkable. It decreased 120 gradually from May and then remained stable. The tangential diameter of cambial cells achieved the maximum of 19.8 μm on April 22nd. The tangential diameter was greater in the period from April to May. It tended to decrease in June, and then was kept in a steady state. Changes of radial diameter and tangential diameter were basically the same. 125 2.2 Seasonal formation of the secondary xylem Xylem cells were produced in mid-April, but not until April 21st, one week after the growth of leaves, only 2 layers immature xylem cells appeared (including the expanding vessel elements and wood fibers). Larger vessel elements and thin-walled wood fibers (early wood) were formed during the cambium’s higher activity (May 7th) (Fig. 1, b). Small differentiated vessel elements 130 and thick-walled and flattened fibers (latewood) were visible adjacent to the cambium when its activity was at its lowest (October 7th and October 15th) (Figs. 1, e and f). The seasonal changes of the xylem cells during the active phase can be seen in Figs. 4 to 7. The layers of immature xylem cells (vessel elements and wood fibers) increased rapidly in May, and reached their maximum (about 20 layers) in mid-June, followed by a decline (Figs. 4 and 6). 135 The cell wall of differentiated immature xylem cells in mid-August was thicker than that of the previous months. The immature xylem cells were not been found on September 30th (Figs. 4 and 6), indicating that the cambium has stopped differentiating the xylem cells. In late-May (Figs. 5 and 7), it began to show the mature xylem cells which had multi-level thickened secondary wall, therefore, it can be speculated that the maturity of xylem cells need 30–40 days to complete. From 140 May to September, the total number (about 170-180 layers) of xylem cells increased significantly and it was maintained until September 30th (Figs 5 and 7), until the leaves started to turn yellow (Table 1). 0 5 10 15 20 25 May June July August September Sampling time Immature wood fiber number/N first stage second stage third stage forth stage Fig. 4 The changes of immature wood fiber number 0 20 40 60 80 100 120 140 160 May June July August September October Sampling time Mature wood fiber number/N first stage second stage third stage forth stage Fig. 5 The changes of mature wood fiber number 0 1 2 3 4 5 6 May June July August September Sampling time Immature vessel element number/N first stage second stage third stage forth stage Fig. 6 The changes of immature 150 vessel element number 0 5 10 15 20 25 30 35 40 45 May June July August September October Sampling time Mature vessel element number/N first stage second stage third stage forth stage Fig. 7 The changes of mature vessel element number 2.3 Correlation between cambium and secondary xylem 155 Linear relationship analysis was made between the cambial characteristics and the xylem anatomical features. The measured data was taken in comparison. The most significant relations are highlighted in Table 2. According to the Pearson’s analysis method, the coefficient of correlation between the cambium cell number and the number of immature wood fiber and immature vessel element were 0.321 and 0.233, respectively. The results showed that the cambium cell number had a highly significant positive correlation with the number 160 of immature wood fiber and immature vessel element. It also can be seen from the results that the number of mature vessel element had an extremely significant negative correlation with the radial diameter of the cambium cell and the tangential diameter of the cambium cell, and the correlation coefficients were -0.121 and -0.192, respectively. The number of mature wood fibers had a significant negative correlation 165 with the tangential diameter of the cambium cells and the coefficient was -0.098. There was a significant negative correlation between the cambium cell number and the number of mature vessel element, and the coefficient was -0.181. Tab. 2 Linear Correlation Analysis between Cambium and Xylem Number of immature wood fiber Number of mature wood fiber Number of immature vessel element Number of mature vessel element Cambium cell number 0.321** －0.054 0.233** －0.181* Cambium cell R-diameter －0.080 －0.063 －0.046 －0.121** Cambium cell T-diameter －0.042 －0.098* －0.008 －0.192** Cambium cell number 0.321** －0.054 0.233** －0.181* 170 ** Correlation is significant at the 0.01 levels (2-tailed); * correlation is significant at the 0.05 levels (2-tailed). The periodicity of cambium activity of deciduous trees in the temperate region was usually apparent. A complete cycle of the seasonal changes was studied in the ultrastructure of the vascular cambium of Populus×euramericana cv. ‘74/76’. The cambial cells reactivated before bud 175 swelling. However, initiation of cambial activity with cell proliferation, and an increase of immature xylem cells, took place about a week later than bud sprouting in 2010. The cambial cells shape, division, and arrangement in trees are commonly considered to be indirectly controlled by indole acetic acid (IAA) streams produced from buds and new leaves [25-27]. The results in the present study are coincident with this assertion. Furthermore, the phenomenon of new xylem 180 production after new leaves appear evidently supported the assumption that growth substances from growing leaves are necessary for continuous normal formation of xylem. It was also suggested that the differentiation of xylem tracheary elements was induced by the polar transport of auxin streams from developing leaves [27, 28]. The TEM observations showed clearly that cambial cells division and the formation of new 185 phloem cells started before the production of xylem [29]. More details about the changes of cambial cells at the onset of cambial activity have been reported [29-31]. It has been shown [32] that when the fusiform initial cells and their derived cells grow to be the tubular cells, they would be necessarily subject to certain factors. The environment near or within a cell determines its differentiation and development, and this internal environment is mainly composed of plant hormones with different 190 capacity and proportion [33]. In addition, seasonal external factors such as sunlight, moisture, nutrients, and temperature will be significantly modified the xylem phenotype [34]. Therefore, the morphology of xylem cell is a unified reflection of its own genetic characteristics and variable regularity influenced by the environment. 3 Conclusions 195 In this paper the cambium presented a seasonal cyclical pattern of activity and dormancy. The active phase of Populus×euramericana cv. ‘74/76’ was from early April to late September in 2010. The highest cambial activity was observed on April 30th to September 22nd, when the trees had mature leaves. The beginning of the reduction of cambial activity to a minimum was on September 30th, when the trees underwent partial leaf fall. From May to September, the total number (about 170 to 180 layers) 200 of xylem cells increased significantly, and it was maintained until September 30th, when the leaves started to turn yellow. The accumulation of xylem cells was closely related to the features of cambium, in particular to the layers of cambium. Acknowledgements 205 The authors are grateful for the support of the Research Fund for the Doctoral Program of Higher Education of China, Grant. No. 200800220006. References [1] CHEN H M, HAN J J, CUI J M, et al. Modification of cambial cell wall architecture during cambium 210 periodicity in Populus tomentosa Carr[J]. Trees, 2010, 24: 533-540. [2] BARNETT J R. Seasonal variation in the ultrastructure of the cambium in New Zealand grown Pinus radiata D[J]. Don. Ann Bot., 1973, 37: 1005-1115. [3] LARSON P R. The Vascular Cambium[M]. Berlin: Springer-Verlag, 1994. [4] RAO K S, RAJPU K S. Cambial activity and development of wood in Acacia nilotica (L.) Del. growing in 215 different forest of Gujarat State [J]. Flora, 2000, 165-171. [5] CATESSON A M. Cambial ultrastructure and biochemistry: Changes in relation to vascular tissue differentiation and the seasonal cycle[J]. Int. J Plant Sci.,1994, 155: 251-261. [6] IQBAL M. The cambial derivatives[M]. Berlin: Gebruder Borntraeger,1995. [7] FARRAR J J, EVERT R F. Seasonal changes in the ultrastructure of the vascular cambium of Robinia 220 pseudoacacia [J]. Trees, 1997, 11: 191-202. [8] RENSING K H, SAMUELS A L. Cellular changes associated with rest and quiescence in winter-dormant vascular cambium of Pinus contorta [J]. Trees, 2004, 18: 373-380. [9] JACOBY G C. 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