全文获取类型
收费全文 | 613篇 |
免费 | 24篇 |
国内免费 | 6篇 |
专业分类
643篇 |
出版年
2023年 | 12篇 |
2022年 | 5篇 |
2021年 | 11篇 |
2020年 | 11篇 |
2019年 | 16篇 |
2018年 | 9篇 |
2017年 | 12篇 |
2016年 | 3篇 |
2015年 | 9篇 |
2014年 | 23篇 |
2013年 | 22篇 |
2012年 | 12篇 |
2011年 | 13篇 |
2010年 | 10篇 |
2009年 | 25篇 |
2008年 | 23篇 |
2007年 | 29篇 |
2006年 | 24篇 |
2005年 | 20篇 |
2004年 | 22篇 |
2003年 | 18篇 |
2002年 | 13篇 |
2001年 | 9篇 |
2000年 | 14篇 |
1999年 | 20篇 |
1998年 | 8篇 |
1997年 | 18篇 |
1996年 | 14篇 |
1995年 | 17篇 |
1994年 | 14篇 |
1993年 | 25篇 |
1992年 | 15篇 |
1991年 | 15篇 |
1990年 | 15篇 |
1989年 | 9篇 |
1988年 | 8篇 |
1987年 | 10篇 |
1986年 | 16篇 |
1985年 | 8篇 |
1984年 | 11篇 |
1983年 | 3篇 |
1982年 | 5篇 |
1981年 | 11篇 |
1980年 | 3篇 |
1979年 | 6篇 |
1978年 | 4篇 |
1977年 | 5篇 |
1973年 | 3篇 |
1972年 | 5篇 |
1971年 | 5篇 |
排序方式: 共有643条查询结果,搜索用时 15 毫秒
131.
A. Mialová J. urkovi M. Mamoová T. Priwitzer A. Lengyelová D. Hladká & A. Lux 《Plant biology (Stuttgart, Germany)》2009,11(5):686-693
Changes in anatomical organisation of the leaf, photosynthetic performance and wood formation were examined to evaluate the temporal and spatial patterns of acclimatisation of micropropagated slow-growing black mulberry ( Morus nigra L.) plantlets to the ex vitro environment. Leaf structure differentiation, the rates of net photosynthesis (Pn ), transpiration (E) and stomatal conductance (gs ), and secondary xylem growth were determined in the course of a 56-day acclimatisation. Differentiation of palisade parenchyma was observed 7 days after transfer. At this stage, the rates of Pn , E and gs reached maximum values, after which the rates of all three gas exchange parameters gradually decreased. The highest proportion of woody area occupied by vessels was also observed 7 days after transfer. An important feature of developing woody tissue is the difference in patterns of vessel distribution from the characteristic differentiation patterns of earlywood and latewood vessels in mature wood of ring-porous trees. Vessels with lumen areas over 3000 μm2 were only differentiated in acclimatised plantlets, whereas vessels in stems sampled on days 0 and 7 had very small lumen areas of up to 560 μm2 . Full acclimatisation, observed 56 days after transfer to the ex vitro environment, was associated with the rapid growth of new in vivo formed leaves, very low rates of E and gs , and much increased secondary xylem tissue within the stem area. 相似文献
132.
Anatomical diversity of Indian rattan palms (Calamoideae) in relation to biogeography and systematics 总被引:2,自引:0,他引:2
ANCY MATHEW K. M. BHAT 《Botanical journal of the Linnean Society. Linnean Society of London》1997,125(1):71-86
Of the 13 genera and 600 species of the subdivision Calamoideae, only four genera– Calamus, Daemonorops, Korthalsia , and Plectocomia –represent the Indian rattans which are found in three major regions: Western Ghats of Peninsular India, Andaman and Nicobar islands and north and north-eastern India. Detailed anatomical survey of 42 species shows considerable differences among the four genera. The vascular bundle in Calamus, Daemonorops and Korthalsia is characterized by a solitary metaxylem vessel and two phloem fields, while Plectocomia shows 1–2 metaxylem vessels and a single phloem field. The mechanical tissues show diversity in Korthalsia and Plectocomia with sclereids as a yellow cap on the outer side of the fibrous sheaths of vascular bundles. The size of the different cells, the diameter of the metaxylem vessel in particular, appears to be related to species habit, geography and stem size. The Andaman and Nicobar islands with equable temperature and high humidity provide the best environment for cane growth–the widest vessels are in canes from this region. While altitudinal influence on vessel diameter appears to be relatively small, the higher latitude is associated with narrow and short vessel elements. With the exception of C. erectus , an erect species with the thickest stem, vessel diameter shows positive correlation with stem diameter. Vessel perforations are simple or rarely scalariform. Climbing palms which grow to enormous heights generally have wider vessels with simple perforations, an adaptation for conductive efficiency. Based on these results, the implications of stem anatomy for rattan biogeography, systematics and identification are discussed, and identification keys to species presented. 相似文献
133.
A hydrological tracer study of water uptake depth in a Scots pine forest under two different water regimes 总被引:2,自引:0,他引:2
Little is known about the vertical distribution of water uptake by trees under different water supply regimes, the subject of this study, conducted in a Scots pine stand on sandy loam in northern Sweden. The objective was to determine the water uptake distribution in pines under two different water regimes, desiccation (no precipitation) and irrigation (2?mm day?1 in July and 1?mm day?1 in August), and to relate the uptake to water content, root and soil texture distributions. The natural 18O gradient in soil water was exploited, in combination with two added tracers, 2H at 10?cm and 3H at 20?cm depth. Extraction of xylem sap and water from the soil profile then enabled evaluation of relative water uptake from four different soil depths (humus layer, 0–10, 10–25 and 25–55?cm) in each of two 50-m2 plots per treatment. In addition, water content, root biomass and soil texture were determined. There were differences in vertical water uptake distribution between treatments. In July, the pines at the irrigated and desiccated plots took up 50% and 30%, respectively, of their water from the upper layers, down to 25?cm depth. In August, the pines on the irrigated plots took up a greater proportion of their water from layers below 25?cm deep than they did in July. In a linear regression, the mean hydraulic conductivity for each mineral soil horizon explained a large part of the variation in relative water uptake. No systematic variation in the residual water uptake correlated to the root distribution. It was therefore concluded that the distribution of water uptake by the pines at Åheden was not a function of root density in the mineral soil, but was largely determined by the unsaturated hydraulic conductivity. 相似文献
134.
Daily and seasonal variation in water relations of macchia shrubs and trees in France (Montpellier) and Turkey (Antalya) 总被引:2,自引:0,他引:2
Based upon two different research studies in the mediterranean regions of France and Turkey, drought resistance strategies were investigated in a broad group of species. The diurnal and seasonal patterns of the water relations of different lifeforms from the thermo-mediterranean to submediterranean lifezones were compared. Three sites near Montpellier, in Southern France, and five sites near Antalya, Turkey were used for this comparison. Xylem pressure potential and relative stomatal aperture were the key water relations parameters collected in France while these parameters as well as osmotic potential and leaf conductance were studied in Turkey.From the 26 different study species investigated in France, 7 distinct types of stomatal control were observed, with the deciduous lifeforms showing the least control, the sclerophyllous and coniferous evergreens the greatest control and the malacophyllous shrublets intermediate levels of control. Predawn water potential values provided a means of classifying species according to their temporal and spatial utilization of site water reserves. The comparison of turgor potentials (difference between water and osmotic potentials) gave an insight into leaf adaptations to site moisture. Species with high predawn water potentials generally maintain positive turgor even at midday during the summer, whereas species with low predawn values were frequently at zero turgor even at predawn. Phlomis grandiflora was the most extreme species with mid-summer predawns and midday water potentials of –6 MPa and osmotic potentials never more negative than –2.4 MPa. 相似文献
135.
Shuya Yano Yong Zhang Shinji Miwa Yasunori Tome Yukihiko Hiroshima Fuminari Uehara 《Cell cycle (Georgetown, Tex.)》2014,13(13):2110-2119
The phase of the cell cycle can determine whether a cancer cell can respond to a given drug. We report here on the results of monitoring of real-time cell cycle dynamics of cancer cells throughout a live tumor intravitally using a fluorescence ubiquitination cell cycle indicator (FUCCI) before, during, and after chemotherapy. In nascent tumors in nude mice, approximately 30% of the cells in the center of the tumor are in G0/G1 and 70% in S/G2/M. In contrast, approximately 90% of cancer cells in the center and 80% of total cells of an established tumor are in G0/G1 phase. Similarly, approximately 75% of cancer cells far from (>100 µm) tumor blood vessels of an established tumor are in G0/G1. Longitudinal real-time imaging demonstrated that cytotoxic agents killed only proliferating cancer cells at the surface and, in contrast, had little effect on quiescent cancer cells, which are the vast majority of an established tumor. Moreover, resistant quiescent cancer cells restarted cycling after the cessation of chemotherapy. Our results suggest why most drugs currently in clinical use, which target cancer cells in S/G2/M, are mostly ineffective on solid tumors. The results also suggest that drugs that target quiescent cancer cells are urgently needed. 相似文献
136.
Michael Nahm Andreas Matzarakis Heinz Rennenberg Arthur Geßler 《Trees - Structure and Function》2007,21(1):79-92
During the growing season of the exceptionally dry and warm year 2003, we assessed seasonal changes in nitrogen, carbon and
water balance related parameters of mature naturally grown European beech (Fagus sylvatica L.) along a North–South transect in Europe that included a beech forest stand in central Germany, two in southern Germany
and one in southern France. Indicators for N balance assessed at all four sites were foliar N contents and total soluble non-protein
nitrogen compounds (TSNN) in xylem sap, leaves and phloem exudates; C and water balance related parameters determined were
foliar C contents, δ13C and δ18O signatures. Tissue sampling was performed in May, July and September. The N related parameters displayed seasonal courses
with highest concentrations during N remobilization in May. Decreased total foliar N contents as well as higher C/N ratios
in the stands in central Germany and southern France compared to the other study sites point to an impaired N nutrition status
due to lower soil N contents and precipitation perception. TSNN concentrations in leaves and phloem exudates of all study
sites were in ranges previously reported, but xylem sap content of amino compounds in July was lower at all study sites when
compared to literature data (c. 1 μmol N mL−1). In September, TSNN concentrations increased again at the two study sites in southern Germany after a rain event, whereas
they remained constant at sites in central Germany and southern France which hardly perceived precipitation during that time.
Thus, TSNN concentrations in the xylem sap might be indicative for water balance related N supply in the beech trees. TSNN
profiles at all study sites, however, did not indicate drought stress. Foliar δ13C, but not foliar C and δ18O followed a seasonal trend at all study sites with highest values in May. Differences in foliar δ13C and δ18O did not reflect climatic differences between the sites, and are attributed to differences in altitude, photosynthesis and
δ18O signatures of the water sources. Except of low TSNN concentrations in the xylem sap, no physiological indications of drought
stress were detected in the trees analysed. We suppose that the other parameters assessed might not have been sensitive to
the drought events because of efficient regulation mechanisms that provide a suitable physiological setting even under conditions
of prolonged water limitation. The uniform performance of the trees from southern France and central Germany under comparably
dry climate conditions denotes that the metabolic plasticity of mature beech from the different sites studied might be similar. 相似文献
137.
《Reproductive biology》2019,19(4):412-420
Von Willebrand disease (VWD) affects blood coagulation and correlates with angiodysplasia. Data on VWD-affected women point to slightly increased miscarriage rates. We aimed to investigate the impact of VWD on angiogenesis in the uteroplacental unit of pregnant pigs of a model of VWD type 1 (T1). Uteri, placentae, and embryos were harvested at time of placentation (day 29 to 31) from four sows (two wildtype (WT) and two heterozygous for a von Willebrand factor (VWF) mutation diagnosed with T1). T1 sows were bred to a T1 boar creating embryos of three different genotypes: WT, T1 or homozygous for the VWF mutation corresponding with VWD type 3 (T3). Uteroplacental tissues were examined histologically. Embryos were genotyped. Gene expression of angiogenic factors possibly related to VWF was determined by quantitative real-time PCR. Corresponding protein expression was analyzed by immunohistochemistry. Genotyping revealed 35.3% WT, 52.9% T1 and 5.9% T3 embryos (5.9% not classified confidently). No histological alterations were found. Gene expression of VEGF was significantly increased in T1 placentae while expression of ANG1, ANG2, TIE2, and ITGB3 was significantly reduced, confirmed on protein level for different cell types. TIE2/TIE1 ratios were significantly lower in T1 placentae. Distribution of embryo genotypes indicates selection favoring the WT. Significant expression differences of angiogenic factors in placentae suggest influence of VWF on these factors during placentation, although angiodysplasia was not observed. The alterations concerning VEGF/VEGFR-2 signaling, integrin expression and the ANG/TIE system may influence angiogenesis and vascular adaptation during placentation and thus the overall outcome of pregnancy. 相似文献
138.
The objective of the present study was to examine the functional coordination among hydraulic traits, xylem characteristics
and gas exchange rates across three deciduous Euphorbiaceae tree species (Hevea brasiliensis, Macaranga denticulata and Bischofia javanica) and three evergreen Euphorbiaceae tree species (Drypetes indica, Aleurites moluccana and Codiaeum variegatum) from a seasonally tropical forest in south-western China. The deciduous tree species were more vulnerable to water stress-induced
embolism than the evergreen tree species. However, the deciduous tree species generally had higher maximal rates of sapwood
and leaf-specific hydraulic conductivity (K
S and K
L), respectively. Compared with the evergreen tree species, the deciduous tree species, however, possessed a lower density
of sapwood and a wider diameter of xylem vessels. Regardless of leaf phenology, the hydraulic vulnerability and conductivity
were significantly correlated with sapwood density and mean vessel diameter. Furthermore, the hydraulic vulnerability was
positively correlated with water transport efficiency. In addition, the deciduous tree species exhibited higher maximal photosynthetic
rates (A
max) and stomatal conductance (g
max), but lower water use efficiency (WUE). Interestingly, the A
max, g
max and WUE were strongly correlated with K
S and K
L across the deciduous and evergreen tree species. These results suggest that xylem structure, rather than leaf phenology,
accounts for the difference in hydraulic traits between the deciduous tree species and the evergreen tree species. Meanwhile,
our results show that there is a significant trade-off between hydraulic efficiency and safety, and a strong functional correlation
between the hydraulic capacity and gas exchange rates across the deciduous and evergreen tree species. 相似文献
139.
Ward NL Van Slyke P Dumont DJ 《Biochemical and biophysical research communications》2004,323(3):937-946
The angiopoietins are a family of growth factors critical for development and maintenance of the vasculature. The primary amino acid sequence of the angiopoietins predicts that they are comprised of a coiled-coiled and a fibrinogen-like domain. The coiled-coiled domain mediates ligand multimerization, whereas the fibrinogen domain engages the receptor. This multimerization is required to elicit a ligand-mediated biological effect via activation of their receptor Tie2. In vitro and in vivo knockout studies have suggested that the angiopoietins are chemotactic for endothelial cells. We were interested in ascertaining whether the angiopoietins have this activity within the animal proper. To accomplish this we engineered a dominant-interfering form of angiopoietin (Ang) 1, called Ang1cc. Ang1cc contains the coiled-coiled domain, which can heterodimerize with other angiopoietins produced in the same cell. We show that Ang1cc can inhibit Tie2 activation and can inhibit Ang1 activity in vitro and in vivo. 相似文献
140.
Molecular distinction between arteries and veins 总被引:6,自引:0,他引:6
The vertebrate vascular system is essential for the delivery and exchange of gases, hormones, metabolic wastes and immunity factors. These essential functions are carried out in large part by two types of anatomically distinct blood vessels, namely arteries and veins. Previously, circulatory dynamics were thought to play a major role in establishing this dichotomy, but recently it has become clear that arterial and venous endothelial cells are molecularly distinct even before the output of the first embryonic heartbeat, thus revealing the existence of genetic programs coordinating arterial-venous differentiation. Here we review some of the molecular mechanisms involved in this process.The first two authors contributed equally to this work 相似文献