全文获取类型
收费全文 | 15835篇 |
免费 | 1390篇 |
国内免费 | 1346篇 |
专业分类
18571篇 |
出版年
2024年 | 45篇 |
2023年 | 234篇 |
2022年 | 477篇 |
2021年 | 767篇 |
2020年 | 636篇 |
2019年 | 718篇 |
2018年 | 707篇 |
2017年 | 512篇 |
2016年 | 688篇 |
2015年 | 1037篇 |
2014年 | 1243篇 |
2013年 | 1271篇 |
2012年 | 1472篇 |
2011年 | 1316篇 |
2010年 | 888篇 |
2009年 | 731篇 |
2008年 | 787篇 |
2007年 | 703篇 |
2006年 | 688篇 |
2005年 | 557篇 |
2004年 | 484篇 |
2003年 | 515篇 |
2002年 | 400篇 |
2001年 | 245篇 |
2000年 | 211篇 |
1999年 | 205篇 |
1998年 | 138篇 |
1997年 | 113篇 |
1996年 | 116篇 |
1995年 | 109篇 |
1994年 | 93篇 |
1993年 | 57篇 |
1992年 | 79篇 |
1991年 | 64篇 |
1990年 | 60篇 |
1989年 | 44篇 |
1988年 | 29篇 |
1987年 | 24篇 |
1986年 | 34篇 |
1985年 | 25篇 |
1984年 | 13篇 |
1983年 | 7篇 |
1982年 | 8篇 |
1981年 | 5篇 |
1980年 | 3篇 |
1979年 | 3篇 |
1976年 | 2篇 |
1970年 | 1篇 |
1962年 | 2篇 |
1950年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
41.
繁缕和无瓣繁缕六个居群的数值分析 总被引:6,自引:0,他引:6
对繁缕(Stellaria media)和无瓣繁缕(S.apetala)的6个居群的57个性状进行Q-聚类和R-聚类的研究。结果表明:(1)Q-聚类中,用一条结合线,可以把繁缕的4个居群聚为一类,无瓣繁缕的2个居群聚为一类。这一结果支持肥繁缕和无瓣繁缕划分为两个物种;(2)R-聚类中,发现了呈现完全正相关、极大正相关和极大负相关的性状,并根据R-聚类的结果,运用一条适当的结合线,把繁缕和无瓣繁缕的57个性状划为5个类群,并分析了各性状的分类学意义。 相似文献
42.
The Chinese egret is a globally endangered species. Here we describe a set of primer pairs to amplify its entire mtDNA. The polymerase chain reaction products (1000–2000 bp) were successfully amplified by using this primer set and were then sequenced and aligned. The contiguous mtDNA sequences of the Chinese egret were assembled to be a circular molecule (17 579 bp). This primer set was also confirmed to be useful for six other species of ardeid birds. The versatility of this primer set will provide a groundwork for further studies on the genetic structure and molecular evolution of the ardeid species. 相似文献
43.
Response of rice (Oryza sativa) with root surface iron plaque under aluminium stress 总被引:1,自引:0,他引:1
BACKGROUND AND AIMS: Rice (Oryza sativa) is an aquatic plant with a characteristic of forming iron plaque on its root surfaces. It is considered to be the most Al-tolerant species among the cereal crops. The objective of this study was to determine the effects of root surface iron plaque on Al translocation, accumulation and the change of physiological responses under Al stress in rice in the presence of iron plaque. METHODS: The japonica variety rice, Koshihikari, was used in this study and was grown hydroponically in a growth chamber. Iron plaque was induced by exposing the rice roots to 30 mg L(-1) ferrous iron either as Fe(II)-EDTA in nutrient solution (6 d, Method I) or as FeSO(4) in water solution (12 h, Method II). Organic acid in root exudates was retained in the anion-exchange resin and eluted with 2 m HCl, then analysed by high-performance liquid chromatography (HPLC) after proper pre-treatment. Fe and Al in iron plaque were extracted with DCB (dithionite-citrate-bicarbonate) solution. KEY RESULTS AND CONCLUSIONS: Both methods (I and II) could induce the formation of iron plaque on rice root surfaces. The amounts of DCB-extractable Fe and Al on root surfaces were much higher in the presence of iron plaque than in the absence of iron plaque. Al contents in root tips were significantly decreased with iron plaque; translocation of Al from roots to shoots was significantly reduced with iron plaque. Al-induced secretion of citrate was observed and iron plaque could greatly depress this citrate secretion. These results suggested that iron plaque on rice root surfaces can be a sink to sequester Al onto the root surfaces and Fe ions can pre-saturate Al-binding sites in root tips, which protects the rice root tips from suffering Al stress to a certain extent. 相似文献
44.
Revegetation is a traditional practice widely used for soil and water conservation on the Loess Plateau in China. However, there has been a lack of reports on soil microbial–biochemical indices required for a comprehensive evaluation of the success of revegetation systems. In this study, we examined the effects of revegetation on major soil nutrients and microbial–biochemical properties in an artificial alfalfa grassland, an enclosed natural grassland, and an artificial shrubland (Caragana korshinskii), with an abandoned cropland as control. Results showed that at 0–5, 5–20, and 20–40 cm depths, soil organic carbon, alkaline extractable nitrogen and available potassium were higher in natural grassland and artificial shrubland compared with artificial grassland and abandoned cropland. Soil microbial biomass C (Cmic) and phosphorous (Pmic) substantially decreased with depth at all sites, and in abandoned cropland was significantly lower than those of natural grassland, artificial grassland, and artificial shrubland at the depth of 0–5 cm. Soil microbial biomass N (Nmic) was higher in artificial shrubland and abandoned cropland compared with that in natural and artificial grasslands. Both Cmic and Pmic were significantly different between the 23‐year‐old and the 13‐year‐old artificial shrublands at the 0–5 cm depth. The activities of soil invertase, urease, and alkaline phosphatase in natural grassland and artificial shrubland were higher than those in artificial grassland and abandoned cropland. This study demonstrated that the regeneration of both natural grassland and artificial shrubland effectively preserved and enhanced soil microbial biomass and major nutrient cycling, thus is an ecologically beneficial practice for recovery of degraded soils on the Loess Plateau. 相似文献
45.
46.
47.
极端耐盐放线菌白色普氏菌YIM 90005T四氢嘧啶及5-羟基四氢嘧啶合成相关基因的克隆 总被引:1,自引:0,他引:1
摘要:【目的】 为了研究耐盐放线菌对高盐环境的适应机理。【方法】 用HPLC定量检测了极端耐盐、丝状产孢放线菌——白色普氏菌(Prauserella alba) YIM 90005T在不同盐浓度下胞内相容性溶质的种类和含量。【结果】 结果发现,四氢嘧啶和5-羟基四氢嘧啶是其主要的相容性溶质。在培养基NaCl浓度为10%时,四氢嘧啶在胞内累积浓度最大,为18.77 μg/mg干菌体重。之后随NaCl浓度的升高,胞内的四氢嘧啶含量逐渐减少,而5-羟基四氢嘧啶的含量逐渐增加,在该菌耐受的最高NaCl浓度下(24% w/v),胞内5-羟基四氢嘧啶含量达到最大值,为22.98 μg/mg干菌体重。设计兼并引物,利用染色体步移,克隆得到四氢嘧啶及5-羟基四氢嘧啶合成相关基因ectABCD。序列分析表明,ectABCD位于一个操纵子中。进一步对不同NaCl浓度培养条件下ectB,D的表达量进行定量分析,结果表明该基因簇表达量随着培养基中NaCl浓度的增加而增大。【结论】 研究结果证实5-羟基四氢嘧啶是P. alba YIM 90005T在极高盐浓度条件下起渗透调节及保护的相容性溶质。 相似文献
48.
Endostatin gene therapy enhances the efficacy of paclitaxel to suppress breast cancers and metastases in mice 总被引:5,自引:0,他引:5
Li J Dong X Xu Z Jiang X Jiang H Krissansen GW Sun X 《Journal of biomedical science》2008,15(1):99-109
Chemotherapy combined with antiangiogenic therapy is more effective than chemotherapy alone. The aim of this study was to
investigate whether endostatin, a potent anti-angiogenic agent, could enhance the efficacy of paclitaxel to combat breast
cancer. An expression plasmid encoding mouse endostatin (End-pcDNA3.1) was constructed, which produced intense expression
of endostatin and inhibited angiogenesis in the chorioallantoic membrane assay. 4T1 breast tumors were established in BALB/c
mice by subcutaneous injection of 1 × 105 4T1 cells. The End-pcDNA3.1 plasmid diluted in the transfection reagent FuGENETM was injected into the tumors (around 100 mm2), and paclitaxel was injected i.p. into the mice. Endostatin gene therapy synergized with paclitaxel in suppressing the growth
of 4T1 tumors and their metastasis to the lung and liver. Both endostatin and paclitaxel inhibited tumor angiogenesis and
induced cell apoptosis. Despite the finding that endostatin was superior to paclitaxel at inhibiting tumor angiogenesis, paclitaxel
was nevertheless more effective at inducing tumor apoptosis. The combination of paclitaxel and endostatin was more effective
in suppressing tumor growth, metastases, angiogenesis, and inducing apoptosis than the respective monotherapies. The combinational
therapy with endostatin and paclitaxel warrants future investigation as a therapeutic strategy to combat breast cancer. 相似文献
49.
采用人结核分枝杆菌(Mycobacterium tuberculosis TB)染色体DNA为模板,选择位于插入片段IS6110中884~865和568~588碱基对处的两个片段为引物,扩增出317bp的特异性片段.将其克隆进pUCl9载体。酶切图谱分析和DNA序列测定证实为目的片段。该片段经DIG标记,分别与11种分枝杆菌DNA进行Southern杂交,结果证明只与人型复合分枝杆菌发生杂交反应。利用该对引物建立的PcR检测拄术对74份结核病痰液标本进行检测,并与临床细菌快速培养结果相比较,发现48份临床阳性均为PcR阳性,在26份临床阴性标本中亦发现11份PCR检测阳性。将标本PCR产物与克隆探针进行杂交,显示两者结果完全一致。说明PCR检测体系结果可靠,其灵敏度明显高于目前临床所采用的方法,可作为一种常规技术用于结核病的临床检测。 相似文献
50.