全文获取类型
收费全文 | 26334篇 |
免费 | 2090篇 |
国内免费 | 2114篇 |
专业分类
30538篇 |
出版年
2024年 | 73篇 |
2023年 | 343篇 |
2022年 | 876篇 |
2021年 | 1427篇 |
2020年 | 969篇 |
2019年 | 1142篇 |
2018年 | 1116篇 |
2017年 | 836篇 |
2016年 | 1189篇 |
2015年 | 1650篇 |
2014年 | 1896篇 |
2013年 | 2069篇 |
2012年 | 2453篇 |
2011年 | 2091篇 |
2010年 | 1288篇 |
2009年 | 1100篇 |
2008年 | 1281篇 |
2007年 | 1126篇 |
2006年 | 987篇 |
2005年 | 856篇 |
2004年 | 740篇 |
2003年 | 667篇 |
2002年 | 570篇 |
2001年 | 488篇 |
2000年 | 431篇 |
1999年 | 421篇 |
1998年 | 266篇 |
1997年 | 280篇 |
1996年 | 263篇 |
1995年 | 249篇 |
1994年 | 226篇 |
1993年 | 139篇 |
1992年 | 207篇 |
1991年 | 147篇 |
1990年 | 131篇 |
1989年 | 111篇 |
1988年 | 74篇 |
1987年 | 95篇 |
1986年 | 57篇 |
1985年 | 56篇 |
1984年 | 44篇 |
1983年 | 31篇 |
1982年 | 30篇 |
1981年 | 20篇 |
1980年 | 8篇 |
1979年 | 6篇 |
1975年 | 4篇 |
1974年 | 2篇 |
1973年 | 6篇 |
1953年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
本文研究一类含人口迁移的性传染病模型,应用摄动方法和定性方法研究平衡点的稳定性和迁移率的影响。普举例说明。 相似文献
62.
长年饲养在高温(28—30℃)环境中雌性中华大蟾蜍,它们的卵母细胞可以长足,但经激素处理时,生发泡不破裂,仅显示成熟过程早期阶段的变化。值得注意的是,在孕酮刺激后的高温卵卵质中,出观了一种能诱发低温卵恢复减数分裂的物质,称作为“依赖冬眠因子的促成熟物质”(HF-MPS)。HF-MPS 与MPF 有不少相似之处,如孕酮处理后,它们在卵质中出现的时间相仿,它们的形成均不依赖于转录水平,而是依赖于翻译水平的蛋白质合成活动;但亦存在不同之处,如MPF 诱发低温卵GVBD 时程不受温度影响,而HF-MPS 在10℃环境中,诱发低温卵GVBD 的时间明显延缓;MPF 不仅能诱发低温卵GVBD,而且同样能诱发高温卵GVBD,然而,HF-MPS 只能诱发低温卵GVBD。由此表明,MPF 和HF-MPS 似乎是截然不同的两类活性蛋白质。高温卵缺少低温诱发产生的“冬眠因子”,所以不能恢复cdc 2基因的转录活动,不能实现MPF 自身催化扩增作用,不能保证孕酮处理后的卵母细胞完成正常成熟的全过程变化。足见,低温是中华大蟾蜍卵母细胞恢复减数分裂过程中的必要条件,是导致中华大蟾蜍现有区域分布的内在原因之一。 相似文献
63.
64.
Microtubule dynamics and organization are important for plant cell morphogenesis and development. The microtubule-based motor protein kinesins are mainly responsible for the transport of some organelles and vesicles, although several have also been shown to regulate microtubule organization. The ARMADILLO REPEAT KINESIN (ARK) family is a plant-specific motor protein subfamily that consists of three members (ARK1, ARK2, and ARK3) in Arabidopsis thaliana. ARK2 has been shown to participate in root epidermal cell morphogenesis. However, whether and how ARK2 associates with microtubules needs further elucidation. Here, we demonstrated that ARK2 co-localizes with microtubules and facilitates microtubule bundling in vitro and in vivo. Pharmacological assays and microtubule dynamics analyses indicated that ARK2 stabilizes cortical microtubules. Live-cell imaging revealed that ARK2 moves along cortical microtubules in a processive mode and localizes both at the plus-end and the sidewall of microtubules. ARK2 therefore tracks and stabilizes the growing plus-ends of microtubules, which facilitates the formation of parallel microtubule bundles. 相似文献
65.
J. W. Moon D. A. Bailey Jr. E. Fallahi R. G. Jensen G. Zhu 《Physiologia plantarum》1990,80(4):612-618
The relationships between increasing nitrogen fertilization and growth, maximum CO2 assimilation and the initial slope of the CO2 response curve were studied in 2 ecotypes of wild strawberry, Fragaria chiloensis (L.) Duchn. Nitrogen accumulation of CA11, an ecotype from a low-nutrient dune site, was greater at all nitrogen concentrations than that of RCP37, an ecotype from a higher-nutrient strand site. Maximum CO2 assimilation, total Rubisco activity, dry weight, and initiation of leaves and crowns were higher in CAI1 than RCP37 as nitrogen treatment was increased from 0 to 200 mg l-1 , whereas these parameters were lower in CAl1 when fertilized at 300 mg T1 , but not in RCP37. The mean leaf area of CA11 was greater than RCP37 when grown with no supplemental nitrogen, but mean leaf area of the 2 lines was similar under nitrogen fertilization. Maximum CO2 assimilation and carboxylation efficiency increased with increasing leaf nitrogen in both clones. At equivalent concentrations of leaf nitrogen, RCP37 had higher CO2 assimilation and carboxylation efficiency than CA11 and the difference between the 2 clones increased as ieaf nitrogen increased. Thus, RCP37 had a higher photosynthetic nitrogen use efficiency than CA11. However, at a given applied nitrogen level, CA11 allocated more nitrogen to a unit of leaf area so that photosynthetic rates were higher than RCP37, except at the highest application of 300 mg l-1 . The high nitrogen accumulation capacity and resource allocation to fruiting structures (crowns) in CA11 leads us to suggest that this clone may possess genes that could increase fruit yield in cultivated strawberry. 相似文献
66.
Xiang-yun Zhu 《Nordic Journal of Botany》2003,23(3):373-384
The pollen morphology of Gueldenstaedtia gansuensis. G. gracilis, G. henryi, G. monophylla. G. mutijlora, G. stenophylla. and G. verna and Tibetia liangshanensis, T. tongolensis, T. yadongensis. T. coelestis, and T yunnanensis are reported for the first time. The seed morphology of G. gracilis, G. maritima. G. monophylla, G. mutiflora, G. taihangensis, and G. verna and L coelestis, T. himalaica, T. yunnanensis, and T. yadongensis are firstly described here. In pollen morphology, the differences of pollen grains of Gueldenstaedtia and Tibetia are as follows: Gueldenstaedtia with pollen grains 3–colporate, psilate, and shapes spheroidal, sometimes subprolate, prolate or oblong; and Tibetia with pollen grains 3– and 4–colporate, perforate, shapes spheroidal, sometimes subprolate or prolate. These results, combined with the data of the basic chromosome number x=7 of Gueldenstaedtia and x=8 of Tibetia, support that the two genera should be recognized as two distinct genera, which are consistent with their morphological characters: Gueldenstaedtia with 2 upper lobes of calyx free, stipules free, adnate to petiole, and Tibetia with 2 upper lobes of calyx connate, stipules connate and opposite to leaves. In Tibetia, two types of pollen grains, 3– and 4–colporate pollen grains, are found. Regarding seed morphology: Gueldenstaedtia has circular depression, irregular circular depression or irregular circular reticulation on the surface; Tibetia has smooth surface. The differences in seed morphology of the two genera also support that they should be kept separate. The pollen morphology supports that G. gansuensis, G. gracilis, G. multiflora, G. stenophylla, and G. verna should be reduced into one species consistent with their morphological characteristics. The pollen grains of G. henryi are different from those of the other species in having wide colpi. 相似文献
67.
Detection of Salmonella typhi by polymerase chain reaction 总被引:1,自引:0,他引:1
A rapid and sensitive method for detection of Salmonella typhi would help in preventing the spread of outbreaks and in clinical diagnosis. In order to develop unique PCR primers to detect Salm. typhi , ribosomal RNA genes from Salm. typhi (Rawlings) were cloned in pUC18. The resulting clone was confirmed by sequencing. The cloned DNA fragment contained the 5S, part of the 23S rRNA genes and the 5S-23S spacer region (EMBL/GenBank accession No. U04734).
It was expected that the 5S-23S spacer region is divergent unlike the highly conserved 23S+5S genes. This was confirmed by comparison with the rRNA gene sequences in the EMBL/GenBank database. A pair of PCR primers specific for Salm. typhi was obtained, based on this spacer region sequence. The specificity of this pair of primers was tested with 54 Salm. typhi strains (of 27 different phage types). All these Salm. typhi strains showed the positive 300 bp PCR product with this pair of primers. Six other Salmonella species as well as six other non- Salmonella bacteria were tested and none showed the 300 bp PCR product. The sensitivity of the detection level was 0·1 pg of pure Salm. typhi genomic DNA, or approximately 40 Salm. typhi cells in a spiked food sample. This pair of primers therefore has the potential for development into a diagnostic tool for the rapid diagnosis of typhoid fever. 相似文献
It was expected that the 5S-23S spacer region is divergent unlike the highly conserved 23S+5S genes. This was confirmed by comparison with the rRNA gene sequences in the EMBL/GenBank database. A pair of PCR primers specific for Salm. typhi was obtained, based on this spacer region sequence. The specificity of this pair of primers was tested with 54 Salm. typhi strains (of 27 different phage types). All these Salm. typhi strains showed the positive 300 bp PCR product with this pair of primers. Six other Salmonella species as well as six other non- Salmonella bacteria were tested and none showed the 300 bp PCR product. The sensitivity of the detection level was 0·1 pg of pure Salm. typhi genomic DNA, or approximately 40 Salm. typhi cells in a spiked food sample. This pair of primers therefore has the potential for development into a diagnostic tool for the rapid diagnosis of typhoid fever. 相似文献
68.
69.
国家自然科学基金评审程序剖析──1993年和1994年植物学科面上基金项目分析陈峰,朱大保(中国科学院武汉植物研究所武汉430074)(国家自然科学基金委员会生命科学部北京100083)关键词基金,评审,科研立项,学科ANALYSLSFORJUDGE... 相似文献
70.