全文获取类型
收费全文 | 31281篇 |
免费 | 2872篇 |
国内免费 | 3932篇 |
出版年
2024年 | 84篇 |
2023年 | 411篇 |
2022年 | 883篇 |
2021年 | 1641篇 |
2020年 | 1152篇 |
2019年 | 1457篇 |
2018年 | 1316篇 |
2017年 | 1011篇 |
2016年 | 1281篇 |
2015年 | 1986篇 |
2014年 | 2421篇 |
2013年 | 2521篇 |
2012年 | 3092篇 |
2011年 | 2703篇 |
2010年 | 1759篇 |
2009年 | 1627篇 |
2008年 | 1793篇 |
2007年 | 1663篇 |
2006年 | 1382篇 |
2005年 | 1250篇 |
2004年 | 995篇 |
2003年 | 884篇 |
2002年 | 739篇 |
2001年 | 530篇 |
2000年 | 440篇 |
1999年 | 428篇 |
1998年 | 297篇 |
1997年 | 224篇 |
1996年 | 210篇 |
1995年 | 177篇 |
1994年 | 168篇 |
1993年 | 118篇 |
1992年 | 138篇 |
1991年 | 133篇 |
1990年 | 126篇 |
1989年 | 123篇 |
1988年 | 98篇 |
1987年 | 98篇 |
1986年 | 80篇 |
1985年 | 77篇 |
1984年 | 52篇 |
1983年 | 34篇 |
1982年 | 39篇 |
1981年 | 30篇 |
1979年 | 29篇 |
1978年 | 23篇 |
1977年 | 22篇 |
1976年 | 31篇 |
1974年 | 22篇 |
1972年 | 26篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
中国某些野生和栽培茶的核型研究 总被引:9,自引:0,他引:9
研究了茶的4个变种和广东野生毛叶茶等共12个材料的核型。所有材料的染色体数目均是2n=30,为二倍体。所有中国大叶变种(越南大叶除外)(Cametlia sinensis var. macrophylla)和阿萨姆大叶变种(C. sinensis var. assamica)均具比较对称或原始的“2A”核型;中国小叶变种(C.sinensis var.bohea)(“铁观音”品种除外),掸部变种(C.sinensis var. shan form)和广东野生毛叶茶(C. ptilophylla)均具较不对称或较进化的“2B”核型。根据核型特征,植物习性和地理分布,作者认为中国四川和云南可能是茶的起源中心,向东或北迁移,演变为中国小叶变种;向南移则演变为阿萨姆变种和掸部变种。 相似文献
92.
93.
94.
95.
Eutrophication of Lakes Leman and Neuchâtel (Switzerland) indicated by oligochaete communities 总被引:1,自引:1,他引:0
Claude Lang 《Hydrobiologia》1984,115(1):131-138
In 1978–80, oligochaete communities of meso-eutrophic Lake Léman (Lake of Geneva) were compared to those of mesotrophic Lake Neuchâtel. Worm species were classified into three groups corresponding to their increasing tolerance to eutrophication: (1) oligotrophic species, mostly Peloscolex velutinus, Stylodrilus heringianus; (2) mesotrophic species, mostly Potamothrix vejdovskyi, P. bedoti; (3) eutrophic species, mostly Potamothrix hammoniensis, P. heuscheri, Tubifex tubifex. In both lakes, eutrophic species constituted the bulk of the communities in terms of absolute abundance. However, relative abundance of mesotrophic and eutrophic species was higher in Lake Léman; oligotrophic species were more important in Lake Neuchâtel. These data confirmed the trophic classification of lakes based on chemical parameters. The number of zero values, which perturbated statistical analysis, was reduced by using species groupings instead of isolated species. Thus, making the lakes more comparable even if different species were present in each one. Relative density values based on all samples were distributed among 4 density classes for the 3 species groupings. The 12 resulting frequencies described the community structure expressed in terms of eutrophication. Furthermore, these frequencies may be used for comparison of eutrophication levels in several lakes. 相似文献
96.
Angiotensin biosynthesis and concentrations in brain of normotensive and hypertensive rats 总被引:3,自引:0,他引:3
We report here on the extraction and characterization of angiotensin I (ANG I) and angiotensin II (ANG II) from the brain of rats. High pressure liquid chromatography (HPLC) with different mobile phases combined with specific radioimmunoassays (RIA) proved to be a powerful tool for peptide characterization in biological samples; (Ile5)-ANG I, (Ile5)-ANG II and (Ile5)-ANG III could clearly be identified in cerebrospinal fluid (CSF), incubated in vivo and in vitro with renin, in total brain extracts, as well as in hypothalamus (HT), medulla oblongata (MO), cerebellum (CER) and cortex (CO). Angiotensin cleaved from CSF angiotensinogen and angiotensin extracted from brain showed retention times identical to those of plasma angiotensin and synthetic standard peptides, indicating that their amino acid sequence is probably identical. ANG I and ANG II were highest in the HT and lowest in the CO. Following bilateral nephrectomy (NX) both ANG I and ANG II persisted at control levels. Young 10 week old spontaneously hypertensive rats (SHRSP) showed significantly lower ANG I and ANG II concentrations in the HT compared with Wistar Kyoto rats (WKY). Intracerebroventricular (i.c.v.) administration of the converting enzyme inhibitor captopril caused a significant increase in ANG 1 in nephrectomized SHRSP but not in WKY. These differences were not found in 40 week old SHRSP. The data show that ANG I and ANG II are synthetized in the brain of rats. The lower concentrations and the enhanced accumulation of ANG I after converting enzyme blockade in nephrectomized young SHRSP indicate an increased turnover of angiotensin in hypertensive rats. 相似文献
97.
98.
Mice were fed an agar-based diet without an additional source of water for 5 weeks. In comparison with a similar group of mice fed a commercial diet and water ad libitum, there were no significant changes in bodyweight. 相似文献
99.
Mice were injected for 1-2 months daily with 10 mg immunoglobulin G (IgG) from four patients with Lambert-Eaton myasthenic syndrome (LEMS); control mice were injected with pooled human IgG from normal donors. Gastrocnemius muscles were homogenised for the assay of acetylcholine (ACh), choline acetyltransferase (ChAT), and cholinesterase (ChE). The ACh, ChAT, and ChE contents of gastrocnemius muscles from "LEMS mice" were about the same as the control values, which were 180 pmol, 40 nmol X h-1 (37 degrees C), and 15 mumol X h-1 (37 degrees C), respectively. Hemidiaphragms were treated with an irreversible ChE inhibitor (Soman) and incubated at 20 degrees C for estimation of ACh release. Resting ACh release from experimental muscles was reduced by about 25% (P2 less than 0.05) and the release evoked by 3 s-1 nervous stimulation by 50% (P2 less than 0.05). On the other hand, 50 mM KCl-induced transmitter release was not abnormal in LEMS mice. The findings indicate that IgG antibody from patients with LEMS may bind to nerve terminal determinants that are involved in quantal and nonquantal ACh release. 相似文献
100.
Electron microscopy of bacteriophage phi 6 nucleocapsid: three-dimensional image analysis. 总被引:7,自引:5,他引:2
下载免费PDF全文
![点击此处可从《Journal of virology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Electron micrographs of bacteriophage phi 6 nucleocapsids, negatively stained by neutralized phosphotungstate and tilted in a goniometer specimen cartridge, proved that the three distinct morphologies seen in the electron microscope are merely three different aspects of a single nucleocapsid structure and strongly suggested that this structure is dodecahedral. 相似文献