全文获取类型
收费全文 | 44038篇 |
免费 | 4155篇 |
国内免费 | 32篇 |
出版年
2021年 | 551篇 |
2020年 | 346篇 |
2019年 | 424篇 |
2018年 | 561篇 |
2017年 | 521篇 |
2016年 | 936篇 |
2015年 | 1603篇 |
2014年 | 1744篇 |
2013年 | 2214篇 |
2012年 | 2777篇 |
2011年 | 2854篇 |
2010年 | 1867篇 |
2009年 | 1801篇 |
2008年 | 2615篇 |
2007年 | 2635篇 |
2006年 | 2557篇 |
2005年 | 2537篇 |
2004年 | 2462篇 |
2003年 | 2425篇 |
2002年 | 2372篇 |
2001年 | 502篇 |
2000年 | 385篇 |
1999年 | 547篇 |
1998年 | 626篇 |
1997年 | 435篇 |
1996年 | 387篇 |
1995年 | 419篇 |
1994年 | 403篇 |
1993年 | 409篇 |
1992年 | 347篇 |
1991年 | 314篇 |
1990年 | 333篇 |
1989年 | 246篇 |
1988年 | 300篇 |
1987年 | 273篇 |
1986年 | 271篇 |
1985年 | 332篇 |
1984年 | 375篇 |
1983年 | 337篇 |
1982年 | 435篇 |
1981年 | 433篇 |
1980年 | 417篇 |
1979年 | 266篇 |
1978年 | 298篇 |
1977年 | 263篇 |
1976年 | 273篇 |
1975年 | 212篇 |
1974年 | 279篇 |
1973年 | 250篇 |
1972年 | 159篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
Design and operation of a completely automated Beckman microsequencer 总被引:11,自引:0,他引:11
A unique, efficient, and inexpensive system has been designed and built for the automatic conversion of anilinothiazolinone derivatives extracted from a Beckman spinning-cup sequencer with subsequent on-line high-pressure liquid chromatography separation of the phenylthiohydantoin derivatives. The Auto Converter-Auto Sampler system is controlled by a tape programmer or microprocessor and operates by transfer of the sample from the conversion vial into an HPLC injection loop by nitrogen pressure. Incorporation of a minor programming change on the sequencer allows the introduction of nitrogen vapor into the spinning cup during phenylisothiocyanate coupling. These modifications have resulted in a completely automated subnanomole protein sequencer. 相似文献
102.
Richard E. Litz 《Journal of nematology》1986,18(1):18-22
In vitro procedures are playing a major role in plant breeding. Embryo rescue, either through the culture of excised embryos derived from incompatible crosses or by means of ovule culture, has been a standard procedure for the introgression of genes conferring disease resistance into economically important plants. Somatic hybridization (i.e., protoplast fusion) has also been demonstrated to have some potential in obtaining hybrids that result from very wide interspecific and intergeneric crosses. Wide crosses have also been achieved by means of in vitro pollination of excised ovaries or ovules. Tissue culture-induced variability in regenerated plant (i.e., somaclonal variation) appears to be an effective way of obtaining undirected genetic change that can enhance disease resistance and yield and alter the growth habit of crops that are normally propagated vegetatively (e.g., potato) or by seed (e.g., tomato). In the near future, the isolation and sequencing of genes that confer resistance to specific plant pathogens will be possible, and transfer of this information between species will become a reality. 相似文献
103.
Richard L. Hughson 《CMAJ》1980,122(10):1115-1119
104.
Concentration factor and biological half-life of 54Mn were determined in three species representing an ecologically and economically important food chain. Green algae (Chlorella spp.), Daphnia magna and yellow perch (Perca flavescens) were exposed to 54Mn in water and assayed for 54Mn uptake. Steady state concentration factors computed from the laboratory data for algae, Daphnia and perch were 4230, 17 000 and 11, respectively. Respective biological half-lives were 1.6, 1.2 and 8.3 days. 相似文献
105.
106.
LOCALIZATION OF MYELIN-ASSOCIATED GLYCOPROTEIN 总被引:1,自引:0,他引:1
107.
Jorge F. Toro-Vazquez Juan Morales-Rueda V. Ajay Mallia Richard G. Weiss 《Food biophysics》2010,5(3):193-202
In this research, we studied the relationship between the molecular structure of (R)-12-hydroxyoctadecanamide, (R)-N-propyl-12-hydroxyoctadecanamide, and (R)-N-octadecyl-12-hydroxyoctadecanamide and the thermo-mechanical properties of their 2% (wt/wt) organogels developed using safflower
oil high in oleic acid (HOSFO) as the liquid phase. Candelilla wax (CW), a well-known edible gelling additive whose main component
is hentriacontane, also was studied for comparative purposes. The results obtained show that the attractive interactions (i.e.,
hydrogen bonding and dipolar interactions) between amide groups and between hydroxyl groups present in the amides resulted
in organogels with higher melting temperature, heat of melting, and crystallization parameters than those found in the CW
organogel. The rheological parameters associated to the strength of the amide or CW-based gels developed in HOSFO (i.e., yield
stress and elastic modulus) seem to be associated with the nature of amide groups (i.e., primary or secondary amide) and the
increase in the length of the self-assembly molecular unit (i.e., L value determined by X-ray diffraction) and therefore to the extent of London dispersion forces along the hydrocarbon chain.
The creep and recovery measurements allowed an evaluation among the internal structures of the different organogels and demonstrated
that independent of the hydrogen bonding and dipolar interaction provided by the amide and the hydroxyl groups, the increase
in the hydrocarbon chain length results in higher organogel resistance to deformation and higher instant recovery capacity.
However, the stabilization of the self-assembly unit through polar groups (i.e., –CONH2 in HOA) reduces organogel elasticity but provides a higher extended recovery capacity. The results reported in this investigation
showed some relationships between gelator structure and the thermo-mechanical properties of low-molecular-mass organic gelator
amides. Our long-term objective is to understand the organogelation process to eventually develop trans-free vegetable oil-based food products with novel textures for the consumers. 相似文献
108.
Abstract Theory predicts that compared with active searchers, ambush foragers should have lower rates of energy intake, slower growth, and higher survival rates. We tested these predictions with data on two species of sympatric, saurophagous, small‐bodied, viviparous elapid snakes: the broad‐headed snake, Hoplocephalus bungaroides, and the small‐eyed snake, Rhinoplocephalus nigrescens. Demographic parameters and growth curves for both species were estimated from a long‐term (9 years) mark‐recapture study in Morton National Park, south‐eastern Australia. The ambush predator (H. bungaroides) displayed slower juvenile growth and later maturation (5 years for males, 6 years for females) than did the active forager (R. nigrescens, 3 years). Litter sizes were similar in both species, but reproductive frequency was higher in R. nigrescens (90–100%) than in H. bungaroides (50%). Juvenile survival was lower in the active searcher (31%) than in the ambush forager (55%), but adult survivorship was similar (74% vs 82%). Our results support the hypothesis that ambush foragers display ‘slow’ life history traits, but additional phylogenetically independent comparisons are needed to evaluate the generality of this pattern. 相似文献
109.
Abstract: The phosphorylcholine concentration of rat brain rises and falls in response to parallel changes in the concentration of circulating choline. A single oral dose of choline chloride (20 mmol/kg) elevated whole-brain concentrations of both choline and phosphorylcholine 5 h after administration; a greater proportion of exogenously administered choline was retained by the brain in its phosphorylated form than as the free arnine. Striatal phosphorylcholine concentrations were elevated within 2 h of choline administration and continued to be significantly greater than control values for up to 34 h after treatment. The response of striatal choline levels to exogenous choline was of shorter duration than that of phosphorylcholine and was correlated with a significant increase in striatal acetylcholine concentrations. The consumption of a choline-free diet for 7 days lowered both serum choline and striatal phosphorylcholine concentrations, but had no effect on striatal choline or acetylcholine. These results suggest that choline kinase is unsaturated by its substrate in vivo and may thus serve to modulate the response of brain choline concentrations to alterations in the supply of circulating choline. 相似文献
110.