全文获取类型
收费全文 | 3354篇 |
免费 | 279篇 |
国内免费 | 194篇 |
出版年
2024年 | 14篇 |
2023年 | 42篇 |
2022年 | 83篇 |
2021年 | 99篇 |
2020年 | 140篇 |
2019年 | 172篇 |
2018年 | 157篇 |
2017年 | 170篇 |
2016年 | 163篇 |
2015年 | 150篇 |
2014年 | 201篇 |
2013年 | 278篇 |
2012年 | 124篇 |
2011年 | 213篇 |
2010年 | 151篇 |
2009年 | 178篇 |
2008年 | 151篇 |
2007年 | 146篇 |
2006年 | 149篇 |
2005年 | 133篇 |
2004年 | 84篇 |
2003年 | 93篇 |
2002年 | 77篇 |
2001年 | 75篇 |
2000年 | 40篇 |
1999年 | 44篇 |
1998年 | 40篇 |
1997年 | 41篇 |
1996年 | 23篇 |
1995年 | 26篇 |
1994年 | 25篇 |
1993年 | 25篇 |
1992年 | 34篇 |
1991年 | 15篇 |
1990年 | 21篇 |
1989年 | 11篇 |
1988年 | 16篇 |
1987年 | 9篇 |
1986年 | 7篇 |
1985年 | 31篇 |
1984年 | 64篇 |
1983年 | 31篇 |
1982年 | 34篇 |
1981年 | 21篇 |
1980年 | 13篇 |
1979年 | 6篇 |
1978年 | 3篇 |
1977年 | 1篇 |
1975年 | 1篇 |
1972年 | 2篇 |
排序方式: 共有3827条查询结果,搜索用时 15 毫秒
971.
This study examines sporophytic and gametophytic vigor in wild and cultivated varieties of Cucurbita pepo L. and their hybrids in order to determine whether hybrid vigor extends to the microgametophyte generation. It also examines
the variation in sporophytic and gametophytic vigor to discern the non-genetic influences of pollen provisioning by the sporophyte
on pollen performance from the genetic influences of the microgametophyte’s own genotype on pollen performance. A cultivated
and a wild C. pepo and their F1 and the F2 generations were grown under field conditions and flower and fruit production were monitored over
one summer. We found that the four types of plants differed significantly in the number of male and female flowers and the
number of fruits they produced. The F1 plants produced significantly more male flowers and marginally more female flowers
and fruits than the parental lines. To estimate gametophytic vigor pollen was germinated in vitro and pollen tube length measured
after 30 min. We found that pollen tubes from the F1 plants had significantly greater growth than tubes from the parental
lines or the F2 generation, indicating that hybrid vigor extends to the microgametophytic generation. By partitioning the
variance of pollen tube growth into ’within’ and ’among’ plant components of variation, we were able to show that the genotype
of the microgametophyte influences pollen performance in vitro, but that expression of hybrid vigor in the microgametophyte
is likely to be due to an environmental effect related to provisioning of the pollen grains during development.
Received: 16 April 1998 / Revision accepted: 18 August 1998 相似文献
972.
Natalia N. Kudryavtseva Alexis V. Sofin Michal M. Sikorski Vassily I. Romanov Andrzej B. Legocki 《Plant Physiology and Biochemistry》1998,36(12):907-911
A unique feature of the symbiotic association between legume plants and rhizobia is the plant-derived membrane which separates the symbionts within root nodule; this membrane is termed the peribacteroid membrane (PBM). Although this membrane plays a vital role in facilitating transport and other processes in nodules, little is known about the proteins that are associated with and are an integral part of it. The objective of this work was to apply modern methods of protein purification to the characterisation of proteins of peribacteroid membrane from nodules of yellow lupine (Lupines luteus). The 17-kDa protein was isolated from purified peribacteroid membrane using size exclusion and ion exchange chromatography (FPLC). The N-terminal amino acid sequence of this protein was determined; the sequence does not match any of the previously reported lupine and other legume sequences. Following detergent solubilisation of purified peribacteroid membrane, integral proteins of 15 to 20 kDa were purified by size exclusion chromatography. 相似文献
973.
重组大肠杆菌YK537/pSB┐TK高密度培养过程中有机酸的RP┐HPLC快速分析 总被引:4,自引:1,他引:3
采用高压液相色谱法(HPLC),利用C18反相柱快速检测重组大肠杆菌YK537/pSB-TK高密度发酵过程中细菌代谢产生的各种有机酸。分析表明,经乙醚和水两次萃取处理后能去除样品中大部分的干扰峰,可获得理想的分离效果,并能定量测出发酵过程中各种有机酸的积累。不同培养方式的结果表明用甘油代替葡萄糖作为细菌生长的碳源可减少乙酸的积累,降低其对细菌生长和外源蛋白表达的抑制作用,从而更易实现重组大肠杆菌的高表达、高密度培养。 相似文献
974.
Qi-Kun Zhao 《Primates; journal of primatology》1994,35(1):57-68
In five groups of seasonally provisioned Tibetan macaques (Macaca thibetana) at Mt. Emei, males were sampled for wounds as an indicator of their competition for females during about 80 days in the
1987 mating season. Quantitative data on intergroup transfer were collected in a period between June 1986 and December 1987.
The young adult (YA) males, the most active age-class in mating activity and intergroup transfer, received most of the wounds.
Wounds tended to appear more in the front of body for YA and subadults (SA) than they did for middle-old aged (MO) males.
This implies that some of the MO males were more active and aggressive in the fights. During the 1.5 year period, 5/6 of the
YA and 5/17 of the MO males made intergroup shifts. Although YA males faced a high risk of receiving wounds at transfer, they
usually rose in rank. On the other hand, the MO males transferred more smoothly but dropped in rank. The peripheral SA males,
which rarely emigrated in the population, were an active component in determining the wounding rate, and the rate and direction
of male migration. Three SA immigrants died of severe attacks made by resident males in 1988 and 1991. Adult sex ratios and
their variations were considerably reduced with male nonrandom shifts and better conservation of the population. 相似文献
975.
Analytical and micropreparative peptide mapping by high performance liquid chromatography/electrospray mass spectrometry of proteins purified by gel electrophoresis. 总被引:4,自引:3,他引:1 下载免费PDF全文
D. Hess T. C. Covey R. Winz R. W. Brownsey R. Aebersold 《Protein science : a publication of the Protein Society》1993,2(8):1342-1351
We report the use of microbore reverse-phase high performance liquid chromatography connected on-line to an electrospray mass spectrometer for the separation/detection of peptides derived by proteolytic digestion of proteins separated by polyacrylamide gel electrophoresis. A small fraction (typically 10% of the total) of the peptides eluting from the column was diverted through a flow-splitting device into the ion source of the mass spectrometer, whereas the majority of the peptide samples was collected for further analyses. We demonstrate the feasibility of obtaining reproducible peptide maps from submicrogram amounts of protein applied to the gel and good correlation of the signal detected by the mass spectrometer with peptide detection by UV absorbance. Furthermore, independently verifiable peptide masses were determined from subpicomole amounts of peptides directed into the mass spectrometer. The method was used to analyze the 265-kDa and the 280-kDa isoforms of the enzyme acetyl-CoA carboxylase isolated from rat liver. The results provide compelling evidence that the two enzyme isoforms are translation products of different genes and suggest that these approaches may be of general utility in the definitive comparison of protein isoforms. We furthermore illustrate that knowledge of peptide masses as determined by this technique provides a major advantage for error-free data interpretation in chemical high-sensitivity peptide sequence analysis. 相似文献
976.
Evidence is emerging on how whole-body powered exoskeleton (EXO) use impacts users in basic occupational work scenarios, yet our understanding of how users learn to use this complex technology is limited. We explored how novice users adapted to using an EXO during gait. Six novices and five experienced users completed the study. Novices completed an initial training/familiarization gait session, followed by three subsequent gait sessions using the EXO, while experienced users completed one gait session with the EXO. Spatiotemporal gait measures, pelvis and lower limb joint kinematics, muscle activities, EXO torques, and human-EXO interaction forces were measured. Adaptations among novices were most pronounced in spatiotemporal gait measures, followed by joint kinematics, with smaller changes evident in muscle activity and EXO joint torques. Compared to the experienced users, novices exhibited a shorter step length and walked with significantly greater anterior pelvic tilt and less hip extension. Novices also used lower joint torques from the EXO at the hip and knee, and they had greater biceps femoris activity. Overall, our results may suggest that novices exhibited clear progress in learning, but they had not yet adopted motor strategies similar to those of experienced users after the three sessions. We suggest potential future directions to enhance motor adaptations to powered EXO in terms of both training protocols and human-EXO interfaces. 相似文献
977.
Kunle Ibukun Olatayo Paul T. Mativenga Annlizé L. Marnewick 《Journal of Industrial Ecology》2023,27(2):601-623
Despite the promotion of plastic recycling to sustainably manage plastic waste and advance the circular economy, existing plastic recycling systems globally are largely experiencing low performance and growth. To transition to world-class plastic material recycling and circularity, defining the metrics that impact the performance of a plastic recycling system is crucial. Bringing together existing literature, this study developed a conceptual framework, comprised of eight key performance metrics, for benchmarking recycling success or assessing the degree to which the performance of any plastic recycling system is optimal. Through a value chain approach, the specific performance metrics relevant to each stage of the plastic recycling system, their objectives, and the actors characterizing the system were analyzed in detail. Also, specific maturity models were developed to measure the performance of any plastic recycling system. This framework provides essential knowledge for related stakeholders to inform further development of plastic recycling and a circular economy. 相似文献
978.
Fernando Pulido Bastien Castagneyrol Francisco Rodríguez-Sánchez Yónatan Cáceres Adara Pardo Eva Moracho Johannes Kollmann Fernando Valladares Johan Ehrlén Alistair S. Jump Jens-Christian Svenning Arndt Hampe 《Global Ecology and Biogeography》2023,32(6):842-854
Aim
Range shifts are expected to occur when populations at one range margin perform better than those at the other margin, yet no global trend in population performances at range margins has been demonstrated empirically across a wide range of taxa and biomes. Here we test the prediction that, if impacts of ongoing climate change on performance in marginal populations are widespread, then populations from the high-latitude margin (HLM) should perform as well as or better than central populations, whereas low-latitude margin (LLM) populations should perform worse.Location
Global.Time period
1995–2019.Major taxa studied
Plants and animals.Methods
To test our prediction, we used a meta-analysis to quantify empirical support for asymmetry in the performance of high- and low-latitude margin populations compared to central populations. Performance estimates (survival, reproduction, or lifetime fitness) for populations occurring in their natural environment were derived from 51 papers involving 113 margin-centre comparisons from 54 species and 705 populations from the Americas, Europe, Africa and Australia. We then related these performance differences to climatic differences among populations. We also tested whether patterns are consistent across taxonomic kingdoms (plants vs animals) and across realms (marine vs terrestrial).Results
Populations at margins performed significantly worse than central populations, and this trend was primarily driven by the low-latitude margin. Although the difference was of small magnitude, it was largely consistent across biological kingdoms and realms. Differences in performance were weakly (p = .08) related to the difference in average temperatures between central and marginal populations.Main conclusions
The observed asymmetry in performance in marginal populations is consistent with predictions about the effects of global climate change, though further research is needed to confirm the effect of climate. It indicates that changes in demographic rates in marginal populations can serve as early-warning signals of impending range shifts. 相似文献979.
Øystein H. Opedal Rocío Pérez-Barrales Vinícius L. G. Brito Nathan Muchhala Miquel Capó Agnes Dellinger 《American journal of botany》2023,110(6):e16200
Pollen plays a key role in plant reproductive biology. Despite the long history of research on pollen and pollination, recent advances in pollen-tracking methods and statistical approaches to linking plant phenotype, pollination performance, and reproductive fitness yield a steady flow of exciting new insights. In this introduction to the Special Issue “Pollen as the Link Between Phenotype and Fitness,” we start by describing a general conceptual model linking functional classes of floral phenotypic traits to pollination-related performance metrics and reproductive fitness. We use this model as a framework for synthesizing the relevant literature, highlighting the studies included in the Special Issue, and identifying gaps in our understanding and opportunities for further development of the field. The papers that follow in this Special Issue provide new insights into the relationships between pollen production, presentation, flower morphology, and pollination performance (e.g., pollen deposition onto stigmas), the role of pollinators in pollen transfer, and the consequences of heterospecific pollen deposition. Several of the studies demonstrate exciting experimental and analytical approaches that should pave the way for continued work addressing the intriguing role of pollen in linking plant phenotypes to reproductive fitness. 相似文献
980.
Gel-filtration-purified R-phycoerythrin and phycocyanin from the filamentous phase of Bangia atropurpurea were subjected to high performance anion exchange chromatographic separation. The purified R-phycoerythrin was a mixture of three charged-isomers that were well resolved in a Bio-Gel MA7P column by elution with a NaCl gradient in phosphate buffer. These were tentatively called phycoerythrin charged-isomers and were also separable from phycocyanin and allophycocyanin by the same system. Hence, it is suggested that the ion exchange chromatographic and the sample preparation methods presented in this report can be used to distinguish the various natures of biliproteins in red algae in addition to the polyacrylamide gel electrophoresis technique. Isomers of R-phycoerythrin have the same absorption and emission spectra and were studied for their subunit compositions by a PRP-3 reverse phase column chromatography. All three charged-isomers have the same and subunits in common but differ in their subunit. 相似文献