首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   3765篇
  免费   302篇
  国内免费   163篇
  2024年   5篇
  2023年   82篇
  2022年   75篇
  2021年   126篇
  2020年   133篇
  2019年   140篇
  2018年   128篇
  2017年   148篇
  2016年   129篇
  2015年   141篇
  2014年   193篇
  2013年   285篇
  2012年   129篇
  2011年   149篇
  2010年   144篇
  2009年   205篇
  2008年   205篇
  2007年   206篇
  2006年   173篇
  2005年   161篇
  2004年   143篇
  2003年   105篇
  2002年   108篇
  2001年   90篇
  2000年   79篇
  1999年   71篇
  1998年   61篇
  1997年   47篇
  1996年   66篇
  1995年   52篇
  1994年   56篇
  1993年   42篇
  1992年   50篇
  1991年   38篇
  1990年   35篇
  1989年   25篇
  1988年   16篇
  1987年   24篇
  1986年   13篇
  1985年   25篇
  1984年   25篇
  1983年   13篇
  1982年   20篇
  1981年   21篇
  1980年   11篇
  1979年   11篇
  1978年   6篇
  1977年   5篇
  1976年   5篇
  1974年   3篇
排序方式: 共有4230条查询结果,搜索用时 15 毫秒
101.
The incredible diversity of plant mating systems has fuelled research in evolutionary biology for over a century. Currently, there is broad concern about the impact of rapidly changing pollinator communities on plant populations. Very few studies, however, examine patterns and mechanisms associated with multiple paternity from cross‐pollen loads. Often, foraging pollinators collect a mixed pollen load that may result in the deposition of pollen from different sires to receptive stigmas. Coincident deposition of self‐ and cross‐pollen leads to interesting mating system dynamics and has been investigated in numerous species. But, mixed pollen loads often consist of a diversity of cross‐pollen and result in multiple sires of seeds within a fruit. In this issue of Molecular Ecology, Rhodes, Fant, and Skogen ( 2017 ) examine how pollinator identity and spatial isolation influence multiple paternity within fruits of a self‐incompatible evening primrose. The authors demonstrate that pollen pool diversity varies between two pollinator types, hawkmoths and diurnal solitary bees. Further, progeny from more isolated plants were less likely to have multiple sires regardless of the pollinator type. Moving forward, studies of mating system dynamics should consider the implications of multiple paternity and move beyond the self‐ and cross‐pollination paradigm. Rhodes et al. ( 2017 ) demonstrate the importance of understanding the roles that functionally diverse pollinators play in mating system dynamics.  相似文献   
102.
BOSCH, J., 1992. Floral biology and pollinators of three co-occurring Cistus species (Cistaceae). The pollination of three species of Cistus, Cistus albidus L., Cistus salvifolius L. and Cistus monspeliensis L. is studied. Insect visitors were censused and some were captured to analyse pollen distribution on their bodies. Floral phenology, breeding systems, some aspects of floral biology (pollen and nectar production, pollen-ovule ratios) and floral morphology were also studied for each species. Flowers of C. albidus produce more pollen and nectar than the other two species studied, and attract more insects, especially those with higher nutritional requirements. All three species are highly self-incompatible, but C. salvifolius and C. monspeliensis show some features that hinder xenogamy, and they have lower pollen-ovule ratios than C. albidus. As a result, this latter species apparently receives higher levels of cross-pollination, but at the cost of a higher investment to attract and reward reliable pollinators.  相似文献   
103.
Abstract

In this 12th issue of News from the Biological Stain Commission (BSC) under the heading of Regulatory affairs, the Biological Stain Commission’s International Affairs Committee presents information from the meetings of ISO/TC 212/WG 1 Quality and competence in the medical laboratory and ISO/TC 212/WG 3 In vitro diagnostic products both held on 2 – 3 June 2010, plus information on the second plenary meeting of ISO/TC 212 Clinical laboratory testing and in vitro diagnostic test systems held on 4 June 2010. All meetings took place in Seoul, Republic of Korea. Finally, information is provided concerning the 25th meeting of CEN/TC 140 In vitro diagnostic medical devices held on 23 June 2010 in Berlin, Germany.  相似文献   
104.
Despite the many advances in tissue engineering approaches, scientists still face significant challenges in trying to repair and replace soft tissues. Nature-inspired routes involving the creation of polymer-based systems of natural origins constitute an interesting alternative route to produce novel materials. The interest in these materials comes from the possibility of constructing multi-component systems that can be manipulated by composition allowing one to mimic the tissue environment required for the cellular regeneration of soft tissues. For this purpose, factors such as the design, choice, and compatibility of the polymers are considered to be key factors for successful strategies in soft tissue regeneration. More recently, polysaccharide-protein based systems have being increasingly studied and proposed for the treatment of soft tissues. The characteristics, properties, and compatibility of the resulting materials investigated in the last 10 years, as well as commercially available matrices or those currently under investigation are the subject matter of this review.  相似文献   
105.
106.
107.
108.
Phenotyping mouse model systems of human disease has proven to be a difficult task, with frequent poor inter‐ and intra‐laboratory replicability, particularly in behavioral domains such as social and cognitive function. However, establishing robust animal model systems with strong construct validity is of fundamental importance as they are central tools for understanding disease pathophysiology and developing therapeutics. To complete our studies of mouse model systems relevant to autism spectrum disorder (ASD), we present a replication of the main findings from our two published studies of five genetic mouse model systems of ASD. To assess the intra‐laboratory robustness of previous results, we chose the two model systems that showed the greatest phenotypic differences, the Shank3/F and Cntnap2, and repeated assessments of general health, activity and social behavior. We additionally explored all five model systems in the same framework, comparing all results obtained in this three‐yearlong effort using informatics techniques to assess commonalities and differences. Our results showed high intra‐laboratory replicability of results, even for those with effect sizes that were not particularly large, suggesting that discrepancies in the literature may be dependent on subtle but pivotal differences in testing conditions, housing enrichment, or background strains and less so on the variability of the behavioral phenotypes. The overall informatics analysis suggests that in our behavioral assays we can separate the set of tested mouse model system into two main classes that in some aspects lie on opposite ends of the behavioral spectrum, supporting the view that autism is not a unitary concept.  相似文献   
109.
摘要 目的:探讨经会阴实时三维盆底超声评估不同分娩方式对产后女性前腔室结构和盆膈裂孔的影响。方法:回顾性分析2015年1月至2019年10月在我院接受检查的100例产妇的诊治资料。根据分娩方式的不同,将患者分为经阴道分娩组(n=55)和剖宫产分娩组(n=45)。比较两组产妇在静息状态和Valsalva状态下的前腔室和盆膈裂孔超声参数。结果:在静息状态下,两组的膀胱颈位置、逼尿肌厚度、膀胱后角和尿道倾斜角相比无差异(P>0.05)。经阴道分娩组在Valsalva状态下的膀胱颈移动度和尿道旋转角均大于剖宫产分娩组,尿道内口漏斗形成率和膀胱膨出率均高于剖宫产分娩组(P<0.05)。在Valsalva状态下,经阴道分娩组的盆膈裂孔前后径、左右径、面积和周长均大于剖宫产分娩组(P<0.05);在静息状态下,两组的上述指标相比,差异均无统计学意义(P>0.05)。结论:应用经会阴实时三维盆底超声技术观察产妇前腔室结构和盆膈裂孔的参数变化,可评估产妇盆底功能受损的程度,经阴道分娩对其影响较大。  相似文献   
110.
The competitiveness and sustainability of low input cost dairy production systems are generally supported by efficient use of pasture in the diets. Therefore, pasture intake directly affects overall efficiency of these systems. We aimed to assess feeding and grazing management main factors that affect pasture dry matter intake (DMI) in commercial dairy farms during the different seasons of the year. Fortnightly visits to 28 commercial dairies were carried out between June 2016 and May 2017 to record production and price, supplement offered and price, pasture access time (PAT), herbage mass (HM) and allowance (HA). Only farms with the most contrasting estimated pasture DMI per cow (eDMI) were compared as systems with high (HPI; N = 8) or low (LPI; N = 8) pasture DMI. Despite a lower individual milk production in HPI than LPI (19.0 v. 23.3 ± 0.7 l/cow, P < 0.01), daily margin over feeding cost was not different between groups (3.07 v. 2.93 ± 0.15 U$S/cow for HPI and LPI, respectively). During autumn and winter, HPI cows ingested more pasture than LPI cows (8.3 v. 4.6 and 5.9 v. 2.9 ± 0.55 kg DM/cow per day, respectively, P < 0.01) although PAT, HM and HA were similar between groups. Both groups offered high supplementation levels during these seasons, even though greater in LPI than HPI (14.7 v. 9.7 ± 0.7 kg DM supplement/cow per day, respectively, P < 0.01). On the other hand, differences between groups for both pasture and supplement DMI were more contrasting during spring and summer (13.1 v. 7.3 ± 0.5 and 4.0 v. 11.4 ± 0.4 kg DM/cow per day for HPI and LPI, respectively, P < 0.01), with higher PAT in both seasons (P < 0.05) and higher HA during summer in HPI than LPI (P < 0.01). Unlike LPI, during these seasons HPI adjusted offered supplement according to HA, achieving a higher pasture eDMI and making more efficient use of available pastoral resource than LPI. As there was no grazing limiting condition for pasture harvesting in either group, the main factor affecting pasture DMI was a pasture by supplement substitution effect. These results reinforce the importance of an efficient grazing management, and using supplements to nutritionally complement pasture intake rather than as a direct way to increase milk production.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号