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   6811篇
  免费   771篇
  国内免费   488篇
  2024年   11篇
  2023年   124篇
  2022年   126篇
  2021年   283篇
  2020年   398篇
  2019年   509篇
  2018年   382篇
  2017年   273篇
  2016年   285篇
  2015年   287篇
  2014年   364篇
  2013年   557篇
  2012年   303篇
  2011年   286篇
  2010年   279篇
  2009年   328篇
  2008年   355篇
  2007年   307篇
  2006年   295篇
  2005年   267篇
  2004年   223篇
  2003年   197篇
  2002年   202篇
  2001年   150篇
  2000年   109篇
  1999年   129篇
  1998年   113篇
  1997年   110篇
  1996年   63篇
  1995年   92篇
  1994年   79篇
  1993年   60篇
  1992年   62篇
  1991年   51篇
  1990年   40篇
  1989年   39篇
  1988年   36篇
  1987年   40篇
  1986年   31篇
  1985年   42篇
  1984年   28篇
  1983年   14篇
  1982年   30篇
  1981年   19篇
  1980年   19篇
  1979年   15篇
  1978年   11篇
  1977年   8篇
  1976年   14篇
  1973年   6篇
排序方式: 共有8070条查询结果,搜索用时 31 毫秒
101.
动物群落是构成城市绿地生态系统的关键要素,声景作为野生动物重要的生态信息,掌握其时空变化及其影响因素,对于指导城市绿地景观设计与生物多样性保护具有重要意义。本文以Web of Science数据库的核心合集2005–2022年收录的67篇研究文献为对象,综合梳理与分析了城市绿地动物声景的时空模式及其驱动因素。城市绿地动物声景在空间上表现出环境空间梯度和植被空间结构的差异,动物声音多样性随海拔、纬度、城市化程度的降低以及植被类型和高度的增加呈现升高趋势。时间尺度呈现出昼夜、季节和年度变化差异,表现为鸟类在黎明和黄昏合唱、昆虫和两栖动物在夜间鸣叫以及季节性和年度性发声规律等。影响城市动物声景模式的因素主要包括植被、环境、人为干扰和动物自身驱动等。动物声景作为当前声景生态学研究的热点之一,面临大时空尺度演变规律研究不足、动物声景分析有限等挑战,建议未来着重开展多时空尺度变化规律研究、创新动物声景分析方法、定量解析影响因素及其响应机制、建立全球动物声景数据库等。  相似文献   
102.
No-salt flowthrough hydrophobic interaction chromatography (HIC) has been shown to effectively remove process and product-related impurities from bioprocess streams. In this publication, a panel of six antibodies has been used to demonstrate operating principles for the application of no-salt flowthrough HIC in antibody purification processes. The results indicate that no-salt flowthrough HIC provides robust aggregate clearance across operating conditions including flow rate, and variations in resin ligand density. Additionally, HMW reduction has an optimal pH range relative to the isoelectric point of each molecule and high molecular weight (HMW) reduction can be improved by altering the total protein load and/or HMW concentration to drive binding of high molecular weight species to the resin.  相似文献   
103.
摘要 目的:探讨超声引导下多点髂筋膜间隙阻滞联合不同剂量右美托咪定在老年全髋关节置换术(THA)中的应用效果。方法:选取2019年11月-2022年12月北京积水潭医院贵州医院择期行THA的老年患者120例,按照随机数字表法分为A组(n=40)、B组(n=40)、C组(n=40),三组均接受超声引导下多点髂筋膜间隙阻滞治疗,在此基础上,A组、B组、C组分别给予0.5 μg/kg、1.0 μg/kg、1.5 μg/kg右美托咪定注射液。对比三组临床指标、血流动力学指标、应激反应指标、脑神经损伤情况,观察三组围术期间不良反应发生情况。结果:B组、C组的阻滞消退时间、拔管时间长于A组,首次下床时间短于A组(P<0.05)。B组、C组的阻滞消退时间、拔管时间、首次下床时间对比无差异(P>0.05)。三组麻醉诱导后即刻(T1)~手术结束时(T3)时间点心率(HR)、平均动脉压(MAP)升高后降低(P<0.05)。B组、C组的T1~T3时间点HR、MAP均低于A组,且B组低于C组(P<0.05)。三组T3时间点皮质醇(Cor)、白介素-6(IL-6)、肿瘤坏死因子-α(TNF-α)升高(P<0.05)。B组、C组的T3时间点Cor、IL-6、TNF-α均低于A组,且B组低于C组(P<0.05)。B组、C组的T3时间点中枢神经特异性蛋白(S100β)均低于A组,且B组低于C组(P<0.05)。B组、C组的T3时间点脑源性神经生长因子(BDNF)均高于A组,且B组高于C组(P<0.05)。三组不良反应发生率组间对比无差异(P>0.05)。结论:超声引导下多点髂筋膜间隙阻滞联合1.0 μg/kg右美托咪定在THA中具有更好的麻醉效果,可缩短首次下床时间,减轻机体应激反应、血流动力学波动以及脑部神经损伤。  相似文献   
104.
Endemic species are important components of regional biodiversity and hold the key to understanding local adaptation and evolutionary processes that shape species distributions. This study investigated the biogeographic history of a relict conifer Pinus bungeana Zucc. ex Endl. confined to central China. We examined genetic diversity in P. bungeana using genotyping-by-sequencing and chloroplast and mitochondrial DNA markers. We performed spatial and temporal inference of recent genetic and demographic changes, and dissected the impacts of geography and environmental gradients on population differentiation. We then projected P. bungeana's risk of decline under future climates. We found extremely low nucleotide diversity (average π 0.0014), and strong population structure (global FST 0.234) even at regional scales, reflecting long-term isolation in small populations. The species experienced severe bottlenecks in the early Pliocene and continued to decline in the Pleistocene in the western distribution, whereas the east expanded recently. Local adaptation played a small (8%) but significant role in population diversity. Low genetic diversity in fragmented populations makes the species highly vulnerable to climate change, particularly in marginal and relict populations. We suggest that conservation efforts should focus on enhancing gene pool and population growth through assisted migration within each genetic cluster to reduce the risk of further genetic drift and extinction.  相似文献   
105.
The aims of this study were to investigate the diet and relative abundance of fruit bats in a lowland Malaysian rain forest and to test the hypothesis that the local assemblage structure of fruit bats varies significantly over time in relation to the availability of food. In total, 352 fruit bats of eight species were captured during 72,306 m2 mist‐net hours of sampling between February 1996 and September 1999. Three species of fruit bats (Balionycteris maculita, Chironax melanocephalus, and Cynopterus brachyotis) that fed on a wide range of “steady state” and “big bang” food resources were captured continuously throughout the study period, with no significant variation in capture rates over time. In contrast, five species that fed exclusively or almost exclusively on “big bang” food resources were sampled intermittently, with significant temporal variation in the capture rates of two species (Cynopterus horsfieldi and Megaerops ecaudatus). Significant variation in the capture rates of the remaining three species (Dyacopterus spadiceus, Eonycteris spelaea, and Rousettus amplexicaudatus) could not be detected due to small sample sizes. Since ephemeral “big bang” food resources were only sporadically available within the study area and were associated with large canopy trees and strangler figs, these results suggest that food abundance, or the availability of specific food items, may be important factors limiting local fruit bat species diversity in old‐growth Paleotropical rain forest. Thus, only three fruit bat species were locally resident within the forest throughout the study period. Therefore, further studies on the ranging behavior and habitat requirements of Malaysian fruit bats are required to assess the adequacy of existing reserves and protected areas.  相似文献   
106.
Abstract The recent development of automatically operating, inexpensive vertical-looking radar (VLR) for entomological purposes has made it practical to carry out routine, automated monitoring of insect aerial migration throughout the year. In this paper we investigate whether such radars might have a role in monitoring and forecasting schemes designed to improve the management of the Brown Planthopper (BPH), Nilaparvata lugens , and of associated rice pest species in China. A survey of the literature revealed that these insects typically migrate at altitudes between 300 to 2 000 m above ground level, but calculations based on BPH radar scattering cross-sections indicated that the maximum altitude at which they individually produce signals analysable by current VLRs is only ˜ 240 m. We also show that coverage over most of the flight altitudes of BPH could be achieved by building a VLR using a wavelength of 8.8 mm instead of the 3.2 cm of existing VLR, but that such a radar would be expensive to build and to operate. We suggest that a more practical solution would be to use a 3.2 cm VLR as a monitor of the aerial movement of the larger species, from which the migration of rice pests in general might be inferred.  相似文献   
107.
Abstract: Six species of Nabidae (Heteroptera) were collected by standardized sweep net sampling in alfalfa fields in Thuringia, Germany, from 1993 to 1995: Nabis pseudoferus , N. ferus , N. brevis , N. major , Nabicula flavomarginata and Aptus mirmicoides . Colonization of a newly cultivated field was studied over a 3-year period. The density of all the studied nabid species was low (less than five individuals per 100 sweeps) and not related to time since colonization started, or to the distance from the margin of the field. Macropterous species were able to colonize the whole field within one season. The density of one macropterous species, N. pseudoferus , varied between the years of study and was mainly affected by the harvest regime. The brachypterous species reached the margin within one season but for density it took three seasons to reach satiated values also in the centre of the field. The abundance of the brachypterous N. brevis was significantly different both between years and sampling sites. This indicates the importance of the surroundings on the succession of this species. Nabis major , a fully winged species, showed a migration pattern intermediate to macropterous and brachypterous nabids. These results suggest that the total abundance of nabid predators cannot be predicted by time or distance from the expansion source (shelter belts). The abundance of brachypterous nabid individuals can be predicted from time since colonization but is best analysed at the species level.  相似文献   
108.
实验分别在出生后4周龄的幼年和成年鲁氏菊头蝠(Rhinolophusrouxi)上进行。使用移动声刺激装置,高频喇叭可在动物头部前方水平方向180度、垂直方向60度的范围内移动。玻璃微电极记录单个神经元的听反应。实验考察了幼年和成年动物下丘神经元的听空间特性,共观察了301个神经元,其中幼年动物148个,成年动物153个。结果表明,4周龄的幼年动物下丘听神经元已表现出方向敏感性,即每个听神经元均有一个特定的最佳反应中心和反应域。但神经元听反应中心在听空间的分布相当弥散,大多数位于对侧水平方向20—80度、垂直方向上下15度范围内。而成年动物听神经元反应中心的分布则相当集中,局限地分布于对侧水平方向28-50度,垂直方向0—10度范围内,两者构成明显差异。  相似文献   
109.
Summary Malignant features in three glioma cell lines were studied in four defined media of various complexity. The cell lines D37MG, D54MG, and GaMG were able to grow in monolayer culture in all media examined, and as multicellular tumor spheroids in the two most nutrient-rich media. In the defined media, none of the cell lines were able to migrate in a migration assay on poly-D-lysine-coated plastic surfaces. Flow cytometric analysis of the GaMG cell line demonstrated no medium-dependent selection of subclones of glioma cells in spheroids cultured for 30 d. Morphological diversity of spheroids varied according to the supplementation of the media. The capacity of glioma cells to invade cellular rat brain aggregates was intact in the media examined. However, glioma migration was severely inhibited by the lack of specific serum components. This study demonstrates that glioma growth and invasion was heterogenously preserved in the defined media used. Depending on the assay to be used in the study of glioma cell behavior, the degree of medium supplementation has to be considered.  相似文献   
110.
Summary Ladsin is a laminin-like cell-adhesive scatter factor with potent cell motility-stimulating ability and was purified from serum-free conditioned medium of a malignant human gastric adenocarcinoma cell line STKM-1. To test its possible role in tumor angiogenesis, we investigated its effect on primary culture of endothelial cells (human umbilical vein endothelial cells) and endothelial cell line ECV304 in this study. Cell adhesion and motility effects of ladsin were observed in both types of endothelial cells. In cell-attachment assay, ladsin interacted with integrin α3β1 that was expressed on the endothelial cell surface. In Boyden chambers, ladsin stimulated both directed and random migration of ECV304 cells. Ladsin induced repair of artificial wounds generated in ECV304 cell monolayers by stimulating cell migration. Ladsin did not affect the growth rate of ECV304 cells at a low cell density but significantly increased the saturation cell density. These results suggest that ladsin may be involved in the adhesion and migration of endothelial cells under some physiological and pathological conditions.  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