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   8915篇
  免费   822篇
  国内免费   901篇
  2023年   207篇
  2022年   168篇
  2021年   260篇
  2020年   293篇
  2019年   390篇
  2018年   289篇
  2017年   298篇
  2016年   327篇
  2015年   289篇
  2014年   397篇
  2013年   596篇
  2012年   330篇
  2011年   400篇
  2010年   296篇
  2009年   415篇
  2008年   475篇
  2007年   493篇
  2006年   452篇
  2005年   424篇
  2004年   372篇
  2003年   358篇
  2002年   305篇
  2001年   273篇
  2000年   222篇
  1999年   202篇
  1998年   195篇
  1997年   148篇
  1996年   132篇
  1995年   142篇
  1994年   122篇
  1993年   97篇
  1992年   112篇
  1991年   109篇
  1990年   80篇
  1989年   75篇
  1988年   68篇
  1987年   70篇
  1986年   38篇
  1985年   97篇
  1984年   101篇
  1983年   56篇
  1982年   98篇
  1981年   83篇
  1980年   71篇
  1979年   59篇
  1978年   29篇
  1977年   30篇
  1976年   30篇
  1975年   16篇
  1973年   23篇
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
1.
2.
In the estuaries near Falmouth (Cornwall, UK) levels of dissolved copper and zinc are high, due to drainage of copper and tin mines. Phytoplankton species composition in the autumn of 1989 deviated in the metal-contaminated Restronguet Creek from that in other estuarine branches,viz. Fal, Tresillian and Percuil. In the riverine part of Restronguet Creek (Carnon River)Euglena mutabilis, known as an acidophilic (pH 3) metal-resistant flagellate, occurred at micromolar Cu and Zn, whereas in the clean riversChlamydomonas sp. andOocystis sp. occurred at nanomolar Cu and Zn. An ordination analysis revealed the following patterns in Cu, Zn and phytoplankton species composition in the poly- and euhaline waters. Seston-bound Zn and dissolved Zn were in equilibrium,Katodinium rotundatum is Zn-tolerant, andSkeletonema costatum occurred in water with high contents of Cu in seston. However, none of the variables in these patterns correlated at a significant level (p>0.05). The results show that algae-metal interactions are complicated, and that statistical correlations foundin situ need experimental verification.Communication no. 542 of the Delta Institute for Hydrobiological Research.  相似文献   
3.
The amount of variation in species composition among sampling units or beta diversity has become a primary tool for connecting the spatial structure of species assemblages to ecological processes. Many different measures of beta diversity have been developed. Among them, the total variance in the community composition matrix has been proposed as a single‐number estimate of beta diversity. In this study, I first show that this measure summarizes the compositional variation among sampling units after nonlinear transformation of species abundances. Therefore, it is not always adequate for estimating beta diversity. Next, I propose an alternative approach for calculating beta diversity in which variance is substituted by a weighted measure of concentration (i.e., an inverse measure of evenness). The relationship between this new measure of beta diversity and so‐called multiple‐site dissimilarity measures is also discussed.  相似文献   
4.
Pollination ofDianthus gratianopolitanus was studied in a population of the Swiss Jura mountains. Pollinators of this plant species are reported here for the first time. The flowers were not only visited by butterflies as postulated in the literature, but also by diurnal hawkmoths (Macroglossum stellatarum) and by diurnal and nocturnal noctuid moths. — Nectar is sucrose-dominant, the sugar concentration is moderate but the amino acid concentration is high. Nectar characteristics correspond well with the syndrome ofLepidoptera-pollinated flowers. — Field observations and flower characters (colour, range of the calyx length) suggest thatDianthus gratianopolitanus is an intermediate species in the transition of butterfly to moth pollination. — Lack of reproductive success inDianthus gratianopolitanus can not be attributed to lack of suitable pollinators.  相似文献   
5.
Inducible plant defense is a beneficial strategy for plants, which imply that plants should allocate resources from growth and reproduction to defense when herbivores attack. Plant ecologist has often studied defense responses in wild populations by biomass clipping experiments, whereas laboratory and greenhouse experiments in addition apply chemical elicitors to induce defense responses. To investigate whether field ecologists could benefit from methods used in laboratory and greenhouse studies, we established a randomized block‐design in a pine‐bilberry forest in Western Norway. We tested whether we could activate defense responses in bilberry (Vaccinium myrtillus) by nine different treatments using clipping (leaf tissue or branch removal) with or without chemical treatment by methyljasmonate (MeJA). We subsequently measured consequences of induced defenses through vegetative growth and insect herbivory during one growing season. Our results showed that only MeJA‐treated plants showed consistent defense responses through suppressed vegetative growth and reduced herbivory by leaf‐chewing insects, suggesting an allocation of resources from growth to defense. Leaf tissue removal reduced insect herbivory equal to the effect of the MeJa treatments, but had no negative impact on growth. Branch removal did not reduce insect herbivory or vegetative growth. MeJa treatment and clipping combined did not give an additional defense response. In this study, we investigated how to induce defense responses in wild plant populations under natural field conditions. Our results show that using the chemical elicitor MeJA, with or without biomass clipping, may be a better method to induce defense response in field experiments than clipping of leaves or branches that often has been used in ecological field studies.  相似文献   
6.
从最高人民法院的一个典型案例出发,探讨药物组合物封闭式权利要求保护范围的解读及其专利侵权判定标准,比较其他国家 的相关规定和判例,并对药物组合物封闭式权利要求的专利授权、确权审查、侵权判定以及申请文件撰写技巧提出见解,以供借鉴和参考。  相似文献   
7.
华北地区针叶林下凋落物层化学性质的研究   总被引:10,自引:0,他引:10  
森林凋落物中贮积了大量的有机、无机养分物质,它是森林土壤自然肥力的重要来源之一。在森林生态系统中,养分物质的内部循环主要是通过凋落物来实现的。在分解、转化过  相似文献   
8.
Fluorescent biosensors are powerful tools for the detection of biochemical events inside cells with high spatiotemporal resolution. Biosensors based on fluorescent proteins often suffer from issues with photostability and brightness. On the other hand, hybrid, chemical–genetic systems present unique opportunities to combine the strengths of synthetic, organic chemistry with biological macromolecules to generate exquisitely tailored semisynthetic sensors.  相似文献   
9.
Strong selection to secure paternity in polyandrous species leads to the evolution of numerous chemicals in the male's seminal content. These include antiaphrodisiac pheromones, which are transmitted from the male to the female during mating to render her unattractive to subsequent males. An increasing number of species have been shown to use these chemicals. Herein, I examine the taxonomic distribution of species using antiaphrodisiac pheromones, the selection pressures driving their evolution in both males and females, and the ecological interactions in which these pheromones are involved. The literature review shows a highly skewed distribution of antiaphrodisiac use; all species currently known to use them are insects with the exception of the garter snakes Thamnophis sirtalis parietalis and T. radix. Nonetheless, many taxa have not yet been tested for the presence of antiaphrodisiacs, in groups both closely and distantly related to species known to express them. Within the Insecta, there have been multiple cases of convergent evolution of antiaphrodisiac pheromones using different chemical compounds and methods of transmission. Antiaphrodisiacs usually benefit males, but their effect on females is variable as they can either prevent them from mating multiple times or help them reduce male harassment when they are unreceptive. Some indirect costs of antiaphrodisiacs also impact both males and females, but more research is needed to determine how general this pattern is. Additional research is also important to understand how antiaphrodisiacs interact with the reproductive biology and sexual communication in different species.  相似文献   
10.
Abstract. Desiccant and soap pest control products were tested against Ixodes scapularis nymphs on sod, pachysandra, landscaping stones and pinebark wood-chips in a laboratory study. High mortality (91–100%) was obtained with the silica-based desiccant Drione and Safer's insecticidal soap (SIS) treatments except for SIS treated woodchips. SIS and Drione contain 0.2 and 1% pyrethrins, respectively. SIS may be absorbed by the woodchips and not picked up by the nymphs as mortality decreased to 78%. Diatomaceous earth and Dri-die reduced nymphs by only 10–41%; neither desiccant contains insecticides. Nymphal mortality increased 15–17% after 20 or 100 ml of water was sprayed over sod plugs treated with SIS or Drione. The addition of isopropyl alcohol (ROH) to SIS increased the efficacy of SIS against nymphal I. scapularis on treated sod plugs but not on treated landscaping substrates. Drione, SIS, SIS-ROH and chlorpyrifos 50WP treated landscaping stones and pinebark woodchips resulted in 88–95% and 72–96% nymphal mortality, respectively. Unexpectedly, these substrates and treatments affected tick movement as well. Effective pest control products applied to xeric landscaping substrates present in maintained beds and borders near wooded areas could deter tick movement and provide significant levels of tick control.  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