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   11152篇
  免费   886篇
  国内免费   981篇
  13019篇
  2024年   20篇
  2023年   178篇
  2022年   216篇
  2021年   319篇
  2020年   326篇
  2019年   382篇
  2018年   322篇
  2017年   425篇
  2016年   474篇
  2015年   454篇
  2014年   471篇
  2013年   642篇
  2012年   413篇
  2011年   430篇
  2010年   401篇
  2009年   599篇
  2008年   568篇
  2007年   657篇
  2006年   506篇
  2005年   453篇
  2004年   374篇
  2003年   358篇
  2002年   317篇
  2001年   314篇
  2000年   311篇
  1999年   278篇
  1998年   247篇
  1997年   231篇
  1996年   213篇
  1995年   234篇
  1994年   189篇
  1993年   182篇
  1992年   202篇
  1991年   166篇
  1990年   140篇
  1989年   130篇
  1988年   97篇
  1987年   116篇
  1986年   86篇
  1985年   96篇
  1984年   73篇
  1983年   39篇
  1982年   71篇
  1981年   54篇
  1980年   60篇
  1979年   46篇
  1978年   37篇
  1977年   22篇
  1976年   29篇
  1973年   20篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
171.
The sex pheromone of Phyllonorycter ringoniella (Matsumura) (Lepidoptera: Gracillariidae) has been identified to be a blend of (Z)‐10‐tetradecenyl acetate (Z10‐14:OAc) and E4,Z10‐tetradecadienyl acetate (E4,Z10‐14:OAc) in Japan, Korea, and China. However, the commercial product based on previous results is not attractive enough to be used for monitoring and controlling apple leafminer populations in the field. We re‐investigated the attractiveness of the two pheromone components, singly and in blends, in apple orchards in Shangdong and Shaanxi, the main apple‐growing provinces in China. Our results revealed that Z10‐14:OAc alone was not attractive to Pringoniella male moths in the field, but E4,Z10‐14:OAc alone not only was strongly attractive but caught more males than any of the blends of Z10‐14:OAc and E4,Z10‐14:OAc tested. The most attractive blend ratios differed slightly for the two locations. No clear dose–response relationship was obtained for the 2:8 blend of Z10‐14:OAc and E4,Z10‐14:OAc. However, the dose–response field study of E4,Z10‐14:OAc alone showed that 1 mg per lure achieved the highest moth catch. These findings differ from the previous report of the best pheromone blend in China. Our data showed that E4,Z10‐14:OAc is the major component of the pheromone of Pringoniella.  相似文献   
172.
通过线粒体控制区序列的分析,研究采自中国南海及东海5个群体102尾细鳞鯻的遗传多样性。发现在962 bp序列中有205个变异位点,其中135个为简约信息位点,共定义102个单倍型。中国近海细鳞鯻总体呈现出较高的遗传多样性特征(Hd=1.000,Pi=0.022),其中博鳌最高(Hd=1.000,Pi=0.028),平潭最低(Hd=1.000,Pi=0.014)。不同地理群体间无明显分化,基因交流频繁(Fst=-0.014—0.041,P0.05);中性检验均为显著负值,推测在16.9万年—5.06万年前,即中-晚更新世出现种群扩张。系统邻接树和单倍型网络图均出现3个显著分化的谱系(谱系间Fst=0.508—0.698,P0.001;净遗传距离Da=0.024—0.031),且各谱系中均有不同地理来源的群体。3个谱系间分歧时间大约在1.07百万年—0.24百万年前,推测可能是更新世冰期边缘海的出现导致群体隔离而产生分化。谱系A(Lineage A)包含85.3%的个体,其总体遗传多样性较高(Hd=1.000,Pi=0.012),其中平潭最高(Hd=1.000,Pi=0.014),合浦最低(Hd=1.000,Pi=0.010);群体间Fst在-0.021—0.068之间,P0.005;AMOVA分析显示只有1.97%的变异来自于种群间,表明群体间也无明显分化;中性检验均为显著负值,推测在25.4万年—7.6万年前出现种群扩张。中国近海细鳞鯻主要受到中-晚更新世海侵和海退的影响而出现种群扩张使得谱系间发生二次接触,最终形成具有显著谱系结构但无地理分化的情况。  相似文献   
173.
高会  翟水晶  孙志高  何涛  田莉萍  胡星云 《生态学报》2018,38(17):6136-6142
2016年1—12月,选择闽江河口鳝鱼滩的短叶茳芏湿地、互花米草湿地以及二者的交错带湿地为研究对象,采用定位研究方法探讨了互花米草入侵影响下湿地土壤有效硅含量的时空变化特征。结果表明:互花米草入侵影响下3块湿地土壤有效硅含量随时间推移整体呈波动上升趋势;互花米草入侵显著提高了鳝鱼滩湿地30—60 cm土层土壤有效硅含量(P0.01),与短叶茳芏湿地相比,交错带湿地和互花米草湿地30—60 cm土层土壤有效硅含量分别增加了8.56%和19.97%,逐步线性回归分析表明土温和电导是影响其变化的重要因素(P0.01)。研究互花米草入侵影响下湿地土壤有效硅含量的变化特征,对于揭示湿地生态系统生源要素硅生物地球化学循环过程以及互花米草入侵及其扩张机制具有重要意义。  相似文献   
174.
Summary The last decade has witnessed successful applications of plant tissue culture techniques in several crops. During that same period, studies in plant molecular genetics have also grown exponentially. Molecular markers (isozymes, RFLPs, and PCR-based markers such as RAPDs) are now used to study many of the current limitations of tissue culture. They have been used to investigate mechanisms that underlie somaclonal variation in the nuclear, mitochondrial, and chloroplast genomes. One recurrent problem with several tissue culture systems has been the difficulty of determining the origin of regenerants. Molecular markers represent powerful tools to determine precisely the origin of plants derived from microspore or anther culture, protoplast fusion, and other tissue culture studies where this information is important. With improvements in tissue culture techniques, populations of doubled haploid lines have been produced in several major crop species. Doubled haploid populations have proven useful in the production of molecular maps and in tagging important agronomic traits. This review describes the use of molecular markers to address fundamental and practical questions of plant tissue culture, and discusses the potential of improvements in molecular techniques and new molecular markers such as SCAR and STS along with high-resolution mapping strategies.  相似文献   
175.
Fungal spore populations in the outdoor and indoor atmosphere of Ismailia have been studied during the period from March 1992 to May 1993. A total of 23 350 cfu and 73 species were recorded,Cladosporium cladosporioides, Aureobasidium pullulans andAspergillus flavus were the most abundant. The indoor and outdoor mycoflora showed marked quantitative and qualitative differences. In view of count, recorded species could be categorized into three groups as follows: (a) species showing higher counts in out- than indoor, (b) species showing the opposite trend i.e. lower counts in out-door than indoor, (c) species showing approximately equal counts in out- and indoor. Regarding seasonal periodicity, March and either September or October showed the highest count for both normal fungal flora (NFF) and opportunistic fungal flora (OFF). While January and July showed the lowest count of them both, May but not July was the lowest as for outdoor NFF.  相似文献   
176.
Abstract: Chromosomal DNA from Bacillus subtilis , bound on the clay minerals, montmorillonite (Wyoming (W) and Apache County (Ap)) and kaolinite (K), was subjected to the random amplified polymorphic DNA (RAPD) technique. DNA bound on the clays was not amplified with 0.625, 1.875, 6.25, and 12.5 U of Taq DNA polymerase, but amplification occurred when the clay-DNA complexes were diluted 10- and 20-fold or when 21 U of Taq DNA polymerase was added. DNA desorbed from the Ap-DNA and K-DNA equilibrium complexes was amplified with 0.625 U of Taq DNA polymerase, whereas amplification of DNA desorbed from the W-DNA complex occurred only after a 10-fold dilution or when 1.875 U of Taq DNA polymerase was used. These observations indicate that clay minerals differentially affect the amplification process, probably by inhibiting the activity of Taq DNA polymerase.  相似文献   
177.
Vascular epiphyte species exclusively, or at least primarily, germinate and grow on other plants without contact with the soil and, in contrast to mistletoes, without parasitizing their hosts. The last review of the systematic distribution of this diverse group of plants dates back more than two decades. The present study pursues three major goals: (1) it critically discusses conceptual problems arising from the distinction of epiphytes from non‐epiphytes; (2) it presents a compilation of epiphytic diversity derived from a vast number of sources; and (3) it arranges epiphyte diversity in an up‐to‐date taxonomic framework. The resulting compilation, which identifies 27 614 species of vascular epiphytes (including primary hemiepiphytes) representing 913 genera in 73 families, or approximately 9% of extant vascular plant diversity, is meant to be an important tool for studies on the ecology and evolution of epiphytes, but also for comparative studies with a focus on other life forms. © 2013 The Linnean Society of London  相似文献   
178.
为了解黄渤海甲壳类的分类多样性特征, 我们统计了2010-2015年中国水产科学研究院黄海水产研究所调查捕获的黄渤海甲壳类(软甲纲: 十足目与口足目)物种名录。结合历史文献, 进一步系统整理得到黄渤海甲壳类物种总名录。基于这2个名录, 应用分类阶元包含指数(the inclusion index at taxonomic level, TINCLi)、平均分类差异指数(average taxonomic distinctness index, Δ+)和分类差异变异指数(variation in taxonomic distinctness index, Λ+)研究了其分类多样性特征。结果显示: 2010-2015年调查名录中, 甲壳类共93种, 隶属于2目39科66属, 其中10种为新分布种; 对虾科、藻虾科、长臂虾科、梭子蟹科和弓蟹科的物种数最多, 合计占总物种数的38.71%; TINCLi分别为1.41种/属和2.38种/科; Δ+和Λ+分别为50.25和35.20。总名录中, 甲壳类共228种, 隶属于2目53科123属, 其中藻虾科、豆蟹科、对虾科、弓蟹科和鼓虾科的物种数最多, 合计占总物种数的30.70%; TINCLi分别为1.85种/属和4.30种/科, Δ+和Λ+分别为50.18和30.87。对虾科的相对丰富度指数(the relative richness index, Rr)最高(100), 其次是梭子蟹科(71.43)和长臂虾科(62.50), 豆蟹科最低(6.25)。黄渤海甲壳类的平均分类差异指数(Δ+)明显小于鱼类(P < 0.05)。2010-2015年调查的Δ+计算值高于理论值, 且在理论值的95%置信区间内, 说明黄渤海甲壳类群落正处在中等程度的干扰中。  相似文献   
179.
Alicia Valdés  Johan Ehrlén 《Oikos》2018,127(6):825-833
Variation in the intensity of plant–animal interactions over different spatial scales is widespread and might strongly influence fitness and trait selection in plants. Differences in traits among plant individuals have been shown to influence variation in interaction intensities within populations, while differences in environmental factors and community composition are shown to be important for variation over larger scales. However, little is still known about the relative importance of the local environmental context vs. plant traits for the outcome of interactions within plant populations. We investigated how oviposition by the seed‐predator butterfly Phengaris alcon on its host plant Gentiana pneumonanthe was related to host plant traits and to local environmental variation, as well as how oviposition patterns translated into effects on host plant fruit set. We considered the local environmental context in terms of height of the surrounding vegetation and abundance of the butterfly's second host, Myrmica ants. The probability of oviposition was higher in plants that were surrounded by lower vegetation, and both the probability of oviposition and the number of eggs increased in early‐flowering and tall plants with many flowers in the three study populations. Flowering phenology, shoot height and flower production were, in turn, related to higher surrounding vegetation. Myrmica abundance was correlated with vegetation height, but had no effect on oviposition patterns. Oviposition and subsequent seed predation by the caterpillars strongly reduced host plant fruit set. Our results show that plant–animal interactions are context‐dependent not only because the context influences the abundance or the behavior of the animal interactor, but also because it influences the expression of plant traits that affect the outcome of the interaction. The results also demonstrate that heterogeneity in environmental conditions at a very local scale can be important for the outcomes of interactions.  相似文献   
180.
Chromosome number variations play an important role in the genus Medicago. In addition to polyploidy there are cases of dysploidy as evidenced by two basic numbers, x = 8 and x = 7, the latter limited to five annual species having 2n = 14. Annuals are diploid with the exception of Medicago scutellata and Medicago rugosa which have 2n = 30 and are considered the result of crosses between the 2n = 16 and 2n = 14 species. However, this hypothesis has never been tested. This study was carried out to investigate the 2n = 14 and 2n = 30 karyotypes and verify the allopolyploid origin of M. scutellata and M. rugosa. Fluorescence in situ hybridization (FISH) of rDNA probes and genomic in situ hybridization (GISH) were performed. FISH showed that all five diploids with 2n = 14 have one pair of 45S and one pair of 5S rDNA sites. M. scutellata displayed four sites of 45S and four sites of 5S rDNA, while in M. rugosa only one pair of each of these sites was found. GISH did not produce signals useful to identify the presumed progenitors with 14 chromosomes. This result suggests alternative evolutionary pathways, such as the formation of tetraploids (2n = 32) and subsequent dysploidy events leading to the chromosome number reduction.  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