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   1464篇
  免费   96篇
  国内免费   293篇
  2023年   25篇
  2022年   34篇
  2021年   39篇
  2020年   46篇
  2019年   54篇
  2018年   43篇
  2017年   44篇
  2016年   50篇
  2015年   70篇
  2014年   60篇
  2013年   91篇
  2012年   63篇
  2011年   58篇
  2010年   54篇
  2009年   78篇
  2008年   100篇
  2007年   81篇
  2006年   93篇
  2005年   75篇
  2004年   62篇
  2003年   66篇
  2002年   67篇
  2001年   69篇
  2000年   52篇
  1999年   47篇
  1998年   25篇
  1997年   38篇
  1996年   28篇
  1995年   21篇
  1994年   33篇
  1993年   20篇
  1992年   25篇
  1991年   26篇
  1990年   14篇
  1989年   17篇
  1988年   16篇
  1987年   13篇
  1986年   16篇
  1985年   12篇
  1984年   10篇
  1982年   9篇
  1981年   3篇
  1980年   4篇
  1977年   1篇
  1970年   1篇
排序方式: 共有1853条查询结果,搜索用时 218 毫秒
91.
通过云南秋海棠属植物属下5个系统分类组的种类及外来园艺品种共109个组合的有性杂交试验,以系统分类组合、园艺分类组合(不同茎的形态类型组合)分别进行方差分析。结果表明:云南秋海棠属植物属下等级的系统分类组内、组间的有性杂交,以及不同茎的形态类型的有性杂交均无显著差异,亲和力较强而可育性高,可在云南秋海棠属植物属内进行广泛地远缘杂交; 云南产秋海棠属植物原种与外来园艺品种间的有性杂交亲和性弱而可育性低,需选择花粉粒和胚囊正常发育的外来园艺品种大量杂交,并采用切实可行的技术措施克服远缘杂交不育,以期培育新颖奇特的秋海棠属植物新品种。  相似文献   
92.
云南地方稻waxy基因序列多样性分析   总被引:1,自引:0,他引:1  
利用特异引物Wx-F/AG-2对来源于云南省16个州市64个县的252份地方稻种waxy基因中含有微卫星序列(CT)n和第一内含子的序列进行PCR扩增并测序。结果表明在268个碱基的序列内检测到4个变异位点:第一内含子上游56位处(CT)n存在(CT)10、(CT)11、(CT)12、(CT)14、(CT)16、(CT)17、(CT)18、(CT)19、(CT)20、(CT)21等10种变异;(CT)11、(CT)18、(CT)17、(CT)12、(CT)10等5种单倍型是云南优势类型,合计占供试材料的92.85%;籼稻以(CT)10、(CT)11、(CT)12为主,而粳稻以(CT)11、(CT)12、(CT)17和(CT)18为主;第一内含子+1位存在G/T变异,81.75%品种是G,T只出现在(CT)16、(CT)17、(CT)18和(CT)20的品种中,其频率在籼稻中为13.48%,粳稻20.86%,水稻16.17%,陆稻22.35%,粘稻10.47%,糯稻42.62%。+86-88位存在ATA/GTA/A--3种变异;+128位处存在(AATT)5和(AATT)6 2种变异。根据这4个变异位点,可将252个云南地方稻种归为16种单倍型,其中Wx4(32.54%)、Wx9(13.89%)、Wx12(12.7%)、Wx5(12.3%)、Wx1(8.33%)、Wx11(7.94%)是主要类型,合计87.7%,其他类型频率较低。籼/粳亚种、水/陆稻和粘/糯中存在单倍型种类和单倍型频率两方面的差异,籼稻/粳稻、水/陆稻和粘/糯稻各亚种或生态型均有独享的单倍型,共享单倍型频率也存在差异,表明亚种间或生态型间发生了一定的遗传分化。单倍型地理分布分析表明,临沧、普洱单倍型种类最丰富,以之为中心向外扩展,单倍型种类有减少的趋势,第一内含子+1位的T主要分布在临沧、普洱、西双版纳、德宏等南部地区。本研究揭示了云南地方稻种群体waxy基因的变异和分布特点。  相似文献   
93.
Individual specialization in diet or foraging behavior within apparently generalist populations has been described for many species, especially in polar and temperate marine environments, where resource distribution is relatively predictable. It is unclear, however, whether and how increased environmental variability – and thus reduced predictability of resources – due to global climate change will affect individual specialization. We determined the within‐ and among‐individual components of the trophic niche and the within‐individual repeatability of δ13C and δ15N in feathers and red blood cells of individual female southern rockhopper penguins (Eudyptes chrysocome) across 7 years. We also investigated the effect of environmental variables (Southern Annular Mode, Southern Oscillation Index, and local sea surface temperature anomaly) on the isotopic values, as well as the link between stable isotopes and female body mass, clutch initiation dates, and total clutch mass. We observed consistent red blood cell δ13C and δ15N values within individuals among years, suggesting a moderate degree of within‐individual specialization in C and N during the prebreeding period. However, the total niche width was reduced and individual specialization not present during the premolt period. Despite significant interannual differences in isotope values of C and N and environmental conditions, none of the environmental variables were linked to stable isotope values and thus able to explain phenotypic plasticity. Furthermore, neither the within‐individual nor among‐individual effects of stable isotopes were found to be related to female body mass, clutch initiation date, or total clutch mass. In conclusion, our results emphasize that the degree of specialization within generalist populations can vary over the course of 1 year, even when being consistent within the same season across years. We were unable to confirm that environmental variability counteracts individual specialization in foraging behavior, as phenotypic plasticity in δ13C and δ15N was not linked to any of the environmental variables studied.  相似文献   
94.
Correlations between morphological and genetic data provide evidence to delineate species or evolutionarily significant units, which then become the units to conserve in management plans. Here, we examine the distribution and genetic differentiation of two morphotypes of short‐finned pilot whale (Globicephala macrorhynchus) in the Pacific Ocean. Mitochondrial control region sequences from 333 samples were combined with 152 previously published sequences to describe genetic variability globally and population structure in the Pacific. Although genetic variability is low, we found strong differentiation at both broad and local levels across the Pacific. Based on genetics, two types are distributed throughout the Pacific, one predominantly in the eastern Pacific and the other in the western and central Pacific. In the eastern Pacific Ocean, no correlation was found between distribution and sea surface temperature. The two types have broad latitudinal ranges, suggesting their distributions are likely driven by more complex factors, such as prey distribution, rather than sea surface temperature.  相似文献   
95.
Thirty-three angiosperm pollen species are here reported from mid-Cretaceous deposits of the Kachaike Formation, Austral Basin, southern Argentina. Clavatipollenites is the most abundant angiosperm genus, with six well-defined morphological groups recognised on the basis of their reticulum morphology and sculpture. Pollen of eudicots are scarce, represented by tricolpate (Psilatricolpites spp. and Tricolpites spp.), tricolporoidate and tricolporate morphotypes (Dryadopollis spp.). Increasing complexity in the aperture structure is seen throughout the sequence; tricolpate and tricolporoidate forms are recorded in almost all samples, while tricolporate pollen grains are restricted to the middle and upper levels of the unit. The high species richness and abundance of monocolpate-ulcerate angiosperm related to monocots or magnoliids sensu lato recorded in the unit is comparable to that previously recognised in other assemblages from the early and middle Albian of the southern (e.g. Australia) and northern hemispheres (e.g. Western Portuguese basin, Europe). The recorded increase in the number of angiosperm species towards the middle and upper parts of the Kachaike Formation, with the presence of monocolpate, tricolpate, tricolporoidate and tricolporate pollen, suggests an early-early middle Albian age for these parts of the unit, in agreement with the early Albian age proposed for its basal levels on the basis of dinoflagellates.  相似文献   
96.
王文采 《广西植物》2016,36(Z1):148-151
描述了荨麻科楼梯草属2新种,其中发现自贵州的安龙楼梯草与条叶楼梯草近缘,与后者的区别在于此新种的茎和叶无毛,叶片椭圆形或狭倒卵形,较宽,雄苞片无角状突起,雌头状花序无梗,其花序托不等3~4裂,柱头画笔头状; 自广西发现的厚序楼梯草与福贡楼梯草近缘,与后者的区别在于此新种的叶片质地较薄,呈膜质,上面疏被糙伏毛,叶柄较短,长达2 mm或不存在,雄花序梗圆柱形,较长,密被短柔毛,雄总苞由一层不具角状突起的苞片组成。  相似文献   
97.
This review updates the information upon the chemical composition of propolis from all Mediterranean countries as well as their biological properties and applications. The non‐volatile fraction of propolis was characterized by the presence of phenolic acids and their esters and flavonoids. Nevertheless, in some countries, diterpenes were also present: Sicily (Italy), Croatia, Malta, Creta (Greece), Turkey, Cyprus, Egypt, Libya, Algeria and Morocco. The volatile fraction of propolis was characterized by the presence of benzoic acid and its esters, mono‐ and sesquiterpenes, being the oxygenated sesquiterpene β‐eudesmol characteristic of poplar propolis, whereas the hydrocarbon monoterpene α‐pinene has been related with the presence of conifers. Regardless the chemical composition, there are common biological properties attributed to propolis. Owing to these attributes, propolis has been target of study for applications in diverse areas, such as food, medicine and livestock.  相似文献   
98.
为了全面揭示云南蚂蚁多样性规律,采用样地调查法研究了滇东南地区的蚂蚁物种多样性。结果表明:在滇东南地区5个垂直带共采获蚂蚁7亚科57属202种;各样地蚂蚁群落主要指标为:物种数目6~49种(平均24种),个体密度0.7~5 943.6头/m~2(平均560.5头/m~2),多样性指数0.1693~2.6382(平均1.6574),均匀度指数0.0771~0.8022(平均0.5468),优势度指数0.0012~0.9031(平均0.2733);蚂蚁群落间相似性系数0.0333~0.3810,处于极不相似至中等不相似水平。结论认为,滇东南地区栖息着丰富的蚂蚁种类,在生态系统功能和生物多样性保护中具有重要价值。垂直带上蚂蚁物种的丰富度和多样性主要受气温制约,具体表现为纬度和海拔对蚂蚁群落的影响。该地区蚂蚁群落主要指标普遍表现出多域效应现象,除了地形和森林结构因素外,主要受到人类干扰影响,应当结合天然林保护工程对该地区蚂蚁群落加以保护,以便充分发挥其生态功能。  相似文献   
99.
100.
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