全文获取类型
收费全文 | 100685篇 |
免费 | 917篇 |
国内免费 | 885篇 |
专业分类
102487篇 |
出版年
2023年 | 33篇 |
2022年 | 91篇 |
2021年 | 177篇 |
2020年 | 79篇 |
2019年 | 118篇 |
2018年 | 11941篇 |
2017年 | 10755篇 |
2016年 | 7667篇 |
2015年 | 956篇 |
2014年 | 722篇 |
2013年 | 800篇 |
2012年 | 4865篇 |
2011年 | 13415篇 |
2010年 | 12409篇 |
2009年 | 8627篇 |
2008年 | 10277篇 |
2007年 | 11835篇 |
2006年 | 716篇 |
2005年 | 952篇 |
2004年 | 1389篇 |
2003年 | 1422篇 |
2002年 | 1142篇 |
2001年 | 323篇 |
2000年 | 210篇 |
1999年 | 91篇 |
1998年 | 89篇 |
1997年 | 79篇 |
1996年 | 59篇 |
1995年 | 41篇 |
1994年 | 58篇 |
1993年 | 83篇 |
1992年 | 56篇 |
1991年 | 74篇 |
1990年 | 34篇 |
1989年 | 36篇 |
1988年 | 44篇 |
1987年 | 29篇 |
1986年 | 19篇 |
1985年 | 16篇 |
1984年 | 18篇 |
1983年 | 35篇 |
1982年 | 12篇 |
1981年 | 15篇 |
1978年 | 12篇 |
1977年 | 13篇 |
1972年 | 251篇 |
1971年 | 276篇 |
1965年 | 14篇 |
1962年 | 25篇 |
1944年 | 12篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
61.
62.
63.
A new protein crosslinking agent, 2,3-dibromopropionyl-N-hydroxysuccinimide ester, has been synthesized and characterized. The potential use of this compound as a temperature-controllable heterobifunctional crosslinking agent has been investigated using model systems and its reactivity compared with that of chlorambucil-N-hydroxysuccinimide ester. The coupling of14C-labeled phenylethylamine to lysozyme has been used to illustrate the feasibility of the use of this crosslinking agent for the synthesis of immunotoxins. 相似文献
64.
65.
Julie Watrin Anne-Marie Lézine Konrad Gajewski Annie Vincens 《Journal of Biogeography》2007,34(3):489-499
Aim To demonstrate that incorporating the bioclimatic range of possible contributor plants leads to improved accuracy in interpreting the palaeoclimatic record of taxonomically complex pollen types.
Location North Tropical Africa.
Methods The geographical ranges of selected African plants were extracted from the literature and geo-referenced. These plant ranges were compared with the pollen percentages obtained from a network of surface sediments. Climate-response surfaces were graphed for each pollen taxon and each corresponding plant species.
Results Several patterns can be identified, including taxa for which the pollen and plant distributions coincide, and others where the range limits diverge. Some pollen types display a reduced climate range compared with that of the corresponding plant species, due to low pollen production and/or dispersal. For other taxa, corresponding to high pollen producers such as pioneer taxa, pollen types display a larger climatic envelope than that of the corresponding plants. The number of species contained in a pollen taxon is an important factor, as the botanical species included in a taxon may have different geographical and climate distributions.
Main conclusions The comparison between pollen and plant distributions is an essential step towards more precise vegetation and climate reconstructions in Africa, as it identifies taxa that have a high correspondence between pollen and plant distribution patterns. Our method is a useful tool to reassess biome reconstructions in Africa and to characterize accurately the vegetation and climate conditions at a regional scale, from pollen data. 相似文献
Location North Tropical Africa.
Methods The geographical ranges of selected African plants were extracted from the literature and geo-referenced. These plant ranges were compared with the pollen percentages obtained from a network of surface sediments. Climate-response surfaces were graphed for each pollen taxon and each corresponding plant species.
Results Several patterns can be identified, including taxa for which the pollen and plant distributions coincide, and others where the range limits diverge. Some pollen types display a reduced climate range compared with that of the corresponding plant species, due to low pollen production and/or dispersal. For other taxa, corresponding to high pollen producers such as pioneer taxa, pollen types display a larger climatic envelope than that of the corresponding plants. The number of species contained in a pollen taxon is an important factor, as the botanical species included in a taxon may have different geographical and climate distributions.
Main conclusions The comparison between pollen and plant distributions is an essential step towards more precise vegetation and climate reconstructions in Africa, as it identifies taxa that have a high correspondence between pollen and plant distribution patterns. Our method is a useful tool to reassess biome reconstructions in Africa and to characterize accurately the vegetation and climate conditions at a regional scale, from pollen data. 相似文献
66.
67.
68.
Morphological dispersion of introduced Hawaiian finches: evidence for competition and a Narcissus effect 总被引:2,自引:0,他引:2
Summary Morphological analyses were conducted on finch species introduced to Oahu, Hawaii. As many as 25 species of finches representing four families (Emberizidae, Fringillidae, Ploecidae, Estrildidae) have been introduced to Oahu. Of these, 15 species currently have established wild populations. When compared with pools of 25 and 23 species introduced to Oahu, the 15 surviving species are morphologically overdispersed. This result is consistent with the hypothesis that competition has influenced the outcome of these introductions. We also tested the hypothesis that surviving introduced finches in one habitat (sugarcane fields) were morphologically overdispersed. When the seven surviving finch species found in sugarcane were compared with the pools of 25 and 23 species, they too were morphologically overdispersed. However, when the seven sugarcane species were compared with a species pool consisting of only the surviving 15 species found across Oahu, they were not morphologically overdispersed. This result suggests that morphological analyses of community structure based only on comparisons of assemblages of surviving species may be biased by a Narcissus effect. 相似文献
69.
The kinetics of haemolysis of rabbit erythrocytes byCroton tiglium lectin was studied as a function of concentration of the lectin and erythrocytes. The length of the prelytic period decreased with increasing lectin concentrations, indicating that the secondary events at the membrane which follow the binding of the lectin to cell surface carbohydrate receptors are accelerated at higher surface concentrations of the lectin. The rate or extent of haemolysis was not affected by the inclusion of ions like K+, Ca2+ and Mg2+ in the medium or by the substitution of ionic medium by a non-ionic medium. The inhibition of haemagglutination and haemolysis of rabbit red cells byCroton tiglium lectin by antilectin rabbit serum was observed. A possible mechanism of haemolysis by the lectin is discussed. 相似文献
70.
N-Acetyl-D-galactosamine in β-linkage being ubiquitous in cell surface glycoproteins, their interaction with lectins specific for this sugar moiety may be a significant event in cell adhesion phenomena. This article discusses the common β-N-acetyl galactosamine-specific lectins, with particular stress on the lectin from winged beans (Psophocarpus tetragonolobus). 相似文献