全文获取类型
收费全文 | 6365篇 |
免费 | 519篇 |
国内免费 | 3篇 |
专业分类
6887篇 |
出版年
2022年 | 42篇 |
2021年 | 68篇 |
2020年 | 58篇 |
2019年 | 51篇 |
2018年 | 95篇 |
2017年 | 76篇 |
2016年 | 121篇 |
2015年 | 229篇 |
2014年 | 271篇 |
2013年 | 306篇 |
2012年 | 448篇 |
2011年 | 390篇 |
2010年 | 275篇 |
2009年 | 232篇 |
2008年 | 357篇 |
2007年 | 374篇 |
2006年 | 348篇 |
2005年 | 338篇 |
2004年 | 322篇 |
2003年 | 319篇 |
2002年 | 308篇 |
2001年 | 94篇 |
2000年 | 65篇 |
1999年 | 84篇 |
1998年 | 106篇 |
1997年 | 63篇 |
1996年 | 62篇 |
1995年 | 76篇 |
1994年 | 77篇 |
1993年 | 75篇 |
1992年 | 66篇 |
1991年 | 51篇 |
1990年 | 60篇 |
1989年 | 42篇 |
1988年 | 53篇 |
1987年 | 44篇 |
1986年 | 46篇 |
1985年 | 47篇 |
1984年 | 58篇 |
1983年 | 43篇 |
1982年 | 43篇 |
1981年 | 39篇 |
1980年 | 46篇 |
1979年 | 41篇 |
1978年 | 40篇 |
1977年 | 32篇 |
1974年 | 38篇 |
1972年 | 29篇 |
1970年 | 26篇 |
1969年 | 31篇 |
排序方式: 共有6887条查询结果,搜索用时 0 毫秒
991.
992.
To test whether the range of montane Compositae species may be restricted to higher sites because of greater herbivory levels in the lowlands, we transplanted six species, combining them in three species pairs each consisting of a rare montane and a widespread species. Individuals of all species were planted at four sites of different altitude ranging from the lowlands to the subalpine Mt. Brocken in the Harz mountains, Germany. Food choice experiments with three mollusc species indicated that the montane plant was more palatable in the species pair Senecio hercynicus/S. ovatus, but the widespread plant was more palatable in the species pair Petasites albus/Tussilago farfara. In the third pair (Cicerbita alpina/Mycelis muralis), neither species was preferred. In the field, species-specific herbivory levels differed in their amount, in their interaction with plant phenology and in their effect on mortality. They only partially reflected the laboratory food choice results. We found clear differences between the lowest and the highest site for all species, but a continuous decrease in herbivory with altitude was only detected in three of the six species. 相似文献
993.
Ulrich Schulte 《Journal of bioenergetics and biomembranes》2001,33(3):205-212
Proteins specifically involved in the biogenesis of respiratory complex I in eukaryotes have been characterized. The complex I intermediate associated proteins CIA30 and CIA84 are tightly bound to an assembly intermediate of the membrane arm. Like chaperones, they are involved in multiple rounds of membrane arm assembly without being part of the mature structure. Two biosynthetic subunits of eukaryotic complex I have been characterized. The acyl carrier subunit is needed for proper assembly of the peripheral arm as well as the membrane arm of complex I. It may interact with enzymes of a mitochondrial fatty acid synthetase. The 39/40-kDa subunit appears to be an isomerase with a tightly bound NADPH. It is related to a protein family of reductases/isomerases. Both subunits have been discussed to be involved in the synthesis of a postulated, novel, high-potential redox group. 相似文献
994.
Schultz Lee Kerckhoffs L. Huub J. Klahre Ulrich Yokota Takao Reid James B. 《Plant molecular biology》2001,47(4):491-498
The brassinosteriod-deficient lkb mutant of garden pea (Pisum sativum L.) is characterized by an erectoides phenotype (reduced internode length, thickened stems, epinastic leaves), which is rescued by application of exogenous brassinolide. We show that the LKB gene is the Arabidopsis DIMINUTO/DWARF-1 (DIM/DWF1) homologue of pea. The DIM/DWF1 homologue from lkb plants contains a mutation that may result in reduced enzyme function, thus resulting in the previously shown accumulation of 24-methylenecholesterol and a deficiency of its hydrogenated product, campesterol. This ultimately leads to a deficiency of the biologically active brassionolide. The mutation in the lkb sequence cosegregates with the lkb phenotype. Northern analyis of the LKB gene revealed that the gene is ubiquitously expressed around the plant and that there is no evidence for negative feedback regulation of the gene. 相似文献
995.
A high density of X-linked genes for general cognitive ability: a run-away process shaping human evolution? 总被引:7,自引:0,他引:7
Ulrich Zechner Monika Wilda Hildegard Kehrer-Sawatzki Walther Vogel Rainald Fundele Horst Hameister 《Trends in genetics : TIG》2001,17(12):957-701
The incidence of mental disability is 30% higher in males than in females. We have examined entries in the OMIM database that are associated with mental disability and for several other common defects. Our findings indicate that compared with the autosomes, the X chromosome contains a significantly higher number of genes that, when mutated, cause mental impairment. We propose that these genes are involved in the development of cognitive abilities and thus exert a large X-chromosome effect on general intelligence in humans. We discuss these conclusions with regard to the conservation of the vertebrate X-chromosomal linkage group and to human evolution. 相似文献
996.
Sommer Frank; Hansen Thomas; Feuchtmayr Heidrun; Santer Barbara; Tokle Nils; Sommer Ulrich 《Journal of plankton research》2003,25(7):869-871
In a mesocosm study, the appendicularian Oikopleura dioica bloomedafter the reduction of copepod abundance, and in a second treatmentshowed a significantly negative correlation with copepod densities.Calculations, together with field data from the Baltic Sea,suggest that common calanoid copepods may control appendicularianpopulation dynamics. 相似文献
997.
The mortality of the common hamsterCricetus cricetus (Linnaeus, 1758) was analysed in two regions of Germany. Samples of radio tagged individuals and carcasses found in the field
during monitoring procedures were compared. Predation and winter mortality were the main mortality factors, followed by disease
and death caused by agricultural machinery during the harvest or other management measures. The causes of mortality differed
in the two regions, due to different environmental and ecological parameters, including field size, crops, presence of predators,
and agricultural management. The main mortality factors were all directly or indirectly linked to agriculture. Present agricultural
management exacerbates predation and increases winter mortality in this species. Crops with a prolonged vegetation cover and
food supply are crucial for the survival of common hamsters on farmland with intense agricultural management. 相似文献
998.
Species–area relationships (SARs) of European butterfly species (Rhopalocera) appear to follow power functions with Mediterranean butterflies having a much higher slope value (z=0.49) compared to the slope for the northern and eastern European countries (z=0.10). A simulated process of species extinction by a stepwise density dependent random elimination of species affected species–area patterns differently. For Mediterranean countries SAR slopes decreased, for other European countries slopes increased during the extinction process. Comparisons of species numbers before and after extinction with those predicted by a classical SAR approach differed widely and revealed that SARs are not able to predict future species numbers at local scales. For Mediterranean countries the classical SAR approach underestimated the number of species remaining after simulated extinction, for all other European countries SARs highly overestimated species numbers. These contrasting patterns indicate that changes in SAR patterns do not unequivocally point to changes in species diversity or community structure as assumed by current theory. On the other hand, the results strongly indicate that simplified applications of SARs for forecasting might give misimpressions about species loss and future biodiversity if the initial community structure, especially relative densities and numbers of species with restricted range size, are not taken into account. 相似文献
999.
Cytogenetic evolution of human ovarian cell lines associated with chemoresistance and loss of tumorigenicity. 总被引:1,自引:0,他引:1
Stéphanie Struski Martine Doco-Fenzy Michael Koehler Ilse Chudoba Francis Levy Linda Masson Nicole Michel Evelyne Ulrich Nadine Gruson Jean Bénard Gérard Potron Pascale Cornillet-Lefebvre 《Analytical cellular pathology》2003,25(3):115-122
In order to identify genomic changes associated with a resistant phenotype acquisition, we used comparative genomic hybridization (CGH) to compare a human ovarian cell line, Igrov1, and four derived subcell lines, resistant to vincristine and presenting a reversion of malignant properties. Multicolor FISH (Multiplex-FISH and Spectral Karyotype) and conventional FISH are also used to elucidate the karyotype of parental cell line. The drug-resistant subcell lines displayed many chromosomal abnormalities suggesting the implication of different pathways leading to a multidrug resistance phenotype. However, these cell lines shared two common rearrangements: an unbalanced translocation der(8)t(8;13)(p22;q?) and a deletion of the 11p. These chromosomal imbalances could reflected the acquisition of the chemoresistance (der(8)) or the loss of tumorigenicity properties (del(11p)). 相似文献
1000.