全文获取类型
收费全文 | 39969篇 |
免费 | 3585篇 |
国内免费 | 6602篇 |
专业分类
50156篇 |
出版年
2024年 | 211篇 |
2023年 | 953篇 |
2022年 | 1148篇 |
2021年 | 1448篇 |
2020年 | 1532篇 |
2019年 | 1945篇 |
2018年 | 1594篇 |
2017年 | 1621篇 |
2016年 | 1701篇 |
2015年 | 1709篇 |
2014年 | 2198篇 |
2013年 | 2846篇 |
2012年 | 1731篇 |
2011年 | 1977篇 |
2010年 | 1657篇 |
2009年 | 2133篇 |
2008年 | 2218篇 |
2007年 | 2177篇 |
2006年 | 2058篇 |
2005年 | 1888篇 |
2004年 | 1673篇 |
2003年 | 1523篇 |
2002年 | 1347篇 |
2001年 | 1083篇 |
2000年 | 979篇 |
1999年 | 965篇 |
1998年 | 774篇 |
1997年 | 706篇 |
1996年 | 673篇 |
1995年 | 639篇 |
1994年 | 605篇 |
1993年 | 468篇 |
1992年 | 468篇 |
1991年 | 448篇 |
1990年 | 341篇 |
1989年 | 315篇 |
1988年 | 285篇 |
1987年 | 262篇 |
1986年 | 232篇 |
1985年 | 246篇 |
1984年 | 257篇 |
1983年 | 172篇 |
1982年 | 210篇 |
1981年 | 147篇 |
1980年 | 142篇 |
1979年 | 115篇 |
1978年 | 96篇 |
1977年 | 54篇 |
1976年 | 54篇 |
1974年 | 29篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
NICOLAS RAY 《Molecular ecology resources》2005,5(1):177-180
pathmatrix is a tool used to compute matrices of effective geographical distances among samples using a least‐cost path algorithm. This program is dedicated to the study of the role of the environment on the spatial genetic structure of populations. Punctual locations (e.g. individuals) or zones encompassing sample data points (e.g. demes) are used in conjunction with a species‐specific friction map representing the cost of movement through the landscape. Matrices of effective distances can then be exported to population genetic software to test, for example, for isolation by distance. pathmatrix is an extension to the geographical information system (GIS) software arcview 3.x. 相似文献
42.
L. Brzsnyi 《Biometrical journal. Biometrische Zeitschrift》1981,23(3):283-295
The paper deals with the analysis of 2 × 2 table measures and describes several new possibilities for applying them. An estimate is given for HELLMICH's asymmetric K-measure and a possible extension of the contingency concept in a special 2×2 case is presented. The concept introduced and the recommended computer application of the 2×2 table measures is supported by new results obtained for the relationship between jaw joint pains and noises examined earlier by F. SCHMID and C. ZSCHEGE. With a larger number of observations the opportunity arises for a deeper analysis of this dental problem using path analysis. The plan of evaluation and the conncetion between the variables examined is illustrated by a path-diagram. The association tables determined on the basis of the (C) values introduced during the examination of jaw joint disturbances using association analysis draw attention to the special role of the left side of mastication. The paper raises ideas for further generalisations and indicates where further research is needed. 相似文献
43.
MUNENORI NOGUCHI HIROSHI INOUE TOSHIHIRO ATAGO SHOGO NAKAMURA 《The Journal of eukaryotic microbiology》1997,44(2):127-133
ABSTRACT. The effects of organic solvents on the ATPase activity and the sliding disintegration of axonemes from Chlamydomonas were investigated. The axonemal ATPase was markedly activated by methanol accompanying with marked inhibition of the sliding disintegration of axonemes. On the contrary, glycerol inhibited the ATPase activity without serious inhibition of the sliding disintegration. As far as the axonemes are not irreversibly denatured by extremely high concentration of solvents, the effects of solvents both on the ATPase and the ability of sliding are reversible. Therefore, the inhibition of sliding accompanied by the activation of ATPase is probably due to an inability to couple the hydrolysis of ATP to sliding between dynein and microtubule in the presence of methanol. The axonemal ATPase was less sensitive to vanadate inhibition after exposure to methanol. This indicates that methanol makes the dyneinADP.Pi complex unstable and increases product release. On the other hand, glycerol and ethylene glycol seem to stabilize the force generation responsible for the sliding through stabilizing the dynein.ADP.Pi complex. 相似文献
44.
45.
The ability to respond to phytochrome (Pfr, the far-red light absorbing from of phytochrome) with anthocyanin synthesis appears first in some marginal regions of the abaxial epidermis of the mustard cotyledons and from there spreads gradually over the entire tissue (transient phase). The pertinent pattern is independent of environmental influences such as light quality and nutritional culture conditions. The competence for Pfr in the epidermal cells, with regard to the initial action of Pfr (concerning anthocyanin synthesis), appears considerably earlier than the ability for actual anthocyanin synthesis. An electron microscopical study of the ultrastructural changes occurring in vacuoles and plastids of the epidermal cells during the transient phase showed that a correlation only exists between the differentiation of central cell vacuoles, originating from the aleurone vacuoles, and the appearance of the ability to accumulate anthocyanin. It is suggested that the formation of a central cell vacuole is a prerequisite for anthocyanin accumulation in the epidermal cells of the mustard seedling cotyledons.Abbreviations Pr, Pfr
red and far-red absorbing forms of phytochrome
- HS
Hoagland's nutrient solution 相似文献
46.
Summary We report on the size distribution of clones marked by mitotic recombination induced by several different doses of X-rays applied to 72 h oldDrosophila larvae. The results indicate that the radiation significantly reduces the number of cells which undergo normal proliferation in the imaginal wing disc. We estimate that 1000 r reduces by 40–60% the number of cells capable of making a normal contribution to the development of the adult wing. Part of this reduction is due to severe curtailment in the proliferative ability of cells which nevertheless remain capable of adult differentiation; this effect is possibly due to radiation-induced aneuploidy. Cytological evidence suggests that immediate cell death also occurs as a result of radiation doses as low as 100 r. The surviving cells are stimulated to undergo additional proliferation in response to the X-ray damage so that the result is the differentiation of a normal wing. 相似文献
47.
48.
The genetic differentiation among populations of the leaf beetle Chrysolina virgata living in wetlands of Japan was studied based on the sequence data of the mitochondrial cytochrome oxidase subunit I gene region (750 bp). Two distinct lineages of mitochondrial haplotypes were found: one (clade A) consisted of 26 haplotypes distributed over the distribution range of C. virgata between north‐east Honshu and Kyushu, whereas the other (clade B) was monotypic and confined to a small region in north‐east Honshu where it coexisted with clade A. Nested clade analysis for these haplotypes suggested that range expansion and following differentiation due to isolation by distance might have resulted in the present distribution pattern of the haplotypes in clade A. We discuss the evolutionary process leading to the occurrence of two distinct haplotype clades in Japan in terms of repeated colonization from the continent and range expansion and contraction during climatic changes. 相似文献
49.
Aiyi Liu James F. Troendle Kai F. Yu Vivian W. Yuan 《Biometrical journal. Biometrische Zeitschrift》2004,46(6):760-768
We consider estimation after a group sequential test. An estimator that is unbiased or has small bias may have substantial conditional bias (Troendle and Yu, 1999, Coburger and Wassmer, 2001). In this paper we derive the conditional maximum likelihood estimators of both the primary parameter and a secondary parameter, and investigate their properties within a conditional inference framework. The method applies to both the usual and adaptive group sequential test designs. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
50.