全文获取类型
收费全文 | 38406篇 |
免费 | 65篇 |
国内免费 | 94篇 |
专业分类
系统科学 | 190篇 |
丛书文集 | 862篇 |
教育与普及 | 107篇 |
理论与方法论 | 239篇 |
现状及发展 | 17143篇 |
研究方法 | 1540篇 |
综合类 | 17997篇 |
自然研究 | 487篇 |
出版年
2013年 | 252篇 |
2012年 | 509篇 |
2011年 | 1028篇 |
2010年 | 227篇 |
2008年 | 654篇 |
2007年 | 673篇 |
2006年 | 709篇 |
2005年 | 715篇 |
2004年 | 627篇 |
2003年 | 673篇 |
2002年 | 691篇 |
2001年 | 1118篇 |
2000年 | 1029篇 |
1999年 | 689篇 |
1992年 | 652篇 |
1991年 | 542篇 |
1990年 | 567篇 |
1989年 | 579篇 |
1988年 | 584篇 |
1987年 | 586篇 |
1986年 | 592篇 |
1985年 | 738篇 |
1984年 | 569篇 |
1983年 | 486篇 |
1982年 | 413篇 |
1981年 | 407篇 |
1980年 | 532篇 |
1979年 | 1202篇 |
1978年 | 1034篇 |
1977年 | 1019篇 |
1976年 | 719篇 |
1975年 | 769篇 |
1974年 | 1152篇 |
1973年 | 969篇 |
1972年 | 1014篇 |
1971年 | 1223篇 |
1970年 | 1608篇 |
1969年 | 1238篇 |
1968年 | 1213篇 |
1967年 | 1226篇 |
1966年 | 1028篇 |
1965年 | 743篇 |
1964年 | 208篇 |
1959年 | 445篇 |
1958年 | 685篇 |
1957年 | 574篇 |
1956年 | 488篇 |
1955年 | 414篇 |
1954年 | 458篇 |
1948年 | 282篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
12.
13.
14.
15.
16.
The moth Utetheisa ornatrix derives protection against predation from systemic pyrrolizidine alkaloids (PAs) that it sequesters as a larva from its foodplants (Leguminosae, Crotalaria spp.). We here show, in laboratory tests, that Utetheisa deficient in body PA can make up for the chemical shortfall by cannibalizing pupae. We present evidence indicating that cannibalism in larvae is elicited not by hunger, but possibly by PA deficiency itself, and that in making cannibalistic choices larvae prefer PA-containing over PA-free pupae. PAs themselves, either in crystalline form or as additives to food items, proved phagostimulatory to larvae. In nature Utetheisa tend to pupate away from their foodplant, essentially out of reach of larval attack. The threat of cannibalism may have contributed to the evolution of this pupation behavior. 相似文献
17.
P pili in uropathogenic E. coli are composite fibres with distinct fibrillar adhesive tips. 总被引:25,自引:0,他引:25
Escherichia coli is a frequent cause of several common bacterial infections in humans and animals, including urinary tract infections, bacteraemia and bacteria-related diarrhoea and is also the main cause of neonatal meningitis. Microbial attachment to surfaces is a key event in colonization and infection and results mainly from a stereochemical fit between microbial adhesins and complementary receptors on host cells. Bacterial adhesins required for extracellular colonization by Gram-negative bacteria are often minor components of heteropolymeric fibres called pili which must be oriented in an accessible manner in these structures to be able to bind to specific receptor architectures. P pili mediate the binding of uropathogenic E. coli to a digalactoside receptor determinant present in the urinary tract epithelium. We report here that the adhesin is a component of distinct fibrillar structures present at the tips of the pili. These virulence-associated tip fibrillae are thin, flexible polymers composed mostly of repeating subunits of PapE that frequently terminate with the alpha-D-galactopyranosyl-(1-4)-beta-D-galactopyranose or Gal alpha (1-4)Gal binding PapG adhesin. 相似文献
18.
Although theoretical studies show that overcompensatory density-dependent mechanisms can potentially generate regular or chaotic fluctuations in animal numbers, the majority of realistic single-species models of invertebrate populations are not overcompensatory enough to cause sustained population cycles. The possibility that overcompensation may generate cycles or chaos in vertebrate populations has seldom been considered. Here we show that highly overcompensatng density-dependent mortality can generate recurrent population crashes consistent with those observed in a naturally limited population of Soay sheep. The observed interval of three or more years between crashes points to sharp 'focusing' of mortality over a narrow range of population density. 相似文献
19.
Linear models are invariant under non-singular, scale-preserving linear transformations, whereas mean square forecast errors (MSFEs) are not. Different rankings may result across models or methods from choosing alternative yet isomorphic representations of a process. One approach can dominate others for comparisons in levels, yet lose to another for differences, to a second for cointegrating vectors and to a third for combinations of variables. The potential for switches in ranking is related to criticisms of the inadequacy of MSFE against encompassing criteria, which are invariant under linear transforms and entail MSFE dominance. An invariant evaluation criterion which avoids misleading outcomes is examined in a Monte Carlo study of forecasting methods. 相似文献
20.
Humoral immune reactions to heat shock proteins (hsp) from microorganisms are one aspect of microbial infections in humans. The production of antibodies which are specific to epitopes present on procaryotic hsp leads also to the appearance of cross-reactive serum antibodies in the host organism that react with human hsp. This article discusses the consequences of such autoreactive antibodies for the host in context with the development of immune tolerance and autoimmune diseases, especially rheumatoid arthritis (RA), and in experimental animal models for arthritis such as adjuvant arthritis in rats. On the basis of epitope cross-reactivity between hsp and other host proteins, a hypothesis is presented for the development of autoimmune disease following the production of hsp-specific antibodies. 相似文献