全文获取类型
收费全文 | 8387篇 |
免费 | 863篇 |
国内免费 | 810篇 |
出版年
2023年 | 206篇 |
2022年 | 145篇 |
2021年 | 245篇 |
2020年 | 323篇 |
2019年 | 411篇 |
2018年 | 361篇 |
2017年 | 347篇 |
2016年 | 319篇 |
2015年 | 328篇 |
2014年 | 426篇 |
2013年 | 628篇 |
2012年 | 347篇 |
2011年 | 362篇 |
2010年 | 302篇 |
2009年 | 385篇 |
2008年 | 414篇 |
2007年 | 452篇 |
2006年 | 378篇 |
2005年 | 368篇 |
2004年 | 347篇 |
2003年 | 328篇 |
2002年 | 310篇 |
2001年 | 228篇 |
2000年 | 224篇 |
1999年 | 192篇 |
1998年 | 119篇 |
1997年 | 129篇 |
1996年 | 120篇 |
1995年 | 137篇 |
1994年 | 116篇 |
1993年 | 107篇 |
1992年 | 109篇 |
1991年 | 100篇 |
1990年 | 58篇 |
1989年 | 61篇 |
1988年 | 68篇 |
1987年 | 66篇 |
1986年 | 56篇 |
1985年 | 60篇 |
1984年 | 48篇 |
1983年 | 59篇 |
1982年 | 47篇 |
1981年 | 50篇 |
1980年 | 45篇 |
1979年 | 30篇 |
1978年 | 18篇 |
1977年 | 17篇 |
1976年 | 18篇 |
1974年 | 11篇 |
1972年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
A.N. Reshetilov A.B. Medvinsky T.P. Eliseeva V.Yu. Shakhbazian M.A. Tsyganov A.M. Boronin G.R. Ivanitsky 《FEMS microbiology letters》1992,94(1-2):59-62
A method of pH distribution measurements in agar nutrient media containing expanding bacterial populations is described. It is based on measuring pH microsamples taken at different points of the media. The sample volume was 10 microliters. A pH sensitive field effect transistor was used as a measuring electrode. Acidification was found to occur in glucose media, while alkalization occurred in the media containing peptone. 相似文献
2.
Roland Sturm 《Biometrical journal. Biometrische Zeitschrift》1991,33(4):441-454
This paper discusses discrete time proportional hazard models and suggests a new class of flexible hazard functions. Explicitly modeling the discreteness of data is important since standard continuous models are biased; allowing for flexibility in the hazard estimation is desirable since strong parametric restrictions are likely to be similarly misleading. Simulation compare continuous and discrete models when data are generated by grouping and demonstrate that simple approximations recover underlying hazards well and outperform nonparametric maximum likelihood estimates in term of mean squared error. 相似文献
3.
Response of Picea rubens seedlings to intermittent mist varying in acidity, and in concentrations of sulfur-, and nitrogen-containing pollutants 总被引:7,自引:0,他引:7
Jay S. Jacobson Twylene Bethard Laurence I. Heller James P. Lassoie 《Physiologia plantarum》1990,78(4):595-601
Two-year-old red spruce seedlings ( Picea rubens Sarg .) growing in. field chambers were repeatedly exposed to acidic mist with a factorial combination of 3 fluctuating levels of acidity: median pH values of 3.0 (range of 2.5 to 3.5), 3.5 (range of 3.0 to 4.0), and 4.0 (range of 3.5 to 4.5). and 3 ion compositions: sulfate. nitrate and ammonium, and a combination of all 3 ions. The experiment was performed during the growing season over a period of 3.5 months. Mist exposures were intermittent with 5 wet-dry cycles for each 16-h overnight exposure period, Foliar necrosis occurred on seedlings treated with the most acidic mist and was most severe when the mist contained sulfate. At a median pH of 3.5, a value close to that of cloud water occurring in the eastern United States, injury developed with sulfuric acid mist, but not. when the mist contained nitric acid. The combination of high acidity and sulfate significantly decreased volume of aboveground tissues, while high acidity and nitrate increased volume. Root and needle dry weights were not affected. However, high acidity of mist was associated with increased leader shoot length. These results indicate, that there is a risk of foliar injury and changes in growth of red spruce with cloud water at a median acidity of pH 3.5 or below, especially when there are high concentrations of sulfate and low concentrations of nitrate. 相似文献
4.
5.
6.
Sheryl Coombs Ruth A. Conley 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1997,180(4):401-415
Extracellular, single unit recording techniques were used to measure the responses of posterior lateral line nerve fibers
to a 50-Hz dipole source that slowly changed its location along the length of the fish. The flow-field equations for a dipole
source were used to model the pressure gradient pattern and thus, the expected excitation pattern along a linear array of
lateral line receptor organs for different source locations. Finally, excitation patterns were similarly modeled along the
left and right side of the fish's head for actual steps taken by sculpin in approach pathways to the 50-Hz dipole source.
Spatial histograms of posterior lateral line nerve fiber responses to different locations of the dipole source could be predicted
from pressure gradient patterns modeled from the flow-field equations, confirming that the modeling approach applied to behavioral
results was a good predictor of excitation patterns likely to be encoded by the lateral line periphery. An examination of
how modeled excitation patterns changed from one position to the next in typical approach pathways and how patterns differed
between positions from which successful and unsuccessful strikes were launched suggests that approach and strike strategies
can indeed be explained by the information available in excitation patterns. In particular, changes in the spatial distribution
of pressure gradient directions (polarities), available only when the source is lateral (as opposed to directly in front of
the fish), appear to enhance the ability of sculpin to determine source distance. Without such information, misses are more
likely to occur and successful strikes are more likely to be launched from short distances only.
Accepted: 23 October 1996 相似文献
7.
L. V. Belousov 《Russian Journal of Developmental Biology》2008,39(5):307-315
The work of prominent Russian biologist Alexander Gavrilovich Gurwitsch (1874–1954) on the theory of organism development are reviewed. Alexander Gurwitsch introduced the concept of embryonic (morphogenetic, biological, and cellular) field and proposed several revisions of it from 1912 to 1944. Although neither of them can be considered as a final theory of development, his the persistent search for the invariant law that allows the shape (spatial structure) to be proposed for each next developmental stage from the previous shape is of imperishable methodological interest. Alexander Gurwitsch anticipated many ideas of the future theory of self-organization. His theoretical constructions are explicit and experiment-oriented but absolutely not esoteric. They represent a highly important and original contribution to theoretical biology and are an essential step to further development of the ontogenetic theory. 相似文献
8.
Multiple, superimposed Type I interferon (IFN) restriction fragments were resolved following 72–92 h of horizontal electrophoresis. Restriction fragment length polymorphisms (RFLPs) for α IFN (IFNA), β IFN (IFNB), ωIFN (IFNW) and trophoblast IFN (IFNT) genes were identified in Hin dill, Eco RI and Taql digestions from 313 cattle. RFLPs with codominant segregation in cattle pedigrees were considered alleles, and 19 distinct polymorphic Type I IFN loci (5 IFNA, 4 IFNB, 8 IFNW and 2 IFNT) were identified. Allele frequencies and observed heterozygosity values were calculated for each locus and several loci were considered highly informative for linkage analysis. Bovine IFN gene numbers (10 IFNA, 6 IFNB, 20 IFNW and 6 IFNT) were estimated from the number of polymorphic loci plus additional monomorphic hybridizing bands present in Eco RI and Hindlll digestions. Physical linkage of the Type I IFN gene families on bovine chromosome 8 was demonstrated by pulsed field gel electrophoresis (PFGE). Hybridization of two or more IFN probes to similarly sized PFGE fragments suggested the tentative gene family order: IFNA/IFNW-IFNT-IFNB. These studies provide a basis for the development of more detailed genetic and physical maps of the bovine Type I IFNs. 相似文献
9.
D. F. Roberts 《International Journal of Anthropology》1996,11(2-4):185-191
As a conclusion, this paper reviews briefly the content of the volume. The wealth of demographic data has not been adequately
exploited in anthropology; this is why this publication is valuable in showing attempts to apply demographic data in a variety
of anthropological problems. This symposium has explored many interesting points which we recall here. Yet it has also opened
up a whole range of further questions on the material presented as well as in this broad field. Several directions of research
could be developed, for instance, testing among human populations, over long periods, the ecological thoughts of ecosystems
evolving as a cascade of instabilities, rather than a succession of equilibrium states. Let us also recall the pervasive nature
of demographic facts in topics such as the energy cycle or the genetic structure and evolution of human populations. 相似文献
10.