全文获取类型
收费全文 | 1352045篇 |
免费 | 119279篇 |
国内免费 | 1432篇 |
专业分类
1472756篇 |
出版年
2021年 | 18164篇 |
2020年 | 12795篇 |
2019年 | 16362篇 |
2018年 | 17992篇 |
2017年 | 16753篇 |
2016年 | 28503篇 |
2015年 | 43156篇 |
2014年 | 51227篇 |
2013年 | 77664篇 |
2012年 | 39351篇 |
2011年 | 27888篇 |
2010年 | 44658篇 |
2009年 | 45734篇 |
2008年 | 26209篇 |
2007年 | 24421篇 |
2006年 | 29521篇 |
2005年 | 30413篇 |
2004年 | 29868篇 |
2003年 | 27196篇 |
2002年 | 25366篇 |
2001年 | 31749篇 |
2000年 | 28873篇 |
1999年 | 29756篇 |
1998年 | 25496篇 |
1997年 | 25133篇 |
1996年 | 24769篇 |
1995年 | 22908篇 |
1994年 | 22737篇 |
1993年 | 21800篇 |
1992年 | 26216篇 |
1991年 | 24401篇 |
1990年 | 23078篇 |
1989年 | 23952篇 |
1988年 | 22004篇 |
1987年 | 20875篇 |
1986年 | 19756篇 |
1985年 | 21689篇 |
1984年 | 21875篇 |
1983年 | 19416篇 |
1982年 | 20301篇 |
1981年 | 19498篇 |
1980年 | 18264篇 |
1979年 | 18228篇 |
1978年 | 17163篇 |
1977年 | 16633篇 |
1976年 | 15746篇 |
1975年 | 15097篇 |
1974年 | 15499篇 |
1973年 | 15970篇 |
1972年 | 13546篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
311.
Morphometry and composition of aragonite and vaterite otoliths of deformed laboratory reared juvenile herring from two populations 总被引:2,自引:0,他引:2
Vaterite otoliths were sampled from two reared populations (Celtic and Clyde Seas) of juvenile herring Clupea harengus. The crystallography, elemental composition and morphometry were analysed and compared with those of normal aragonite otoliths. The incidence of vaterite otoliths in the juveniles sampled (n = 601) ranged from 7·8% in the Clyde population to 13·9% in the Celtic Sea population, and was 5·5% in the small sample (n = 36) of wild adults examined. In all but one case fish had only one vaterite otolith; the corresponding otolith of the pair was completely aragonite. Although the majority of the juveniles sampled showed craniofacial deformities, there was no link between the skull or jaw malformation and the incidence of vaterite otoliths. All vaterite otoliths had an aragonite inner area, and vaterite deposition began sometime after the age of 90 days. The vaterite otoliths were larger and lighter than their corresponding aragonite partners, and were less dense as a consequence of the vaterite crystal structure. The vaterite areas of the otoliths were depleted in Sr, Na and K. Concentrations of Mn were higher in the vaterite areas. The transition between the aragonite inner areas and the vaterite areas was sharply delineated. Within a small spatial scale (20 μm3) in the vaterite areas, however, there was co‐precipitation of both vaterite and aragonite. The composition of the aragonite cores in the vaterite otoliths was the same as in the cores of the normal aragonite otoliths indicating that the composition of the aragonite cores did not seed the shift to vaterite. Vaterite is less dense than aragonite, yet the concentrations of Ca analysed with wavelength‐dispersive spectrometry (WDS) were the same between the two polymorphs, indicating that Ca concentrations measured with WDS are not a good indicator of hypermineralized zones with high mineral density. The asymmetry in density and size of the otoliths may cause disruptions of hearing and pressure sensitivity for individual fish with one vaterite otolith, however, the presence of vaterite otoliths did not seem to affect the growth of these laboratory reared juvenile herring. 相似文献
312.
N. I. Koshoridze N. G. Aleksidze N. B. Surguladze 《Journal of Evolutionary Biochemistry and Physiology》2003,39(4):446-450
Galactose- and inositol-binding proteins with lectin activity (GL-GAL and GL-I, respectively) were isolated from membranes enriched with cells of chicken brain fractions. Both lectins are glycoproteins of molecular mass 13.5 and 11.5 kDa, respectively; they show a high affinity to EDTA (GL-I) and EGTA (GL-GAL, GL-I), which indicates an important role of Ca+2 in molecular organization of these lectins. In brain glial cells of chick embryos, unlike adult chickens, a soluble form of lectins has been revealed; it is easily extracted with 2 mM EDTA and shows sensitivity to L-lactose, D-galactose, and N-acetyl-D-galactosamine. It is suggested that in the course of embryonal and postembryonal development of the chicken brain, a transformation and qualitative changes of the lectin spectrum occur due to a change of function of glial cells. 相似文献
313.
F. Kato T. Hino A. Nakaji M. Tanaka Y. Koyama 《Molecular & general genetics : MGG》1995,247(3):387-390
In many species of actinomycetes, carotenogenesis can be photoinduced. The capacity to respond to photoinduction is, however unstable and, in various strains of Streptomyces, is lost at a relatively high frequency. In Streptomyces setonii ISP5395, which normally produces no carotenoids, carotenoid-producing mutants can be obtained following protoplast regeneration. We report here the characterization of a gene, crtS, which was isolated from one such mutant and can confer on wild-type S. setonii ISP5395 cells the capacity to synthesize carotenoids. Sequence analysis of crtS reveals an open reading frame, which shows homology to genes that encode alternative sigma factors in Bacillus subtilis. We propose that crtS encodes a sigma factor which is necessary for the expression of a cryptic gene(s) for carotenoid biosynthesis in S. setonii ISP5395. 相似文献
314.
315.
316.
Marvin J. Allison Guillermo Focacci Calogero Santoro 《American journal of physical anthropology》1982,59(3):299-304
Burials and mummies of dogs from the Arica, Chile, area are described. It is concluded that these dogs were brought to this area 2500 years ago by shepherds from the highlands and are still present, relatively unchanged. 相似文献
317.
318.
James B. Reid 《Animal behaviour》1982,30(4):1212-1216
A young captive rook, Corvus frugilegus, inserted a plug into a plug-hole in its aviary floor so that a pool of water formed, which was used by all four rooks in the aviary for drinking and bathing. The bird was selective about which, of six holes, it chose for insertion of the tool, choosing the appropriate one with respect to the water source. Days on which attempted or successful tool-use occurred were drier than other days, and days of successful tool-uses were also warmer. Availability of fresh water to the birds did not influence the occurrence of tool-use. 相似文献
319.
320.
J. D. MUMFORD 《The Annals of applied biology》1981,97(2):243-252
The results of a survey of sugar beet growers are reported which provide subjective information on farmer perceptions of the hazard from pests and diseases on sugar beet, the control methods available and used, farmers' estimates of the costs and effect of controls, their objectives for pest control, and their attitudes to advice. In addition, objective information from research and field observations on losses and control methods is provided. The basis of pest control decisions and implications from comparisons of subjective and objective perceptions of the various problems and solutions are discussed. 相似文献