全文获取类型
收费全文 | 928788篇 |
免费 | 98832篇 |
国内免费 | 1252篇 |
出版年
2018年 | 19443篇 |
2017年 | 18157篇 |
2016年 | 17859篇 |
2015年 | 15589篇 |
2014年 | 17253篇 |
2013年 | 23506篇 |
2012年 | 31577篇 |
2011年 | 40671篇 |
2010年 | 30972篇 |
2009年 | 26578篇 |
2008年 | 34527篇 |
2007年 | 37080篇 |
2006年 | 23045篇 |
2005年 | 23805篇 |
2004年 | 23275篇 |
2003年 | 22411篇 |
2002年 | 21297篇 |
2001年 | 35348篇 |
2000年 | 35709篇 |
1999年 | 28692篇 |
1998年 | 11136篇 |
1997年 | 11726篇 |
1996年 | 11282篇 |
1995年 | 10611篇 |
1994年 | 10464篇 |
1993年 | 10309篇 |
1992年 | 24054篇 |
1991年 | 23319篇 |
1990年 | 22872篇 |
1989年 | 22198篇 |
1988年 | 20492篇 |
1987年 | 20066篇 |
1986年 | 18425篇 |
1985年 | 18509篇 |
1984年 | 15464篇 |
1983年 | 13459篇 |
1982年 | 10778篇 |
1981年 | 9587篇 |
1980年 | 9131篇 |
1979年 | 14772篇 |
1978年 | 11887篇 |
1977年 | 10733篇 |
1976年 | 10124篇 |
1975年 | 10999篇 |
1974年 | 11803篇 |
1973年 | 11586篇 |
1972年 | 10870篇 |
1971年 | 10061篇 |
1970年 | 8211篇 |
1969年 | 7769篇 |
排序方式: 共有10000条查询结果,搜索用时 640 毫秒
871.
Monitoring the integrity of self: biology of MHC-restriction of virus-immune T cells 总被引:2,自引:0,他引:2
All available evidence indicates that the cytotoxic thymus-derived lymphocyte (T cell), which is lytic for virus-infected target cells in vitro, is also the effector in cell-mediated immunity in vivo. Such T cell show two orders of specificity: for the virus in question, and for a particular self major histocompatibility complex (MHC) glycoprotein. Recirculating T cells amy thus be considered to survey the integrity of self, the self components involved being the cell-surface structures that are recognized as foreign during graft rejection. Virus-infected liver cells are apparently eliminated in much the same way as a transplanted organ. The necessary balance between self-tolerance (absence of autoreactivity) and self-monitoring effector T cell function seems to be established during the process of differentiation in thymus. The molecular nature of the underlying recognition events is, as yet, obscure. 相似文献
872.
Summary Between 3 Nov. 1983 and 9 Apr. 1984, six applications of fertiliser N (ammonium, nitrate or urea) were given to four autumn
sown (26 Oct. 1983)Vicia faba L cultivars, Banner Winter (BW) and Maris Beagle (MBg), cold tolerant cultivars normally sown in the autumn, and Herz Freya
(HF) and Maris Bead (MBd), cold sensitive cultivars more commonly sown in the spring. The effects of additional N were determined
by comparison with plants given zero-N (controls).
Application of N, regardless of form, had no effect on % emergence at the first sampling (15 Dec. 1983); >90% for BW, MBg
and HF, but only 40–60% for MBd. At this time the dry weight, carbon content and nitrogen content of all cultivars was approximately
20% less than that of the seed on planting. No more plants emerged after 15 Dec. 1983. Between 15 Dec. 1983 and 20 Feb. 1984,
all cultivars, regardless of N treatment, showed little change in dry weight, carbon content and nitrogen content but the
proportion of total plant dry weight, carbon content and nitrogen content in the cotyledons decreased while the proportions
in root, stem and leaf tissue increased. On 20 Feb. 1984 there were no N effects. All cultivars but especially BW and MBg,
showed progressive increases in dry weight, carbon content and nitrogen content during the period 20 Feb. 1984 to 8 May 1984.
Pooled results for all four cultivars indicated that on 8 May 1984, plants given ammonium and urea had a greater dry weight,
carbon content and nitrogen content than controls.
At harvest (1–3 Sep. 1984), BW and MBg outyielded (g dw seed m−2) HF and MBd. Pooled results for all cultivars indicated that application of N regardless of form gave increased yield and
an increased N concentration (mg N g−1 dw) in the seed. 相似文献
873.
Quantitative histology of the hypertrophied human heart 总被引:1,自引:0,他引:1
Myocardial hypertrophy accompanies systemic hypertension and aortic stenosis, i.e., pressure overload. In man cardiac failure only appears after years of pressure overload, during which time cardiac function had been maintained. The structural correlates of cardiac failure have been a subject of much interest for many years. Several hypotheses relating alterations in muscle fiber alignment, capillary density, or collagen content have been offered. The application of morphometric techniques has provided essential quantitative information on the structural components of the normal and diseased heart. These data indicate that muscle fiber alignment remains normal in the pressure overloaded heart despite the presence of hypertrophy or the appearance of clinical failure. On the other hand, capillary density is decreased and collagen content is increased in hypertrophied hearts. Chemical studies on collagen concentration however have yielded inconsistent results. The relative contribution of the microcirculation and collagenous structure of the myocardium on its respective O2 availability, mechanical behavior, and deterioration in pump function will require further investigation. 相似文献
874.
G Gradwohl A Mazen G de Murcia 《Biochemical and biophysical research communications》1987,148(3):913-919
The interaction between highly purified poly(ADP-ribose) polymerase from calf thymus and different topological forms of pBR322 DNA has been studied by gel retardation electrophoresis and electron microscopy. We show that: (i) in the absence of nicks on DNA the enzyme has a marked affinity for supercoiled (form I) DNA, (ii) in the presence of single stranded breaks poly(ADP-ribose) polymerase preferentially binds to form II, (iii) in all cases enzyme molecules are frequently located at DNA intersections, (iv) a cooperative binding of the enzyme on DNA occurs. 相似文献
875.
876.
877.
J M Eaton 《BMJ (Clinical research ed.)》1980,281(6242):758-759
878.
879.
Detailed methods are presented for measurement and study of in vivo mutations and in vitro mutagenesis in human lymphocytes. The methods described include preparation of conditioned medium containing interleukin-2, enumeration of mutant clones, in vitro mutagenesis, and expansion of mutant clones for further study. 相似文献
880.
The role of pH, KCl, ATP, water activity, and temperature in ATP synthesis from ADP and Pi was investigated in sarcoplasmic reticulum vesicles isolated from rabbit skeletal muscle. In totally aqueous medium, the synthesis of ATP was inhibited by ATP, KCl, and pH values above 6.5. When the water activity of the medium was decreased by the addition of 30% (v/v) dimethyl sulfoxide, the synthesis of ATP was no longer inhibited by ATP; it was activated by KCl and the optimum pH changed from 6.5 to 7.5. In totally aqueous medium, the concentration of MgCl2 needed for half-maximal synthesis of ATP was found to vary with the temperature of the assay medium; at 35 degrees C it was 1 mM and increased to a value higher than 10 mM when the temperature was decreased to 15 degrees C. In the presence of 30% dimethyl sulfoxide, maximal synthesis of ATP was attained in presence of 0.05 mM MgCl2 at both 15 and 35 degrees C. The hypothesis is raised that in the living cell water structure may play a role in regulating the synthesis of ATP observed during the reversal of the Ca2+ pump of the sarcoplasmic reticulum. 相似文献