全文获取类型
收费全文 | 108484篇 |
免费 | 1762篇 |
国内免费 | 980篇 |
专业分类
111226篇 |
出版年
2023年 | 88篇 |
2022年 | 249篇 |
2021年 | 414篇 |
2020年 | 261篇 |
2019年 | 319篇 |
2018年 | 12098篇 |
2017年 | 10894篇 |
2016年 | 7862篇 |
2015年 | 1374篇 |
2014年 | 1074篇 |
2013年 | 1258篇 |
2012年 | 5382篇 |
2011年 | 13887篇 |
2010年 | 12622篇 |
2009年 | 8812篇 |
2008年 | 10559篇 |
2007年 | 12099篇 |
2006年 | 999篇 |
2005年 | 1201篇 |
2004年 | 1582篇 |
2003年 | 1586篇 |
2002年 | 1289篇 |
2001年 | 552篇 |
2000年 | 430篇 |
1999年 | 267篇 |
1998年 | 173篇 |
1997年 | 116篇 |
1996年 | 110篇 |
1995年 | 91篇 |
1994年 | 80篇 |
1993年 | 130篇 |
1992年 | 188篇 |
1991年 | 162篇 |
1990年 | 138篇 |
1989年 | 155篇 |
1988年 | 123篇 |
1987年 | 133篇 |
1986年 | 121篇 |
1985年 | 118篇 |
1984年 | 103篇 |
1983年 | 101篇 |
1982年 | 95篇 |
1981年 | 93篇 |
1980年 | 89篇 |
1979年 | 99篇 |
1978年 | 99篇 |
1975年 | 74篇 |
1974年 | 74篇 |
1972年 | 300篇 |
1971年 | 314篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
It is widely believed that environmental variability is the main cause for fluctuations in commercially exploited small pelagic
fish populations around the world. Nevertheless, density-dependent factors also can drive population dynamics. In this paper,
we analyzed thirteen years of a relative abundance index of two clupeoids fish populations coexisting in the central-south
area off Chile, namely the common sardine, Strangomera bentincki, and anchovy, Engraulis ringens. We applied the classical diagnostic tools of time series analysis to the observed time-series. Also, the realized per capita
population growth rate was studied with the aim of detecting the feedback structure that is characterizing the population
dynamics of the two species. The analysis suggests that population fluctuations of the two species have an important density-dependent
component, displaying first-order (direct density-dependent) and second-order (delayed density-dependent) simultaneously.
The density-dependent component explained 70.5 and 55.6 % of the realized per capita population growth rate of common sardine
and anchovy, respectively. The deterministic skeleton model showed an asymptotic convergence to equilibrium density. In presence
of a stochastic environment, fluctuations were reproduced for the species showing a component of fluctuation with a period
of 4 year. The intrinsic dynamics of each species is typical of interacting species resulting from trophic interactions. It
is postulated that the second-order dynamics of S. bentincki and E. ringens in central-south Chile, may be the result from interactions with a specialist predator (the fishing fleet), interacting with
exogenous environmental factors. 相似文献
72.
Jacek Kęsy Beata Maciejewska Magdalena Sowa Magdalena Szumilak Krzysztof Kawałowski Maja Borzuchowska Jan Kopcewicz 《Plant Growth Regulation》2008,55(1):43-50
It has been shown that both IAA and ethylene application inhibit flower induction in the short-day plant Pharbitis nil. However application of IAA has elevated ethylene production in this plant, as well. Strong enhancement of ethylene production
is also correlated with the night-break effect, which completely inhibits flowering. In order to determine what the role of
IAA and ethylene is in the photoperiodic flower induction in Pharbitis nil, we measured changes in their levels during inductive and non-inductive photoperiods, and the effects of ethylene biosynthesis
and action inhibitors on inhibition of flowering by IAA. Our results have shown that the inhibitory effect of IAA on Pharbitis nil flowering is not physiological but is connected with its effect on ethylene biosynthesis. 相似文献
73.
74.
75.
The three potent antimitotic vinca alkaloids: vincristine (VCR), vinblastine (VLB), and vindesine (VDS) were compared for their effect in blocking axoplasmic transport in vitro using a desheathed preparation of the peroneal branch of cat sciatic nerve. A range of vinca alkaloid concentrations from 1–100μM was examined. The relative order of potency in blocking axoplasmic transport was VCR > VLB > VDS at a concentration of 25μM. At the higher concentrations block occurred so rapidly that a statistically significant difference between these agents could not be obtained. The relation of vinca block ot the transport mechanism is discussed. 相似文献
76.
Dennis T. Villareal Caroline M. Apovian Robert F. Kushner Samuel Klein 《Obesity (Silver Spring, Md.)》2005,13(11):1849-1863
Obesity causes serious medical complications and impairs quality of life. Moreover, in older persons, obesity can exacerbate the age‐related decline in physical function and lead to frailty. However, appropriate treatment for obesity in older persons is controversial because of the reduction in relative health risks associated with increasing body mass index and the concern that weight loss could have potential harmful effects in the older population. This joint position statement from the American Society for Nutrition and NAASO, The Obesity Society reviews the clinical issues related to obesity in older persons and provides health professionals with appropriate weight‐management guidelines for obese older patients. The current data show that weight‐loss therapy improves physical function, quality of life, and the medical complications associated with obesity in older persons. Therefore, weight‐loss therapy that minimizes muscle and bone losses is recommended for older persons who are obese and who have functional impairments or medical complications that can benefit from weight loss. 相似文献
77.
78.
The initial time of synthesis of globin polypeptide chains in differentiating red cells has been delineated. Three analytical techniques were used, namely, SDS-polyacrylamide gel electrophoresis, acid urea polyacrylamide gel electrophoresis, and two-dimensional gel electrophoresis. The results indicate that globin peptide chains are synthesized in proerythroblasts at the initial time when globin mRNA first enters into the cytoplasm, and there is no apparent time gap between these two events. 相似文献
79.
A case is presented of toxic shock syndrome in a patient with systemic lupus erythematosus. Toxic shock syndrome is rarely reported in patients who are immunosuppressed, perhaps because such patients are often treated vigorously with antibiotics at the earliest sign of infection. The association in this case may have been coincidental. 相似文献
80.
Reinhold Benesch Samuel Waxman Ruth Benesch Charles Baugh 《Biochemical and biophysical research communications》1982,106(4):1359-1363
Pteroyl polyglutamates bind to hemoglobin with an affinity which leaves only a minor fraction of red cell folate in the free state. This could serve as a regulating mechanism for protein synthesis in the differentiating erythroblast. 相似文献