全文获取类型
收费全文 | 100881篇 |
免费 | 1103篇 |
国内免费 | 885篇 |
专业分类
102869篇 |
出版年
2021年 | 114篇 |
2020年 | 70篇 |
2019年 | 98篇 |
2018年 | 11921篇 |
2017年 | 10750篇 |
2016年 | 7617篇 |
2015年 | 854篇 |
2014年 | 612篇 |
2013年 | 688篇 |
2012年 | 4638篇 |
2011年 | 13270篇 |
2010年 | 12268篇 |
2009年 | 8494篇 |
2008年 | 10150篇 |
2007年 | 11743篇 |
2006年 | 621篇 |
2005年 | 843篇 |
2004年 | 1323篇 |
2003年 | 1349篇 |
2002年 | 1109篇 |
2001年 | 411篇 |
2000年 | 313篇 |
1999年 | 183篇 |
1998年 | 127篇 |
1997年 | 124篇 |
1996年 | 118篇 |
1995年 | 86篇 |
1994年 | 86篇 |
1993年 | 121篇 |
1992年 | 137篇 |
1991年 | 154篇 |
1990年 | 120篇 |
1989年 | 87篇 |
1988年 | 97篇 |
1987年 | 97篇 |
1986年 | 69篇 |
1985年 | 88篇 |
1984年 | 90篇 |
1983年 | 93篇 |
1982年 | 67篇 |
1981年 | 71篇 |
1980年 | 57篇 |
1979年 | 47篇 |
1978年 | 53篇 |
1976年 | 49篇 |
1975年 | 50篇 |
1974年 | 47篇 |
1973年 | 45篇 |
1972年 | 284篇 |
1971年 | 314篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
61.
62.
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
63.
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
64.
65.
66.
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
67.
Effects of menstrual cycle on metabolic responses to exercise 总被引:1,自引:0,他引:1
68.
69.
70.
Cultivar and pH effects on competition for nodule sites between isolates of Rhizobium in beans 总被引:2,自引:0,他引:2
Two experiments were carried out to evaluate the effect of acidity on bean-Rhizobium competition for nodule sites. SevenPhaseolus vulgaris host cultivars differing in acid-pH tolerance were grown in sand culture, and irrigated using a sub-irrigation system and nutrient solutions of pH 4.5, 5.0, 5.5, and 6.0. A mixed inoculant of two antibiotically markedRhizobium leguminosarum bvphaseoli strains CIAT899 (acid-tolerant) and CIAT632 (acid-sensitive) was used. The acid-tolerant CIAT899 dominated CIAT632 in nodule occupancy across all cultivars and pH treatments. Although several of the varieties had previously been identified as PH-tolerant, and these cultivars performed better than those reported to be acid sensitive, all showed a marked increase in nodulation and plant development when the pH was raised from 4.5 to 6.0. The second experiment using a modified Leonard jar system varied the inoculation ratio between CIAT899 and UMR1116 (acid-sensitive, inefficient in N2-fixation) and contrasted nodulation response for the bean varieties Preto 143 (pH-tolerant) and Negro Argel (pH-sensitive) at 3 pH treatments (4.5, 5.5, 6.5). There was a significant effect of host cultivar, ratio of inoculation, and pH on the percentage of nodule occupancy by each strain. At low pH CIAT899 had higher nodule occupancy than UM1116 in the variety Negro Argel but had the same percentage of nodulation when the variety was Preto 143. Increasing the cell concentration of UMR1116 produced more inefficient nodules at all treatment combinations and reduced plant growth for both cultivars used. 相似文献