全文获取类型
收费全文 | 108587篇 |
免费 | 1578篇 |
国内免费 | 891篇 |
专业分类
111056篇 |
出版年
2024年 | 24篇 |
2023年 | 98篇 |
2022年 | 251篇 |
2021年 | 470篇 |
2020年 | 243篇 |
2019年 | 322篇 |
2018年 | 12157篇 |
2017年 | 10936篇 |
2016年 | 8001篇 |
2015年 | 1471篇 |
2014年 | 1241篇 |
2013年 | 1432篇 |
2012年 | 5642篇 |
2011年 | 14158篇 |
2010年 | 12799篇 |
2009年 | 8861篇 |
2008年 | 10765篇 |
2007年 | 12221篇 |
2006年 | 1179篇 |
2005年 | 1376篇 |
2004年 | 1723篇 |
2003年 | 1645篇 |
2002年 | 1412篇 |
2001年 | 380篇 |
2000年 | 258篇 |
1999年 | 152篇 |
1998年 | 151篇 |
1997年 | 122篇 |
1996年 | 88篇 |
1995年 | 58篇 |
1994年 | 68篇 |
1993年 | 93篇 |
1992年 | 76篇 |
1991年 | 79篇 |
1990年 | 56篇 |
1989年 | 55篇 |
1988年 | 40篇 |
1987年 | 40篇 |
1986年 | 23篇 |
1985年 | 23篇 |
1984年 | 31篇 |
1983年 | 34篇 |
1982年 | 23篇 |
1981年 | 19篇 |
1980年 | 17篇 |
1975年 | 20篇 |
1974年 | 16篇 |
1972年 | 251篇 |
1971年 | 280篇 |
1962年 | 26篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
61.
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
62.
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
63.
64.
65.
Methylobacterium rhodesianum MB 126 was cultivated using extended cultures without outflow. The feeding regime was based on the pH-regulated synchronous dosages of ammonia, methanol, phosphatc and trace elements according to supposed stoichiometric relations. The acidity of the culture medium was kept constant at pH 6.8, whereas the dissolved oxygen concentration was adjusted at 80% of saturation by autoregulation of the stirrer speed. However, besides testing technical conditions, two types of fermentations were discovered which are described in this paper. Firstly, although at the beginning of the bioprocesses the impeller speed increased up to 2,000 rpm, a decrease of dissolved oxygen down to zero was unavoidable. Secondly, methanol was accumulated temporarily up to 44 g/l and 26 g/l at 23 h of fermentation time and without inhibition of growth at least up to 30 g/l or PHB production. During this accumulation of the carbon substrate, exponential growth phases were detected showing growth rates of μ = 0.20/h and 0.21/h. But then, phases of retarded growth followed, whereas the methanol disappeared either continuously or after a steady level. In the course of a 54-h fermentation period, the synthesized PHB amounted to a content of above 50% of cell dry mass. From this data, a volumetric productivity of 0.4 g PHB/lxh was estimated. Moreover, the growth related yield coefficients were calculated to YX/MeOH = 0.21 and YX/MeOH = 0.14, whereas the product related yield coefficients amounted to YPHB/MeOH = 0.12 and YPHB/MeOH = 0,09. Since the shift down of growth rates as well as the production of PHB agreed in time with partial oxygen limitation (40% oxygen saturation), the competition observed between the tricarboxylic acid cycle and PHB synthesis was discussed. Summarizing the results, it was concluded that the frequently described inhibitory effect of methanol of above 2 g/l seems to be rather an effect of experimentally chosen conditions than of a general physiological phenomenon. Therefore, it could be demonstrated that the toxicity of methanol could be overcome if it was not dosed at different times but simultaneously with other medium components. 相似文献
66.
67.
68.
Andrea Schrödel 《当今生物学》2007,37(5):289-289
69.
The kinetics of haemolysis of rabbit erythrocytes byCroton tiglium lectin was studied as a function of concentration of the lectin and erythrocytes. The length of the prelytic period decreased with increasing lectin concentrations, indicating that the secondary events at the membrane which follow the binding of the lectin to cell surface carbohydrate receptors are accelerated at higher surface concentrations of the lectin. The rate or extent of haemolysis was not affected by the inclusion of ions like K+, Ca2+ and Mg2+ in the medium or by the substitution of ionic medium by a non-ionic medium. The inhibition of haemagglutination and haemolysis of rabbit red cells byCroton tiglium lectin by antilectin rabbit serum was observed. A possible mechanism of haemolysis by the lectin is discussed. 相似文献
70.
N-Acetyl-D-galactosamine in β-linkage being ubiquitous in cell surface glycoproteins, their interaction with lectins specific for this sugar moiety may be a significant event in cell adhesion phenomena. This article discusses the common β-N-acetyl galactosamine-specific lectins, with particular stress on the lectin from winged beans (Psophocarpus tetragonolobus). 相似文献