全文获取类型
收费全文 | 201873篇 |
免费 | 8832篇 |
国内免费 | 1591篇 |
出版年
2021年 | 1396篇 |
2020年 | 955篇 |
2019年 | 1178篇 |
2018年 | 13391篇 |
2017年 | 12060篇 |
2016年 | 9546篇 |
2015年 | 3228篇 |
2014年 | 3275篇 |
2013年 | 4549篇 |
2012年 | 8433篇 |
2011年 | 16907篇 |
2010年 | 15125篇 |
2009年 | 10881篇 |
2008年 | 13413篇 |
2007年 | 14806篇 |
2006年 | 3729篇 |
2005年 | 3687篇 |
2004年 | 3960篇 |
2003年 | 3875篇 |
2002年 | 3410篇 |
2001年 | 4728篇 |
2000年 | 4466篇 |
1999年 | 3341篇 |
1998年 | 1143篇 |
1997年 | 1121篇 |
1996年 | 981篇 |
1993年 | 882篇 |
1992年 | 2515篇 |
1991年 | 2533篇 |
1990年 | 2495篇 |
1989年 | 2254篇 |
1988年 | 2104篇 |
1987年 | 2014篇 |
1986年 | 1849篇 |
1985年 | 1856篇 |
1984年 | 1527篇 |
1983年 | 1338篇 |
1982年 | 965篇 |
1981年 | 902篇 |
1979年 | 1453篇 |
1978年 | 1152篇 |
1977年 | 1029篇 |
1976年 | 1000篇 |
1975年 | 1180篇 |
1974年 | 1279篇 |
1973年 | 1351篇 |
1972年 | 1436篇 |
1971年 | 1322篇 |
1970年 | 937篇 |
1969年 | 959篇 |
排序方式: 共有10000条查询结果,搜索用时 928 毫秒
81.
I V Kudriashova D B Logunov O S Papsuevich R I Kruglikov 《Biulleten' eksperimental'no? biologii i meditsiny》1985,99(1):5-7
Perfusion of the snail (Helix lucorum L.) CNS with DG-AVP (concentration 10(-6) M) in the course of low frequency intracellular stimulation (2-4-minute interval) of the defensive reflex command neurons led to an increase in the excitability. It was expressed both in the reduction of the spike generation latency, in the increased number of spikes in response to fixed stimuli, and in the activation of pacemaker potentials. If DG-AVP was added to the medium during endoneuronal habituation, there was no increase in the excitability. It is supposed that modification of the neuronal excitability may be caused by the DG-AVP effect on the pacemaker mechanism. 相似文献
82.
The ability of an invasive plant to occupy new areas is often attributed to both morphological and physiological plasticities
that allow them to remain viable over a wide range of environmental conditions. Studies addressing the ecological requirements
of Microstegium vimineum often consider soil moisture or soil moisture along with other factors as important explanatory components for the establishment
and persistence of this invasive monocot. However, controlled studies specifically targeting water relations in M. vimineum are needed. Therefore, the purpose of this study was to determine how different water availabilities influence the growth
and physiological performance of M. vimineum. This study utilized experimental microcosms to achieve different water availabilities including low soil moisture (<15%
water), moderate soil moisture (ca. 20–30%), and flooded conditions. While both flooded and low soil moisture resulted in
diminished growth, M. vimineum still survived under these conditions. Physiological processes including C4 metabolism, minimum stress under low water conditions, and the ability to increase tissue rigidity may confer some advantages
to M. vimineum during periods of limiting water conditions. Similarly, the proportionally low root biomass, shallow root structure, and
its ability to maintain stable water relations during flooding and/or soil waterlogging may facilitate M. vimineum’s ability to invade mesic habitats. It is likely, therefore, that the capacity to tolerate both low soil moistures and flooded
conditions has enhanced the ability of M. vimineum to populate disturbed systems in central North Carolina. 相似文献
83.
A particularly vexing phenomenon within invasion ecology is the occurrence of spontaneous collapses within seemingly well-established exotic populations. Here, we assess the frequency of collapses among 68 exotic bird populations established in Hawaii, Puerto Rico, Los Angeles and Miami. Following other published definitions, we define a ‘collapse’ as a decline in abundance of ≥90 % within ≤10 years that lasts for at least 3 years. We show that 44 of the 68 exotic bird populations have exhibited declines at some point within their time series. Sixteen of the populations declined sufficiently to be defined as collapsed. It took on average 3.8 ± 1.8 years for populations to decline into a collapsed state, and this state persisted on average for 7.1 ± 6.3 years across (collapsed) populations. We compared the severity and duration of declines across all 44 declining populations according to taxonomic Order and geographic region. Neither variable explained substantial variation in the metrics of collapse. Our results indicate that severe, rapid, and persistent population declines may be common among exotic populations. We suggest that incorporating the probability and persistence of collapses into management decisions can inform efforts to enact control or eradication measures. We also suggest that applying our approach to other taxa and locations is crucial for improving our understanding of when and where collapses are likely to occur. 相似文献
84.
85.
86.
87.
88.
S. T. Zakhidov T. L. Marshak E. A. Malolina A. Yu. Kulibin I.A. Zelenina S. M. Pavluchenkova V. M. Rudoi O. V. Dement’eva S. G. Skuridin Yu. M. Evdokimov 《Biochemistry (Moscow) Supplemental Series A: Membrane and Cell Biology》2010,4(3):293-296
The effect of gold nanoparticles on mouse epididymal sperm has been studied using the model system of nuclear chromatin decondensation
in vitro. It is shown that the treatment of gametes, preliminary membrane-freed by sodium dodecyl sulfate, in the mediums
containing gold nanoparticles (with diameter ∼2.5 nm) in concentrations 1.0 × 1015 or 0.5 × 1015 particles/ml and following incubation in dithiothreitol solution (DTT) resulted in failure of chromatin decondensation process
and nucleus structure. We conclude that gold nanoparticles possess spermatotoxicity. The mechanism of cytotoxic effect of
gold nanoparticles may be related with their interaction with molecules of double-helix DNA. The model system studied in this
research is applicable for further investigations of cytotoxic effects of nanoparticles of different origin and made of different
metals. 相似文献
89.
90.