全文获取类型
收费全文 | 1189410篇 |
免费 | 99126篇 |
国内免费 | 1654篇 |
专业分类
1290190篇 |
出版年
2021年 | 17653篇 |
2020年 | 12573篇 |
2019年 | 16121篇 |
2018年 | 16912篇 |
2017年 | 15671篇 |
2016年 | 27473篇 |
2015年 | 42080篇 |
2014年 | 50127篇 |
2013年 | 76650篇 |
2012年 | 30872篇 |
2011年 | 17694篇 |
2010年 | 41464篇 |
2009年 | 43581篇 |
2008年 | 17589篇 |
2007年 | 14897篇 |
2006年 | 21507篇 |
2005年 | 22571篇 |
2004年 | 21864篇 |
2003年 | 19702篇 |
2002年 | 17903篇 |
2001年 | 28296篇 |
2000年 | 25070篇 |
1999年 | 26738篇 |
1998年 | 24574篇 |
1997年 | 24363篇 |
1996年 | 23981篇 |
1995年 | 22084篇 |
1994年 | 21984篇 |
1993年 | 20934篇 |
1992年 | 22636篇 |
1991年 | 20831篇 |
1990年 | 19137篇 |
1989年 | 20389篇 |
1988年 | 18544篇 |
1987年 | 17179篇 |
1986年 | 16413篇 |
1985年 | 18402篇 |
1984年 | 19168篇 |
1983年 | 17055篇 |
1982年 | 18766篇 |
1981年 | 18150篇 |
1980年 | 16908篇 |
1979年 | 14912篇 |
1978年 | 15028篇 |
1977年 | 14601篇 |
1976年 | 13841篇 |
1975年 | 12880篇 |
1974年 | 12934篇 |
1973年 | 13425篇 |
1972年 | 11088篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
Tsyganov V. E.; Pavlova Z. B.; Kravchenko L. V.; Rozov S. M.; Borisov A. Y.; Lutova L. A.; Tikhonovich I. A. 《Annals of botany》2000,86(5):975-981
Using ethyl methane sulfonate (EMS) treatment of the seeds ofline SGE, a new mutant of pea (Pisum sativum L.) with alterationsin root development was obtained. The mutant phenotype dependson the density of the growth substrate: on sand (a high densitysubstrate) the mutant forms a small compact curly root systemwhereas on vermiculite (a low density substrate) differencesbetween the root systems of the mutant and wild type plantsare less pronounced. Genetic analysis revealed that the mutantcarries a mutation in a new pea gene designedcrt (curly roots).Gene crt has been localized in pea linkage group V. The mutantline named SGEcrt showed increased sensitivity to exogenousauxin and an increased concentration of endogenous indole-3-aceticacid (IAA) in comparison with the wild type line SGE. Copyright2000 Annals of Botany Company Pisum sativum L., root development, garden pea mutant, curly roots, auxin, environmental stimulus response 相似文献
962.
How reliable are dung counts for estimating elephant numbers? 总被引:4,自引:0,他引:4
R. F. W. Barnes 《African Journal of Ecology》2001,39(1):1-9
Dung counts are the most commonly used techniques for estimating elephant numbers in forests, yet there is considerable scepticism concerning their accuracy. Published accounts of dung counts show that they give estimates similar to those from other methods for vertebrates ranging in size from lizards to elephants. For ungulates, macropods and elephants there are strong correlations between estimates from dung counts and other methods. Thus, dung counts are as accurate or inaccurate as other methods for estimating vertebrate numbers, including elephants. Dung counts for elephants give estimates that are as precise as, and sometimes more precise than, those from aerial surveys of elephants. This is because the variance in dung density is usually low and results in a lower than expected variance for the final elephant estimate when combined with the variances of defecation and decay rates. Dung counts may be more appropriate than aerial surveys for monitoring small or declining elephant populations. 相似文献
963.
J. L. Wilkens T. Kuramoto 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1998,168(7):483-490
The decapod cardiovascular system consists of a single ventricle that pumps blood into seven arteries; previous work has
shown that the outflow distribution patterns of intact animals are variable. In the present study, flow recordings were made
from pairs of arteries in semi-isolated hearts whilst different cardioactive hormones were infused into the heart. Each hormone
(5-hydroxytryptamine, octopamine, dopamine, proctolin and F1) changed the outflow pattern, heart rate and ventricular pressure
in a unique way. The probable sites of hormone action are the cardioarterial valves located at the origin of each artery except
one, the dorsal abdominal. Outflow from the dorsal abdominal is controlled downstream by valves located at the origin of the
segmental lateral arteries. The responses to a particular hormone were sometimes different between the hearts of American
and Japanese lobsters.
Accepted: 11 May 1998 相似文献
964.
965.
966.
N. Jepsen 《Journal of fish biology》2003,63(1):260-262
Radio-tagged pikeperch Stizostedion lucioperca (55–74 cm L T ) were recaptured in a reservoir by anglers 52–55 months after tagging. A total of four fish were recaptured during the summer of 2001. These recaptures are remarkable because of the long period between tagging and recapture and because only one (of 15 potential) tagged pikeperch had been caught in the long period since tagging. 相似文献
967.
Biogeographical change in the tiger, Panthera tigris 总被引:2,自引:0,他引:2
968.
969.
S. M. Al-Zahrani 《Bioprocess and biosystems engineering》1999,21(1):57-60
The effects of mixing, the sodium alginate concentration, and calcium chloride concentration on the release of sulphamethoxazole (model drug) impregnated in calcium alginate beads were investigated and evaluated. The release behaviour of the sulphamethoxazole from the calcium alginate beads was studied in a 0.1N HCl aqueous solution at 37v°C. The release rate of the sulphamethoxazole depends heavily on the type of mixers during the formation of the drug-alginate beads. The highest release rate was achieved when four-bladed rectangular agitator was used while the lowest release was achieved when magnetic stirrer was used. The amount of the released sulphamethoxazole varies slightly with the variation of the alginate concentration. The total release of sulphamethoxazole when 1% w/v solution of sodium alginate was used found to be 80% of the total drug content while 72% and 68% of the total drug content for 1.5% and 2% sodium alginate solutions. Three different calcium chloride concentrations were used (i.e., 5%, 10%, and 15% CaCl2). The effect of the calcium chloride concentration on the release of the sulphamethoxazole is very pronounced. 相似文献
970.
J. M. Fryxell T. Crease A. W. Illius 《Proceedings. Biological sciences / The Royal Society》1999,266(1425):1277
The maintenance of genetic variability in morphological traits that affect fitness is poorly understood. We present a simple Mendelian model of genetic traits affecting foraging efficiency in grazing ungulates, based on a trade-off between rates of energy extraction at low versus high levels of plant abundance. The model suggests that variation in foraging efficiency could be maintained via lottery competition arising as a direct consequence of dynamically unstable interactions between consumers and their food resources. Lottery competition is a plausible mechanism for explaining wide variability in foraging efficiency, such as that documented in unstable Soay sheep populations on the St Kilda archipelago. 相似文献