全文获取类型
收费全文 | 1601821篇 |
免费 | 146155篇 |
国内免费 | 1451篇 |
专业分类
1749427篇 |
出版年
2021年 | 18194篇 |
2019年 | 16184篇 |
2018年 | 19472篇 |
2017年 | 18308篇 |
2016年 | 29191篇 |
2015年 | 42727篇 |
2014年 | 50832篇 |
2013年 | 76922篇 |
2012年 | 46203篇 |
2011年 | 37549篇 |
2010年 | 47362篇 |
2009年 | 47545篇 |
2008年 | 34498篇 |
2007年 | 33639篇 |
2006年 | 36369篇 |
2005年 | 37716篇 |
2004年 | 36547篇 |
2003年 | 33920篇 |
2002年 | 31855篇 |
2001年 | 47772篇 |
2000年 | 45175篇 |
1999年 | 41867篇 |
1998年 | 27117篇 |
1997年 | 26829篇 |
1996年 | 26133篇 |
1995年 | 24284篇 |
1994年 | 23908篇 |
1993年 | 23189篇 |
1992年 | 35464篇 |
1991年 | 34070篇 |
1990年 | 32599篇 |
1989年 | 33039篇 |
1988年 | 30216篇 |
1987年 | 29001篇 |
1986年 | 27307篇 |
1985年 | 28804篇 |
1984年 | 27073篇 |
1983年 | 23927篇 |
1982年 | 22679篇 |
1981年 | 21680篇 |
1980年 | 20275篇 |
1979年 | 23221篇 |
1978年 | 20912篇 |
1977年 | 19643篇 |
1976年 | 18915篇 |
1975年 | 19012篇 |
1974年 | 19856篇 |
1973年 | 20108篇 |
1972年 | 17352篇 |
1971年 | 15764篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
931.
932.
933.
934.
Paul E. Berry 《Brittonia》1999,51(2):214-216
A species erroneously described as aDiospyros is here transferred toLissocarpa, asL. tetramera (Rusby) P. E. Berry. This gives a total of five species currently recognized in Lissocarpaceae. A key to the species and
a discussion of the individual taxa are presented. 相似文献
935.
Studies in biodiversity-ecosystem function and conservation biology have led to the development of diversity indices that take species' functional differences into account. We identify two broad classes of indices: those that monotonically increase with species richness (MSR indices) and those that weight the contribution of each species by abundance or occurrence (weighted indices). We argue that weighted indices are easier to estimate without bias but tend to ignore information provided by rare species. Conversely, MSR indices fully incorporate information provided by rare species but are nearly always underestimated when communities are not exhaustively surveyed. This is because of the well-studied fact that additional sampling of a community may reveal previously undiscovered species. We use the rarefaction technique from species richness studies to address sample-size-induced bias when estimating functional diversity indices. Rarefaction transforms any given MSR index into a family of unbiased weighted indices, each with a different level of sensitivity to rare species. Thus rarefaction simultaneously solves the problem of bias and the problem of sensitivity to rare species. We present formulae and algorithms for conducting a functional rarefaction analysis of the two most widely cited MSR indices: functional attribute diversity (FAD) and Petchey and Gaston's functional diversity (FD). These formulae also demonstrate a relationship between three seemingly unrelated functional diversity indices: FAD, FD and Rao's quadratic entropy. Statistical theory is also provided in order to prove that all desirable statistical properties of species richness rarefaction are preserved for functional rarefaction. 相似文献
936.
937.
C-Type Virus Released from Cultured Human Rhabdomyosarcoma Cells 总被引:44,自引:0,他引:44
R. M. McALLISTER M. NICOLSON M. B. GARDNER R. W. RONGEY S. RASHEED P. S. SARMA R. J. HUEBNER M. HATANAKA S. OROSZLAN R. V. GILDEN A. KABIGTING L. VERNON 《Nature: New biology》1972,235(53):3-6
RD-114 virus, released from human rhabdomyosarcoma cells, has all the characteristics of a mammalian C-type virus. Immunological tests indicate that it differs from all known C-type viruses and is the most likely candidate for a human C-type virus yet described. 相似文献
938.
Populations of the Australian sleepy lizard, Tiliqua rugosa, near Mt. Mary, South Australia carry natural infestations of two tick species Aponomma hydrosauri and Amblyomma limbatum. In field experiments at two sites, 18 km apart, lizards with experimentally increased tick loads had smaller home ranges,
moved shorter distances in a day, and were found basking more but moving less often than lizards from which ticks were experimentally
removed. The results were consistent for adult lizards in two years, and for sub-adults in a third year. Laboratory trials
showed that juvenile lizards that had tick infestations had lower sprint speeds than uninfested siblings, and that adults
with tick infestations had less endurance than those that were uninfested. The results contrast with those of a previous survey
that showed that lizards with high tick loads had greater body size and remained longer at a site, but indicate that there
may be a balance, for lizards, between the fitness advantages in occupying habitats with high-quality resources, and the costs
from parasites that also prefer those habitats.
Received: 02 March 1999 / Accepted: 07 October 1999 相似文献
939.
E H Frieden H G Pollock B G Steinetz A B Rawitch 《Archives of biochemistry and biophysics》1988,266(2):334-342
In addition to B31 (CM-a) and B28 (CM-B) relaxins, acid-acetone extracts of ovaries of pregnant sows contain a third major relaxin species (relaxin C). The major components of relaxin C possess about half the activity of CM-a or CM-B in the guinea pig palpation assay, but are completely inactive in the mouse pubic ligament assay. Its uterotrophic and protein anabolic effects in ovariectomized, estrogen-primed mice, however, are comparable to those of CM-B. Sequence analysis indicates that the two major components of relaxin C, like the other porcine relaxins, consist of two polypeptide chains linked by disulfide bonds. The shorter (A) chains are identical to the A chains of the other porcine relaxins, except for the absence of the N-terminal arginine residue. The B chains display microheterogeneity; the B sequences of the two predominant species are the same as those of the other porcine relaxins through B25, but terminate at valine residue B25 or serine residue B26, respectively. 相似文献
940.
G. F. Katekak 《Biologia Plantarum》1985,27(2-3):92-99
The chemical and stereochemical properties of phytotropins are described in terms of a receptor model, based on their ability
to bind to a receptor isolated from maize coleoptiles. Their root gravitropic properties are correlated with their ability
to bind to the receptor. It is suggested that their ability to inhibit auxin transport, without more, may not be enough to
explain their physiological effects. 相似文献