全文获取类型
收费全文 | 116734篇 |
免费 | 2672篇 |
国内免费 | 900篇 |
专业分类
120306篇 |
出版年
2021年 | 267篇 |
2019年 | 219篇 |
2018年 | 12036篇 |
2017年 | 10872篇 |
2016年 | 7795篇 |
2015年 | 1251篇 |
2014年 | 987篇 |
2013年 | 1303篇 |
2012年 | 5310篇 |
2011年 | 13882篇 |
2010年 | 12779篇 |
2009年 | 8848篇 |
2008年 | 10604篇 |
2007年 | 12214篇 |
2006年 | 1137篇 |
2005年 | 1369篇 |
2004年 | 1938篇 |
2003年 | 1907篇 |
2002年 | 1632篇 |
2001年 | 952篇 |
2000年 | 845篇 |
1999年 | 684篇 |
1998年 | 354篇 |
1997年 | 382篇 |
1996年 | 362篇 |
1995年 | 312篇 |
1994年 | 264篇 |
1993年 | 318篇 |
1992年 | 624篇 |
1991年 | 530篇 |
1990年 | 541篇 |
1989年 | 536篇 |
1988年 | 449篇 |
1987年 | 449篇 |
1986年 | 381篇 |
1985年 | 381篇 |
1984年 | 414篇 |
1983年 | 334篇 |
1982年 | 293篇 |
1981年 | 282篇 |
1980年 | 250篇 |
1979年 | 327篇 |
1978年 | 267篇 |
1977年 | 282篇 |
1976年 | 215篇 |
1975年 | 240篇 |
1974年 | 240篇 |
1973年 | 210篇 |
1972年 | 407篇 |
1971年 | 457篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
31.
32.
The purpose of this review is to provide a global perspective on Oncorhynchus salmonine introductions and put-and-take fisheries based on modern stocking programs, with special emphasis on freshwater
ecosystems. We survey the global introductions of nine selected salmonines of the genus Oncorhynchus: golden trout, cutthroat trout, pink salmon, chum salmon, coho salmon, masu/cherry salmon, rainbow trout/steelhead, sockeye
salmon/kokanee, and chinook salmon. The information is organized on a geographical basis by continent, and then by species
and chronology. Two different objectives and associated definitions of ‘success’ for introductions are distinguished: (a)
seed introduction: release of individuals with the purpose of creating a wild-reproducing, self-sustaining population; and
(b) put-and-take introduction: release of individuals with the purpose of maintaining some level of wild population abundance,
regardless of wild reproduction. We identify four major phenomena regarding global salmonine introductions: (1) general inadequacy
of documentation regarding introductions; (2) a fundamental disconnect between management actions and ecological consequences
of introductions; (3) the importance of global climate change on success of previous and future introductions; and (4) the
significance of aquaculture as a key uncertainty in accidental introductions. We conclude this review with a recognition of
the need to terminate ongoing stocking programs for introduced salmonines worldwide. 相似文献
33.
34.
G. Van Der Velde L. A. Van Der Heijden P. A. J. Van Grunsven P. M. M. Bexkens 《Aquatic Ecology》1982,16(1):51-60
Summary In this paper the leaf-marking method as used for the study of the development and initial decomposition of floating leaves is described and the reliability of the various measurements is tested and/or discussed. Some general results obtained withNymphoides peltata (Gmel.) O. Kuntze in tanks and in the field are presented and crltically discussed. Autolysis followed by microbial decay was in all cases the most important factor by which leaves disintegrated. In the field plots animals were responsible for the disappearance of 22% of the total leaf area produced during a growth season. This is, however, the combined effect of consumption and damage succeeded by microbial decay. Real grazing can be estimated to be no more than 10% of the production of floating leaves. Fungi can have an important role in initial decomposition, especially after the flowering period, as is demonstrated forSeptoria villarsiae Desm. All damage types show temporal and, in the case of animals, also spatial distribution patterns. 相似文献
35.
M A Kersten M J Arninkhof H J Op den Camp L J Van Griensven C van der Drift 《Biochimica et biophysica acta》1999,1428(2-3):260-272
Mycelium of Agaricus bisporus took up methylamine (MA), glutamate, glutamine and arginine by high-affinity transport systems following Michaelis-Menten kinetics. The activities of these systems were influenced by the nitrogen source used for mycelial growth. Moreover, MA, glutamate and glutamine uptakes were derepressed by nitrogen starvation, whereas arginine uptake was repressed. The two ammonium-specific transport systems with different affinities and capacities were inhibited by NH(+)(4), with a K(i) of 3.7 microM for the high-velocity system. The K(m) values for glutamate, glutamine and arginine transport were 124, 151 and 32 microM, respectively. Inhibition of arginine uptake by lysine and histidine showed that they are competitive inhibitors. MA, glutamate and glutamine uptake was inversely proportional to the intracellular NH(+)(4) concentration. Moreover, increase of the intracellular NH(+)(4) level caused by PPT (DL-phosphinotricin) resulted in an immediate cessation of MA, glutamine and glutamate uptake. It seems that the intracellular NH(+)(4) concentration regulates its own influx by feedback-inhibition of the uptake system and probably also its efflux which becomes apparent when mycelium is grown on protein. Addition of extracellular NH(+)(4) did not inhibit glutamine uptake, suggesting that NH(+)(4) and glutamine are equally preferred nitrogen sources. The physiological importance of these uptake systems for the utilization of nitrogen compounds by A. bisporus is discussed. 相似文献
36.
37.
38.
Roberta L. Millstein 《Journal of the history of biology》2008,41(2):339-367
Biologists and philosophers have been extremely pessimistic about the possibility of demonstrating random drift in nature,
particularly when it comes to distinguishing random drift from natural selection. However, examination of a historical case
– Maxime Lamotte’s study of natural populations of the land snail, Cepaea nemoralis in the 1950s – shows that while some pessimism is warranted, it has been overstated. Indeed, by describing a unique signature
for drift and showing that this signature obtained in the populations under study, Lamotte was able to make a good case for
a significant role for␣drift. It may be difficult to disentangle the causes of drift and selection acting in a population,
but it is not (always) impossible. 相似文献
39.
40.
D. Padilla F. Acosta J. A. García F. Real Jose R. Vivas 《Archives of microbiology》2009,191(3):191-198
Hafnia alvei, a Gram negative bacillus related to the Enterobacteriaceae family, is considered an opportunistic pathogen of several animal species and humans. In this communication, we describe
fimbrial-like structures from different strains of H. alvei that cannot be easily ascribed to any of the previously reported fimbrial types in this species (type I or type III). Polymerase
chain reaction (PCR) and immunofluorescence assays were carried out to study fimbriae and flagella in H. alvei strains isolated from different sources. No correlation between the results obtained by PCR and those obtained by phenotypic
methods were found, and the antibodies used gave cross or different recognition patterns of the surface structures present
in these strains. We report as well that strain and growth temperature influence fimbriation and expression of flagella in
human and animal isolates of H. alvei. This study also indicates that the absence of fimbriae have a significant positive influence on the initial adhesion of
H. alvei to human epithelial cells. 相似文献