全文获取类型
收费全文 | 12618篇 |
免费 | 1022篇 |
国内免费 | 9篇 |
专业分类
13649篇 |
出版年
2023年 | 51篇 |
2022年 | 99篇 |
2021年 | 234篇 |
2020年 | 136篇 |
2019年 | 201篇 |
2018年 | 238篇 |
2017年 | 219篇 |
2016年 | 315篇 |
2015年 | 608篇 |
2014年 | 637篇 |
2013年 | 773篇 |
2012年 | 1053篇 |
2011年 | 1014篇 |
2010年 | 634篇 |
2009年 | 580篇 |
2008年 | 816篇 |
2007年 | 842篇 |
2006年 | 722篇 |
2005年 | 718篇 |
2004年 | 679篇 |
2003年 | 630篇 |
2002年 | 635篇 |
2001年 | 116篇 |
2000年 | 75篇 |
1999年 | 126篇 |
1998年 | 153篇 |
1997年 | 125篇 |
1996年 | 113篇 |
1995年 | 115篇 |
1994年 | 98篇 |
1993年 | 75篇 |
1992年 | 84篇 |
1991年 | 54篇 |
1990年 | 58篇 |
1989年 | 60篇 |
1988年 | 33篇 |
1987年 | 36篇 |
1986年 | 39篇 |
1985年 | 38篇 |
1984年 | 48篇 |
1983年 | 34篇 |
1982年 | 59篇 |
1981年 | 39篇 |
1980年 | 43篇 |
1979年 | 27篇 |
1978年 | 31篇 |
1977年 | 32篇 |
1976年 | 18篇 |
1975年 | 17篇 |
1973年 | 13篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
41.
Atmospheric CO2 concentration has increased by 25% over the preindustrial level. A parallel increase in C concentration and decreases in N
concentration and δ13C of plants grown throughout this century have been observed in plant specimens stored in herbaria. We tested our previous
results in a study of 12 more species collected in the western Mediterranean throughout this century (1920–1930, 1945–1955,
and 1985–1990) and tree rings of Quercus pubescens from the same area. These changes were accompanied by apparent increases in condensed tannin concentration. A decreasing trend
in δ15N both in herbarium material and tree rings was also found, indicating that ecosystems might cope with higher plant N demand
by decreasing N losses and increasing N fixation and mineralization. These results may contribute to a better understanding
of the effects of global change on carbon and nitrogen cycling.
Received: 12 November 1995 / Accepted: 17 May 1996 相似文献
42.
Sylvie Leparoux Marc Padrines Yannick Fortun Bernard Colas 《Biotechnology letters》1996,18(2):135-138
Summary Enzymatic O-glycosylation of dipeptide derivatives containing a serine residue in the N or C terminal position and alanine or glycine as the second amino acid was achieved using the transgalactosylation activity of -galactosidase from the Achatina achatina digestive juice. Reactions were performed with lactose as glycosyl donor and the dipeptide ethyl (or methyl) esters N-protected by a benzyloxycarbonyl group (Z) as glycosyl acceptors. Yields of galactosyl-dipeptide derivatives were much higher than those obtained with the E.coli -galactosidase as catalyst. 相似文献
43.
In the short day plant Chenopodium rubrum and the long day plant Nicotiana tabacum cv. Havana 425, adenylate kinase (EC 2.7.4.3) occurs as a family of isoforms, with at least two members localized in the chloroplast representing the main isoforms. In this work, isoforms were separated by anion exchange chromatography and relative isoform activities were compared between vegetative plants and plants induced to flowering. In both species examined, a light regime leading to floral induction resulted in a significant decrease in the activity of one chloroplast isoform. This decrease modified considerably the relative distribution of isoform activities, especially that between the two chloroplast activities. 相似文献
44.
45.
Madan M. Kwatra Jolanda Schreurs Debra A. Schwinn Michael A. Innis Marc G. Caron Robert J. Lefkowitz 《Protein expression and purification》1995,6(6)
To obtain large quantities of pure human β2-adrenergic receptor (β2-AR) needed for structural studies, an efficient method for β2-AR purification was developed using a recombinant receptor with an eight amino acid epitope at its C-terminus. This epitope is recognized by KT3-monoclonal antibody. The epitope tagged β2-AR was expressed in Sf9 cells with a specific activity of 5–20 pmol/mg of membrane protein. The epitope-tagged and wild-type receptors had identical ligand binding properties. The tagged receptor was solubilized using dodecyl-β-maltoside with a quantitative yield. Solubilized epitope-tagged receptors were partially purified by KT3-mAb immunoaffinity in 60–70% yield. Further purification of the receptors on an alprenolol-affinity column resulted in a homogenous preparation with an overall yield of >30%. The purified receptor was concentrated to >1 mg/ml without loss of ligand binding activity. 相似文献
46.
Marc Klein 《Invertebrate neuroscience : IN》1995,1(1):15-24
Up and down-regulation of calcium and potassium conductances are associated with several forms of short-term synaptic modulation.
Detailed investigation of synaptic plasticity in the marine gastropodAplysia, and in other mollusks, indicates that synaptic transmission can be influenced in a number of ways by modulatory neurotransmitters
acting through several second-messenger cascades. Modulation at the synapse itself occurs by means of the regulation of calcium
current as well as through effects on processes directly involved in transmitter mobilization and exocytosis. Modulation of
potassium current plays a major role in controlling neuronal excitability and may contribute to a lesser extent to the regulation
of transmitter release through actions on the resting potential and on action potential configuration. 相似文献
47.
Plasmid transfer to indigenous marine bacterial populations by natural transformation 总被引:3,自引:0,他引:3
Abstract Horizontal gene transfer among microbial populations has been assumed to occur in the environment, yet direct observations of this phenomenon are rare or limited to observations where the mechanism(s) could not be explicitly determined. Here we demonstrate the transfer of exogenous plasmid DNA to members of indigenous marine bacterial populations by natural transformation, the first report of this process for any natural microbial community. Ten percent of marine bacterial isolates examined were transformed by plasmid DNA while 14% were transformed by chromosomal DNA. Transformation of mixed marine microbial assemblages was observed in 5 of 14 experiments. In every case, acquisition of the plasmid by members of the indigenous flora was accompanied by modification (probably from genetic rearrangement or methylation) that altered its restriction enzyme digestion pattern. Estimation of transformation rates in estuarine environments based upon the distribution of competency and transformation frequencies in isolates and mixed populations ranged from 5 × 10−4 to 1.5 transformants/1 day. Extrapolation of these rates to ecosystem scales suggests that natural transformation may be an important mechanism for plasmid transfer among marine bacterial communities. 相似文献
48.
Margaret E. Halloran Marc Bekoff 《Ethology : formerly Zeitschrift fur Tierpsychologie》1994,97(3):236-248
Nesting behaviour of Abert squirrels (Sciurus aberti), including site selection and use, was studied in the foothills of the Rocky Mountains in Boulder County, Colorado. Only females were observed building nests, although both males and females maintained nests once they were built. Communal nesting by Abert squirrels was rare, but the majority of observed nest sharings involved unrelated male and female pairs. A total of 14 variables were used to evaluate the nests (n = 49) inhabited by Abert squirrels from May 1988 to Jun. 1991. All nests were located in Ponderosa pine (Pinus ponderosa) trees. The majority of nests were constructed of twigs and located in the upper one-third of the canopy, near the trunk, on the south-east side of the tree. Trees with nests were predominantly located in closed stands. Nest trees, when compared with unused control trees that were equally accessible to squirrels, were significantly different from control trees in five of nine variables. Nest tree crowns intertwined with a larger number of adjacent tree crowns than did control tree crowns. Nest trees were also significantly taller than control trees, but subdominant to adjacent trees within a stand. Seasonal and diurnal patterns of nest use indicate that Abert squirrels do not choose nest locations on the south-east sides of trees to facilitate behavioural thermoregulation. Rather, Abert squirrels select nest site locations to (1) maximize accessibility and (2) maximize structural stability which may provide protection from wind and rain. 相似文献
49.
50.