全文获取类型
收费全文 | 1913篇 |
免费 | 219篇 |
国内免费 | 296篇 |
专业分类
2428篇 |
出版年
2024年 | 22篇 |
2023年 | 76篇 |
2022年 | 84篇 |
2021年 | 86篇 |
2020年 | 89篇 |
2019年 | 97篇 |
2018年 | 94篇 |
2017年 | 85篇 |
2016年 | 102篇 |
2015年 | 86篇 |
2014年 | 114篇 |
2013年 | 120篇 |
2012年 | 87篇 |
2011年 | 87篇 |
2010年 | 80篇 |
2009年 | 116篇 |
2008年 | 116篇 |
2007年 | 120篇 |
2006年 | 93篇 |
2005年 | 84篇 |
2004年 | 69篇 |
2003年 | 59篇 |
2002年 | 55篇 |
2001年 | 53篇 |
2000年 | 28篇 |
1999年 | 48篇 |
1998年 | 38篇 |
1997年 | 33篇 |
1996年 | 20篇 |
1995年 | 15篇 |
1994年 | 19篇 |
1993年 | 16篇 |
1992年 | 17篇 |
1991年 | 12篇 |
1990年 | 11篇 |
1989年 | 13篇 |
1988年 | 17篇 |
1987年 | 3篇 |
1986年 | 10篇 |
1985年 | 5篇 |
1984年 | 10篇 |
1983年 | 8篇 |
1982年 | 5篇 |
1981年 | 6篇 |
1979年 | 5篇 |
1978年 | 2篇 |
1975年 | 2篇 |
1974年 | 2篇 |
1973年 | 3篇 |
1970年 | 2篇 |
排序方式: 共有2428条查询结果,搜索用时 10 毫秒
1.
The probability that protected areas will deliver their potential for maintaining or enhancing biodiversity is likely to be
maximised if they are appropriately and effectively managed. As a result, governments and conservation agencies are devoting
much attention to the management of protected areas. In the U.K., the demand for performance accountability has resulted in
Public Service Agreements (PSA) that set out targets for government departments to deliver results in return for investments
being made. One such target for England is to ensure that all nationally important wildlife sites are in favourable condition
by 2010. Here, we tested the hypothesis, of potential strategic importance, that the ecological condition of these sites is
predictable from relationships with a range of physical, environmental and demographic variables. We used binary logistic
regression to investigate these relationships, using the results of English Nature’s 1997–2003 condition assessment exercise.
Generally, sites in unfavourable condition tend to be larger in area, located at higher elevations, but with higher human
population density and are more spatially isolated from units of the same habitat. However, despite the range of different
parameters included in our models, the extent to which the condition of any given site could be predicted was low. Our results
have implications for the delivery of PSA targets, funding allocation, and the location of new protected areas. 相似文献
2.
M. Gaudet F. Villani M. Cherubini I. Beritognolo I. Dalla Ragione S. Proietti 《Plant biosystems》2019,153(4):491-497
In central Italy, Prunus cerasus var. austera is cultivated as small stands or scattered trees in marginal areas for the production of jam and wine. Thanks to the healthy attributes of its products and its ability to grow in different environmental conditions, this variety has gained new interest in the development of marginal areas. We assessed the level of the genetic variability of P. cerasus var. austera germplasm from central Italy and identified a ‘core collection’ representative of the present genetic diversity. A total of 161 trees, morphologically identified as var. austera, and one tree, identified as var. caproniana were collected and genotyped by 14 SSRs. Two individuals provided by a commercial plant nursery, one of P. cerasus var. caproniana and one of P. cerasus var. austera, were used as control. Thirteen SSRs presented private alleles in austera. Seven individuals morphologically identified as austera revealed private alleles specific to caproniana. The PCoA and Bayesian clustering analysis showed a main genetic group including var. austera, while a second group included all the caproniana-like genotypes. A core collection of 31 trees (46% of austera genotypes) was selected. This study can be considered as a starting point for future investigations on this variety. 相似文献
3.
4.
The use of plantations to manage extensive tracks of deforested lands in the tropics is a conservation strategy that has recently received considerable attention. Plantation trees can promote seed dispersal by attracting dispersers and creating favorable site conditions, leading to increased germination and establishment of indigenous trees. Subsequently, plantation trees can be harvested for profit or left to senesce, leaving a native tree community. We evaluated the effect of vine, grass, and shrub cutting (weeding) over a 3‐year period on regeneration of indigenous trees subsequent to the removal of plantation softwoods in Kibale National Park, Uganda. Counter to what would be expected if weeding released trees from competition, we found no difference in the total number of stems or in the stems greater than 10 cm diameter at breast height between control and weeded plots; there were more stems greater than 1 cm diameter at breast height in the control plots. For species found in both control and weeded plots, the maximum size of individuals did not differ. At the end of the study, 61 species were found in the control plots and 43 species were found in the weeded plots, and in both types of plots the three most abundant species were the same. The number of species and stems classified as early or middle successional species did not differ between weeded and control plots. The fact that weeding did not promote regeneration of indigenous trees after the removal of plantation trees illustrates the importance of evaluating and field‐testing potential management options. 相似文献
5.
B Griefahn C Künemund M Blaszkewicz K Golka P Mehnert G Degen 《Bioelectromagnetics》2001,22(8):581-588
The present study investigated the hypothesis that a strong extremely low frequency magnetic field partially suppresses the synthesis of melatonin and subsequently elevates the core body temperature. Seven healthy young men (16-22 years) took part in a control and in an exposure session. Three men experienced first the control and then the exposure session, four men experienced the sessions in reverse order. Control sessions were performed as constant routines, where the participants spent 24 hour periods continuously in bed while air temperature was 18 degrees C, illumination less than 30 lux, and the sound pressure level 50 dBA. The exposure sessions differed from that protocol only between 6 pm and 2 am when a strong extremely low frequency magnetic field was continuously applied (16.7 Hz, 0.2 mT). Assuming that the participants were unable to perceive the field consciously, they were blind against the actual condition. Salivary melatonin levels were determined hourly; body core temperatures and heart rates were registered continuously throughout. Neither of these parameters revealed alterations that can be related to the influence of the magnetic field. The present results, taken together with other investigations using that particular field, lead to the hypothesis that the effects most likely, occur, only after repetitive exposures to intermittent fields. 相似文献
6.
C. E. Scanlon N. R. Chalmers M. A. O. Monteiro da Cruz 《International journal of primatology》1989,10(2):123-136
Three wild groups of common marmoset, Callithrix jacchus jacchus,in north-east Brazil, of approximately similar size, had home ranges between 2.5 and 6.5 ha. But their core areas were similar
in size between 1.0 and 1.5 ha, with a monthly area of heavy use between 1.1 and 1.6 ha. The groups were selective in the
use of their home ranges, even though they were small: they used some areas heavily and others lightly. The core areas had
higher densities of trees that produced gum exudates than did other parts of the home ranges. Our data suggest that a group
of marmosets in this habitat may require a minimum of about 50 gum trees in its home range at a minimum density of about 50
trees/ha. In addition, the animals require suitable trees in which to sleep. We suggest that patches of forest with these
desirable properties remain relatively fixed in size and location over the years and that individual animals are constantly
in flux between them. 相似文献
7.
A native high molecular complex (Mr 850000) containing about 50% of the allphycocyanin of the phycobilisome but lacking allophycocyanin B was separated from isolated phycobilisomes by gel electrophoresis. It was designated APCM since the large linker polypeptide LCM was exclusively localized in this complex. The complex exhibited a ?196°C fluorescence emission maximum at 673 nm (671 nm at 25°C). In addition, a core complex (designated APC, Mr≥1000000) consisting of both APCM and AP 680 was isolated by combined gel filtration and linear gradient centrifugation. At 25°C this complex showed dual emission peaks at 670 and 680 nm demonstrating functional independence of the terminal emitters. A complex similar to APCM can be isolated from phycobilisomes of Anabaena variabilis. This is evidence that APCM is the constitutive center of the tricylindrical core of hemidiscoidal cyanobacterial phycobilisomes. Two models summarizing the structural and functional consequences of the results are presented in the discussion. 相似文献
8.
P. M. Abuja M. Schmuck I. Pilz P. Tomme M. Claeyssens H. Esterbauer 《European biophysics journal : EBJ》1988,15(6):339-342
Limited proteolysis (papain) of the cellobiohydrolase I (CBH I, 65 kDa) from Trichoderma reesei led to the seperation of two functional domains: a core protein (55 kDa) containing the active site, and a C-terminal glycopeptide (10 kDa) implicated in binding to the insoluble matrix (cellulose). The quaternary structures of the intact CBH I and its core in solution are now compared by small angle X-ray scattering (SAXS) measurements. The molecular parameters derived for the core (Rg=2.09 nm, Dmax=6.5 nm) and for the intact enzyme (Rg=4.27 nm, Dmax=18 nm) indicate very different shapes. The resulting models show a tadpole-like structure for the intact enzyme where the isotropic part coincides with the core protein and the flexible tail part should be identified with the C-terminal glycopeptide. Thus in this enzyme, functional differentiation is reflected in structural peculiarities.Abbreviations SAXS
small angle X-ray scattering
- SDS-PAGE
SDS-polyacrylamide gel electrophoresis
- IEF-PAG
polyacrylamide gel isoelectric focusing; cellobiohydrolase (CBH, 1,4--glucan cellobio hydrolase (E.C.3.2.1.91))
- Dmax
maximum diameter
- Rg
radius of gyration 相似文献
9.
Peter Alliger Wolfgang Traut Eric Carstens Ellen Fanning 《Biochimica et Biophysica Acta (BBA) - Gene Structure and Expression》1988,951(2-3)
A monkey cell factor that interacts specifically with double- and single-stranded DNA sequences in the early domain of the simian virus 40 (SV40) core origin of replication was identified using gel-retention assays. The protein was enriched over 1200-fold using ion-exchange and affinity chromatography on single-strand DNA cellulose. Binding of protein to mutant origin DNA restriction fragments was correlated with replication activity of the mutant DNAs. Exonuclease footprint experiments on single-stranded DNA revealed prominent pause sites in the early domain of the core origin. The results suggest that this cellular protein may be involved in SV40 DNA replication. 相似文献
10.
Rapid and simple isolation of pure photosystem II core and reaction center particles from spinach 总被引:2,自引:2,他引:0
Peter J. van Leeuwen Maaike C. Nieveen Erik Jan van de Meent Jan P. Dekker Hans J. van Gorkom 《Photosynthesis research》1991,28(3):149-153
Pure and active oxygen-evolving PS II core particles containing 35 Chl per reaction center were isolated with 75% yield from spinach PS II membrane fragments by incubation with n-dodecyl--D-maltoside and a rapid one step anion-exchange separation. By Triton X-100 treatment on the column these particles could be converted with 55% yield to pure and active PS II reaction center particles, which contained 6 Chl per reaction center.Abbreviations Bis-Tris
bis[2-hydroxyethyl]imino-tris[hydroxymethyl]methane
- Chl
chlorophyll
- CP29
Chl a/b protein of 29 kDa
- Cyt b
559
cytochrome b
559
- DCBQ
2,5-dichloro-p-benzo-quinone
- LHC II
light-harvesting complex II, predominant Chl a/b protein
- MES
2-[N-Morpholino]ethanesulfonic acid
- Pheo
pheophytin
- PS H
photosystem II
- QA
bound plastoquinone, serving as the secondary electron acceptor in PS II (after Pheo)
- SDS
sodiumdodecylsulfate 相似文献