全文获取类型
收费全文 | 2322篇 |
免费 | 125篇 |
国内免费 | 1篇 |
出版年
2023年 | 20篇 |
2022年 | 14篇 |
2021年 | 71篇 |
2020年 | 55篇 |
2019年 | 50篇 |
2018年 | 71篇 |
2017年 | 71篇 |
2016年 | 77篇 |
2015年 | 116篇 |
2014年 | 110篇 |
2013年 | 185篇 |
2012年 | 145篇 |
2011年 | 148篇 |
2010年 | 99篇 |
2009年 | 73篇 |
2008年 | 121篇 |
2007年 | 110篇 |
2006年 | 114篇 |
2005年 | 103篇 |
2004年 | 107篇 |
2003年 | 80篇 |
2002年 | 67篇 |
2001年 | 26篇 |
2000年 | 35篇 |
1999年 | 31篇 |
1998年 | 21篇 |
1997年 | 24篇 |
1996年 | 13篇 |
1995年 | 10篇 |
1994年 | 8篇 |
1993年 | 19篇 |
1992年 | 27篇 |
1991年 | 19篇 |
1990年 | 16篇 |
1989年 | 20篇 |
1988年 | 13篇 |
1987年 | 14篇 |
1986年 | 26篇 |
1985年 | 12篇 |
1984年 | 15篇 |
1983年 | 15篇 |
1982年 | 11篇 |
1980年 | 10篇 |
1979年 | 7篇 |
1978年 | 5篇 |
1977年 | 4篇 |
1976年 | 5篇 |
1975年 | 6篇 |
1969年 | 3篇 |
1958年 | 3篇 |
排序方式: 共有2448条查询结果,搜索用时 15 毫秒
1.
Maria João Feio Trefor B. Reynoldson Verónica Ferreira Manuel Augusto S. Graça 《Hydrobiologia》2007,579(1):55-68
We sampled macroinvertebrates at 75 locations in the Mondego river catchment, Central Portugal, and developed a predictive
model for water quality assessment of this basin, based on the Reference Condition Approach. Sampling was done from June to
September 2001. Fifty-five sites were identified as “Reference sites” and 20 sites were used as “Test sites” to test the model.
At each site we also measured 40 habitat variables to characterize water physics and chemistry, habitat type, land use, stream
hydrology and geographic location. Macroinvertebrates were generally identified to species or genus level; a total of 207
taxa were found. By Unweighted Pair Group Method with Arithmetic mean (UPGMA) clustering and analysis of species contribution
to similarities percentage (SIMPER), two groups of reference sites were established. Using Discriminant Analysis (stepwise
forward), four variables correctly predicted 78% of the reference sites to the appropriate group: stream order, pool quality,
substrate quality and current velocity. Test sites’ environmental quality was established from their relative distance to
reference sites, in MDS ordination space, using a series of bands (BEAST methodology). The model performed well at upstream
sites, but at downstream sites it was compromised by the lack of reference sites. As with the English RIVPACS predictive model,
the Mondego model should be continually improved with the addition of new reference sites. The adaptation of the Mondego model
methodology to the Water Framework Directive is possible and would consist mainly of the integration of the WFD typology and
increasing the number of ellipses that define quality bands.
Handling editor: K. Martens 相似文献
2.
S Ikei M Ogawa T Beppu C Ohara K Sakamoto H Sameshima H Arakawa Y Yamaguchi T Yamanaka S Kudo 《Cytokine》1992,4(6):581-584
In an attempt to investigate the interaction between the changes of cytokines and acute phase reactants after transcatheter arterial chemoembolization therapy (TACE), the levels of interleukin 6 (IL-6), interleukin 8 (IL-8), C-reactive protein (CRP) and pancreatic secretory trypsin inhibitor (PSTI) in the blood of patients with unresectable hepatocellular carcinoma (HCC) were measured. Before the therapy, serum IL-6 and plasma IL-8 levels were detectable in 77.8% and 28.5%, respectively, of patients with HCC. Levels of serum IL-6 and plasma IL-8 increased after TACE and reached a peak on day 3 in all patients (18/18) and in 87.5% of patients (12/14), respectively. Both blood levels of IL-6 and IL-8 reached a peak earlier than those of CRP and PSTI did after the therapy. When the maximal values of IL-6 were compared with those of CRP and PSTI, there were significant positive correlations (r = 0.63, P < 0.01 and r = 0.81, P < 0.01, respectively). Similarly, comparisons of the maximal values of IL-8 with those of CRP and PSTI gave a significant correlation (r = 0.68, P < 0.01 and r = 0.67, P < 0.05, respectively). However, no significant correlation was found between the elevation of IL-6 and IL-8. 相似文献
3.
Fine root distribution of trees and understory in mature stands of maritime pine (Pinus pinaster) on dry and humid sites 总被引:2,自引:0,他引:2
Maritime pine (Pinus pinaster) is the main tree cropping species in the Landes of Gascogne forest range in south western France. Soils are nutrient poor, sandy podzosols and site fertility is determined essentially by organic matter content and depth of water table, which is known to limit root growth. We hypothesised, with an ultimate goal of constructing a nutrient uptake model applicable to this region, that the organic top horizons together with the depth of the water table should be the most important parameters related to fine root distribution and presence of associated mycorrhiza. To test this hypothesis, we compared two adult Pinus pinaster stands, contrasting in depth of water table and soil fertility and evaluated fine roots (diameter ≤2 mm) of understory species and fine roots and ectomycorrhizal morphotypes of Pinus pinaster down to 1.2 m, using a soil corer approach. Total fine root biomass of Pinus pinaster was not significantly different between both sites (3.6 and 4.5 t ha−1 for the humid, respectively, dry site), but root distribution was significantly shallower and root diameter increased more with depth at the humid site, presumably due to more adverse soil conditions as related to the presence of a hardpan, higher amount of aluminium oxides and / or anoxia. Fine roots of Pinus pinaster represented only about 30% of total fine root biomass and 15% of total fine root length, suggesting that the understory species cannot be ignored with regards to competition for mineral nutrients and water. A comparison of the ectomycorrhizal morphotypes showed that the humid site could be characterised by a very large proportion of contact exploration types, thought to be more relevant in accessing organic nutrient sources, whereas the dry site had a significantly higher proportion of both long-distance and short-distance exploration types, the latter of which was thought to be more resistant to short-term drought periods. These results partly confirm our hypothesis on root distribution as related to the presence of soil mineral nutrients (i.e. in organic matter), point out the potential role of understory plant species and ectomycorrhizal symbiosis and are a valuable step in building a site-specific nutrient uptake model. 相似文献
4.
Augusto Pessina 《Cytotechnology》1993,12(1-3):127-135
5.
José Augusto Rosemberg 《Mycopathologia》1959,10(4):283-288
Riassunto E' una breve nota descrittiva diIsaria cretacea
van Beyma isolata da terreno di bosco ad Eucalipto.Trattasi di un Ifomicete non ancora segnalato presente in Italia e forse non ancora rinvenuto nel terreno.E' specie auxo-eterotrofa e presenta attività antagonista verso altre specie di funghi.
Summary The present note deals briefly withIsaria cretacea van Beyma, isolated from Eucaliptus soil. The fungus is a Hyphomycete whose presence in Italy was never noticed before and perhaps it is the first time the it is found in soil. ThisIsaria is an auxo-heterotrophic species and shows a certain antagonistic activity against some fungi.
Resumo O presente trabalho é uma nota breve descritiva deIsaria cretacea van Beyma, isolada de terreno de um bosque de Eucaliptos. Tratase de um Ifomicete cuja presença na Italia não foi ainda assinalada e talvez seja a primeira vez que é isolada de terreno.E' uma especie auxo-heterotrofica e apresenta uma certa atividade antagonista sobre outros fungos.相似文献
6.
High-efficiency purification and chemical characterization of B cell stimulatory factor-1/interleukin 4 总被引:5,自引:0,他引:5
J Ohara J E Coligan K Zoon W L Maloy W E Paul 《Journal of immunology (Baltimore, Md. : 1950)》1987,139(4):1127-1134
B cell stimulatory factor-1/interleukin 4, a lymphokine produced by phorbol ester activated-EL-4 thymoma cells, was purified to homogeneity in good yield by a two-step purification procedure, using affinity chromatography and a single subsequent round of reverse-phase high-performance liquid chromatography. The N-terminal sequence of the first 24 amino acids was consistent with that inferred from the nucleotide sequence of BSF-1 cDNA clones. Amino acid composition analysis also agreed well with that predicted from the nucleotide sequence. A rabbit antibody to a peptide corresponding to positions 100 to 113 inferred from the nucleotide sequence of the cDNA clones bound to BSF-1 purified from EL-4 cells. Purified BSF-1 possesses complex N-linked glycosidic side chains as shown by reduction in Mr from 20,000 to approximately 15,000 by endoglycosidase F but not by endoglycosidase H treatment. Removal of these N-linked sugars does not diminish the activity of BSF-1 as a costimulant in the response of B cells to anti-IgM. By contrast, the reduction of disulfide bonds completely destroyed biologic activity. A monoclonal antibody to BSF-1 blocks its binding to cellular receptors and inhibits biological activities, whereas antibody to the BSF-1 peptide (100-113) has neither effect. 相似文献
7.
A Matsuda Y Ohara T Kakutani K Negishi Y Wataya H Hayatsu T Ueda 《Nucleic acids research》1990,18(7):1833-1838
A crude preparation of 2-phenyladenosine was found to be mutagenic in the Ames Salmonella assay. In the purification of this preparation, it was revealed that 2-phenyladenosine itself was nonmutagenic but that 2-(m- and p-nitrophenyl)-adenosines (5m,p) contaminating the sample were the mutagenic principles. A structure-activity relationship study was carried out, and it was found that 5p, 2-(p-nitrophenyl)-adenine (7p), and 2-(p-nitrophenyl)-2'-deoxyadenosine (15p) were strongly mutagenic toward S. typhimurium TA98 and TA100 without metabolic activation, the potency being in the order 15p greater than 7p greater than 5p. The potency of 15p in TA98 was one order of magnitude greater than that of 4-nitroquinoline N-oxide. 15p also showed mutagenicity in the mouse cell line FM3A in culture. 相似文献
8.
9.
The present paper describes a simple technique that hardens the shell of nuts and makes the use of a tool to crack them open more compelling. Walnuts were coated with a dough of sawdust and nontoxic white glue in different combinations; they were tested for hardness by using machines normally used to test different kinds of wood. Data on relative hardness for uncoated walnuts and walnuts coated with dough of two different combinations are presented. The coated walnuts were significantly harder to break than the uncoated ones, whereas no significant difference was found when comparing the hardness of two types of coated walnuts. Furthermore, observations on a captive group of tufted capuchins (Cebus apella) are described. The monkeys needed significantly more time to break open the coated walnuts. Early results show that coated walnuts may favor acquistion of tool use skills in a juvenile capuchin. 相似文献
10.
Trypsin-sensitive neutralization site on VP1 of Theiler''s murine encephalomyelitis viruses. 总被引:6,自引:5,他引:1
Y Ohara A Senkowski J L Fu L Klaman J Goodall M Toth R P Roos 《Journal of virology》1988,62(9):3527-3529
We generated Theiler's murine encephalomyelitis virus mutants resistant to several neutralizing monoclonal antibodies (MAbs) having their epitopes near a trypsin cleavage site of VP1. Neutralization and Western blot (immunoblot) studies suggest that two of the MAbs have identical epitopes that partly overlap the epitope of a third MAb. Sequencing of RNA of the mutants localized the epitopes to a site near the carboxyl end of VP1. The limited diversity of nucleotide changes seen in the mutants and the immunodominance of the site suggest that the carboxyl end of VP1 may have an important function. 相似文献