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101.
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We consider studies of cohorts of individuals after a critical event, such as an injury, with the following characteristics. First, the studies are designed to measure "input" variables, which describe the period before the critical event, and to characterize the distribution of the input variables in the cohort. Second, the studies are designed to measure "output" variables, primarily mortality after the critical event, and to characterize the predictive (conditional) distribution of mortality given the input variables in the cohort. Such studies often possess the complication that the input data are missing for those who die shortly after the critical event because the data collection takes place after the event. Standard methods of dealing with the missing inputs, such as imputation or weighting methods based on an assumption of ignorable missingness, are known to be generally invalid when the missingness of inputs is nonignorable, that is, when the distribution of the inputs is different between those who die and those who live. To address this issue, we propose a novel design that obtains and uses information on an additional key variable-a treatment or externally controlled variable, which if set at its "effective" level, could have prevented the death of those who died. We show that the new design can be used to draw valid inferences for the marginal distribution of inputs in the entire cohort, and for the conditional distribution of mortality given the inputs, also in the entire cohort, even under nonignorable missingness. The crucial framework that we use is principal stratification based on the potential outcomes, here mortality under both levels of treatment. We also show using illustrative preliminary injury data that our approach can reveal results that are more reasonable than the results of standard methods, in relatively dramatic ways. Thus, our approach suggests that the routine collection of data on variables that could be used as possible treatments in such studies of inputs and mortality should become common. 相似文献
108.
Short-term mineralization of maize residues in soils as determined by carbon-13 natural abundance 总被引:2,自引:0,他引:2
Studies of soil organic matter equilibria must include estimates of C turnover. The objective of this study was to provide
data on how the natural 13C abundance method can be used to determine the flow of C from C4 residues and soil organic matter (C3-source) in a short-term incubation. Corn residue was added at a rate of 5.7 mg C g−1 soil to two soils, a clay and a sandy clay loam. During the course of a 35-day incubation in a CO2-free system, CO2-C and 13C natural abundance of the respired CO2 were measured. About 20% of the corn residue-C added was mineralized in both soils as determined from the CO2 respired and the 13C natural abundance of the respired CO2. Mineralization of the added residues was also calculated as the difference of the total amount of the respired CO2-C between the control and the corn residue-treated soils divided by the total amount of corn residue-C. Values were 35% for
the clay soil, and 30% for the sandy clay loam soil. The difference in values calculated from the 13 C natural abundance and the difference method was due to mineralization of the indigenous soil organic C resulting from the
addition of corn residues. Use of the natural 13C abundance method could determine the degree of ‘priming effect’ in soils amended with C4-C residues.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
109.
Ferrie A.M.R. Taylor D.C. MacKenzie S.L. Keller W.A. 《Plant Cell, Tissue and Organ Culture》1999,57(2):79-84
Brassica oleracea accessions possess traits that would be useful in commercial Brassica crops. These traits can be studied
more effectively through the production of doubled haploid plants. Nineteen B. oleracea accessions from several subspecies
possessing significant sn-2 erucic acid were screened for suitability for microspore culture using techniques well established
for Brassica. Fifteen of the 19 accessions produced embryos. Genotypic differences were observed with embryogenesis ranging
from 0 to 3000 embryos/100 buds. Embryogenesis was improved for two of four accessions by initiating cultures in NLN medium
with 17% sucrose, then reducing sucrose to 10% after 48 h. An increase in embryogenesis for the same two accessions was observed
when microspores were cultured at a density of 100 000/ml rather than 50 000 microspores/ml. A culture temperature of 32 °C
for 48 h was beneficial for three of the four accessions when compared to a longer incubation period (72 h) or a higher temperature
(35 °C). One accession line, Bo-1, was found to produce microspore-derived embryos which contained triacylglycerols with significant
proportions of erucic acid at the sn-2 position.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
110.
Toxigenic bacteria and sudden infant death syndrome (SIDS): nasopharyngeal flora during the first year of life 总被引:4,自引:0,他引:4
C.Caroline Blackwell Doris A.C. MacKenzie Valerie S. James Robert A. Elton Abdulaziz A. Zorgani Donald M. Weir Anthony Busuttil 《FEMS immunology and medical microbiology》1999,25(1-2):51-58
Many developmental and environmental risk factors for sudden infant death syndrome (SIDS) are similar to those for susceptibility to respiratory tract infection, and toxigenic bacteria have been implicated in some SIDS cases. We assessed nasopharyngeal flora of healthy infants in relation to risk factors to determine which species best lit the mathematical model proposed for the common bacterial toxin hypothesis and if these findings complemented results obtained from SIDS cases which occurred during the period of the survey. Longitudinal studies were carried out between April 1993 and March 1996 on 253 healthy infants and their mothers. 150 from a multiply deprived area, 103 from an affluent area. Concurrent SIDS infants (37) were screened for nasopharyngeal flora. Among healthy infants < or = 3 months of age, the predominant isolate was Staphylococcus aureus 57% compared with 86% for SIDS infants in that age range (P< 0.02). There were significant associations between isolation of different species from both mother and baby but no association between isolation of any species with: area of residence: parental smoking habits; breast or bottle feeding; symptoms of viral infection: seasonality. We conclude that S. aureus fits the mathematical model for SIDS. Both staphylococci and/or their toxins were identified in a significant proportion of SIDS cases. Isolation of staphylococci from healthy infants was associated with the 2-4-month age range, a risk factor consistently found in all epidemiological studies of SIDS. This might reflect the developmental stage in which 80-90% of infants express the Lewis(a) antigen which we have shown to be one of the receptors for S. aureus. 相似文献