全文获取类型
收费全文 | 4937篇 |
免费 | 495篇 |
国内免费 | 2篇 |
出版年
2021年 | 57篇 |
2020年 | 34篇 |
2019年 | 50篇 |
2018年 | 67篇 |
2017年 | 75篇 |
2016年 | 90篇 |
2015年 | 173篇 |
2014年 | 160篇 |
2013年 | 205篇 |
2012年 | 276篇 |
2011年 | 286篇 |
2010年 | 170篇 |
2009年 | 143篇 |
2008年 | 191篇 |
2007年 | 216篇 |
2006年 | 203篇 |
2005年 | 206篇 |
2004年 | 191篇 |
2003年 | 211篇 |
2002年 | 173篇 |
2001年 | 178篇 |
2000年 | 154篇 |
1999年 | 145篇 |
1998年 | 69篇 |
1997年 | 81篇 |
1996年 | 63篇 |
1995年 | 65篇 |
1994年 | 60篇 |
1993年 | 71篇 |
1992年 | 99篇 |
1991年 | 102篇 |
1990年 | 63篇 |
1989年 | 86篇 |
1988年 | 81篇 |
1987年 | 70篇 |
1986年 | 53篇 |
1985年 | 64篇 |
1984年 | 47篇 |
1983年 | 53篇 |
1982年 | 56篇 |
1981年 | 48篇 |
1980年 | 31篇 |
1979年 | 42篇 |
1978年 | 46篇 |
1977年 | 33篇 |
1975年 | 28篇 |
1974年 | 42篇 |
1973年 | 33篇 |
1972年 | 31篇 |
1971年 | 31篇 |
排序方式: 共有5434条查询结果,搜索用时 15 毫秒
31.
Effects and interactions of varying CaSO4 and NaCl levels on growth and nitrogen fixation ofLeucaena leucocephala K8 were examined. Leucaena was grown in nutrient solution at four levels of CaSO4 (0.5, 1.0, 2.5 and 5.0 mM) and NaCl (1, 25, 50 and 100 mM) in randomized blocks with five replications. While NaCl significantly reduced plant growth, additions of CaSO4 increased plant height, leaf number, and biomass of salt treated plants. For the nonsaline treatments, high CaSO4 levels slightly depressed growth, which contradicts suggestions that Leucaena has a high calcium requirement. A significant
calcium/sodium interaction was not seen for nodule number or weight. Nodule number was significantly depressed by 100 mM NaCl and nodule weight of the salt stressed plants significantly increased as CaSO4 concentration increased from 0.5 to 2.5 mM. Effects of NaCl and CaSO4 on nitrogen content of plant parts were inconclusive. The promotion of Leucaena salinity tolerance by addition of CaSO4 may be attributed to the effect of calcium in maintaing the selective permeability of membranes. 相似文献
32.
Goats and some sheep synthesize a juvenile hemoglobin, Hb C (alpha 2 beta
C2), at birth and produce this hemoglobin exclusively during severe anemia.
Sheep that synthesize this juvenile hemoglobin are of the A haplotype.
Other sheep, belonging to a separate group, the B haplotype, do not
synthesize hemoglobin C and during anemia continue to produce their adult
hemoglobin. To understand the basis for this difference we have determined
the structural organization of the beta- globin locus of B-type sheep by
constructing and isolating overlapping genomic clones. These clones have
allowed us to establish the linkage map 5' epsilon I-epsilon II-psi beta
I-beta B-epsilon III-epsilon IV- psi beta II-beta F3' in this haplotype.
Thus, B sheep lack four genes, including the BC gene, and have only eight
genes, compared with the 12 found in the goat globin locus. The goat
beta-globin locus is as follows: 5' epsilon I-epsilon II-psi beta X-beta
C-epsilon III-epsilon IV-psi beta Z-beta A-epsilon V-epsilon VI-psi beta
Y-beta F3'. Southern blot analysis of A-type sheep reveals that these
animals have a beta- globin locus similar to that of goat, i.e., 12 globin
genes. Thus, the beta-globin locus of B-haplotype sheep resembles that of
cows and may have retained the duplicated locus of the ancestor of cows and
sheep. Alternatively, the B-sheep locus arrangement may be the result of a
deletion of a four-gene set from the triplicated locus.
相似文献
33.
Lise Stengård Hansen 《BioControl》1988,33(1):33-42
The phytoseiid miteAmblyseius barkeri (Hughes) (=Amblyseius mckenziei Sch. & Pr.) was used for biological control ofThrips tabaci Lind. in 7 commercial glasshouses with cucumber (a total of 5780 m2). Predatory mites were introduced 3–4 times in densities ranging from 40 to 300/m2 at each release. In 6 of the 7 glasshouses, control of thrips was satisfactory throughout the growing season. Thrips densities
were kept below 15 individuals per leaf. In 1 glasshouse, thrips damage was seen on the fruits at densities of 25 thrips per
leaf, but the thrips population was quickly reduced and remained at low densities for the next 3 months.
相似文献
34.
Characterization of the Antirrhinum floral homeotic MADS-box gene deficiens: evidence for DNA binding and autoregulation of its persistent expression throughout flower development. 总被引:26,自引:9,他引:17 下载免费PDF全文
Z Schwarz-Sommer I Hue P Huijser P J Flor R Hansen F Tetens W E Lnnig H Saedler H Sommer 《The EMBO journal》1992,11(1):251-263
35.
J. Plet P. K. Pedersen F. B. Jensen J. K. Hansen 《European journal of applied physiology and occupational physiology》1992,65(2):171-177
The purpose of the study was to examine the influence of oxygen-breathing on maximal oxygen uptake (VO2max) and submaximal endurance performance. Six young women and five men rode a cycle-ergometer while breathing compressed air (normoxia, NOX) or a 55% O2 in N2 mixture (hyperoxia, HOX). The VO2max increased significantly by 12% (P less than 0.01) with HOX in the women but not in the men (+4%; nonsignificant). Maximal heart rate was also increased with HOX in the women but not in the men. Endurance time during work to exhaustion at 80% of normoxic VO2max was 41% longer in HOX than in NOX (P less than 0.025) with no significant difference between the men and the women. The variation among individuals was large. The oxygen uptake and respiratory quotient were not different in the two endurance tests, but pulmonary ventilation (VE) and blood lactate concentration were lower in HOX than in NOX, especially during the latter part of the task. Plasma base deficit (BDpl) increased initially by 3.5 mmol.l-1 during HOX and then stabilized. In NOX, a continuous increase was seen and the change was more than twice as large. Relative to BDpl, VE was higher in HOX than in NOX indicating a more efficient ventilatory compensation of the metabolic acidosis. The reduced ventilatory demand and lower metabolic acidosis in HOX in combination with lower relative exercise intensity may have contributed to the longer time to exhaustion. However, the pattern of individual variation suggested that other mechanisms were also involved. 相似文献
36.
Mineralization budgets in sediment microcosms: Effect of the infauna and anoxic conditions 总被引:1,自引:0,他引:1
Abstract A number of sediment incubations were set up to reproduce some of the conditions used by Kristensen and Blackburn [1] and to make a comparison with their results. There were three types of microcosm: aerobic (OX), anaerobic (AN) and aerobic with Nephtys (NOX). In addition to other measurements, dissolved organic nitrogen (DON) pools and fluxes, were measured. The sediment in this experiment contained more particulate organic matter (POM). Nephtys (NOX) had the same effect as Nereis in increasing the rate of mineralization of POC and PON, compared with the OX-cores (2.1 and 2.6 times, respectively). Again, the AN-cores had a higher mineralization rate (loss of POM) than that of the OX-cores, but in addition, mineralization in NOX-cores was not significantly different from AN-cores. It was thus confirmed that anoxic mineralization could be as high, or higher, than the oxic process. Both the temporal patterns of O2 -and and CO2 -fluxes and their magnitudes were very similar to those reported earlier. This contrasts with the higher loss of POM in the present experiment. However, the loss of C in DOC (associated with the measured DON) can account for the extra POM loss. The pore-water profiles of σCO2 and NH4 + were similar to those in the earlier report, and the fluxes of σCO2 , O2 , NH4 + and NO3 − followed the same temporal pattern. 相似文献
37.
Abstract Pseudomonas sp. strain RW611 utilized the ammonium salt of 2-sulfobenzoate as sole source of carbon, sulfur, nitrogen, and energy. The xenobiotic sulfo substituent was dioxygenolytically eliminated as sulfite, which was then slowly oxidized to sulfate. 2,3-Dihydroxybenzoate, which resulted from desulfonation underwent meta -cleavage, mediated by 2,3-dihydroxybenzoate 3,4-dioxygenase activity. This enzyme was inhibited by 3-chlorocatechol and 2,3,4-trihydroxybenzoate. 相似文献
38.
M E Murphy R Kolvenbach M Aleksis R Hansen H Sies 《Free radical biology & medicine》1992,13(2):95-100
To assess whether oxidative stress contributes to the ischemia/reperfusion injury of aortic surgery, the contents of alpha-tocopherol, alpha-tocopheryl quinone, ascorbate, lipid-derived malondialdehyde, protein thiols, cholesterol, and lactate were analyzed in plasma samples from 24 patients subjected to aortic crossclamping. alpha-Tocopherol, ascorbate, and protein thiols decreased during ischemia, whereas alpha-tocopheryl quinone increased in all but two cases, doubling on average in proportion to alpha-tocopherol. Upon reperfusion alpha-tocopherol, ascorbate, and protein thiols remained low, whereas alpha-tocopheryl quinone returned to the preischemic level. Lipid-derived malondialdehyde (a measure of lipid hydroperoxides) increased significantly only during reperfusion. The results suggest that oxidative stress occurs simultaneously with ischemia/reperfusion during aortic surgery, and that measurement of the tocopheryl quinone/tocopherol ratio may shed new light on the underlying pathological events. 相似文献
39.
Recombinant bovine interferon-alpha(I)1 (rBolFN-alpha) may be useful for enhancing fertility in sheep because it has extensive sequence homology with ovine trophoblast protein-1. To test the effectiveness of rBolFN-alpha, several experiments were performed in which bred females were given intramuscular injections of rBolFN-alpha around the time of maintenance of the corpus luteum. Treatment with rBolFN-alpha enhanced the fertility of ewes that were bred via natural service or embryo transfer of whole or demi-embryos. Interferon treatment was successful in enhancing lambing rate if injections were given twice daily from Days 11 to 18, 12 to 14, 12 to 15 or 12 to 16. Overall, the lambing rate for ewes bred via natural service was 94/126 (74.6%) for control ewes and 101/126 (80.2%) for rBolFN-alpha treated ewes. Litter size was not affected by treatment. Interferon treatment was not successful in increasing the lambing rate if given as a single injection on Day 12 or as a series of once-daily injections from Days 11 to 16. These results demonstrate that rBolFN-alpha can increase the lambing rate in ewes. 相似文献
40.