全文获取类型
收费全文 | 8212篇 |
免费 | 630篇 |
国内免费 | 8篇 |
专业分类
8850篇 |
出版年
2023年 | 44篇 |
2022年 | 88篇 |
2021年 | 176篇 |
2020年 | 122篇 |
2019年 | 129篇 |
2018年 | 216篇 |
2017年 | 177篇 |
2016年 | 257篇 |
2015年 | 438篇 |
2014年 | 423篇 |
2013年 | 547篇 |
2012年 | 720篇 |
2011年 | 631篇 |
2010年 | 415篇 |
2009年 | 322篇 |
2008年 | 478篇 |
2007年 | 445篇 |
2006年 | 432篇 |
2005年 | 411篇 |
2004年 | 416篇 |
2003年 | 388篇 |
2002年 | 310篇 |
2001年 | 97篇 |
2000年 | 71篇 |
1999年 | 75篇 |
1998年 | 80篇 |
1997年 | 47篇 |
1996年 | 53篇 |
1995年 | 50篇 |
1994年 | 51篇 |
1993年 | 35篇 |
1992年 | 51篇 |
1991年 | 51篇 |
1990年 | 31篇 |
1989年 | 35篇 |
1988年 | 32篇 |
1987年 | 28篇 |
1986年 | 29篇 |
1985年 | 45篇 |
1984年 | 35篇 |
1983年 | 30篇 |
1982年 | 24篇 |
1981年 | 25篇 |
1980年 | 30篇 |
1979年 | 27篇 |
1978年 | 26篇 |
1977年 | 24篇 |
1976年 | 31篇 |
1975年 | 25篇 |
1974年 | 19篇 |
排序方式: 共有8850条查询结果,搜索用时 0 毫秒
21.
The F2 progeny of a cross between a chromosome 2 multiple marker stock and an adapted cultivar of barley were analyzed for four
morphological markers and electrophoretic patterns of eight leaf isozymes. TheIdh-2 locus was linked to thePer-5 locus (27.96±5.07 cM) and to thee locus (10.26±3.13 cM). Also, thePer-5 ande loci were located on the short arm of chromosome 2. In additionIdh-2 was also located on barley chromosome 2 and was linked to thev locus (13.18±3.56 cM), which is located on the long arm of chromosome 2. Two other marker genes,li andwst,,B, were linked (26.50±5.24 cM) on chromosome 2 but segregate independently of the other loci evaluated.
This project was supported by funds from the U.S.-Spain Joint Committee for Scientific and Technological Cooperation. 相似文献
22.
The tritiated 1 antagonist prazosin [3H]PRZ binds specifically and with high affinity to postsynaptic adrenoceptors in membrane preparations from cerebral cortex. Since adrenoceptors are of protein nature, it was of interest of investigate the possible role of disulfide (—SS—) and sulfhydril (—SH) groups in the binding of [3H]PRZ. Pretreatment of the membranes with the disulfide and sulfhydryl reactivesdl0Dithiothreitol,l-Dithiothreitol, Dithioerythritol or 5,5-Dithiobis-(2-nitrobenzoic acid) (DTNB), alone or in combination with the alkylating agent N-Methylmaleimide (NMM), decreased specific [3HPRZ binding, with minor changes in the non-specific counts. Saturation experiments revealed that all these reagents reduced the affinity of the binding site for [3H]PRZ, as judged by theK
d 25°C, but only the alkylating agent NMM and the oxydizing reagent DTNB produced in addition to the increase inK
d, a decrease of the maximum binding capacity (B
max). The present results provide evidence for a participation of—SS—and/or—SH groups in the recognition site of the 1-adrenoceptor of cerebral cortex. 相似文献
23.
24.
Widespread mortality of the black sea urchin Diadema antillarum occurred in the Caribbean in 1983; beginning in Panama in January, and having its major impact at Barbados in September. Mortality on ten reefs surveyed in Barbados was 93.2%, with the highest being 99.9% and the lowest 86.9%. Mortality on each reef was independent of the pre-mortality density on the reef. Urchins with test diameters between 20 and 40 mm were more severely affected than smaller or larger urchins. Populations on reefs exposed to incoming oceanic water suffered heavier mortality than those on protected reefs. Mean size of urchins was smallest on high density reefs. This may indicate a negative effect of density on urchin growth. At post-mortality densities, urchins may grow faster and reach sexual maturity sooner. 相似文献
25.
Abstract. Early subcultures of human embryonic lung fibroblasts are exceptional, as they grow far beyond confluence before growth ceases: the stationary dish may well contain 3-10 monolayer equivalents. Maximal growth rates, however, occur at about one-sixth confluence when doubling times are 15-20 hr; a density at which cell contacts begin to become frequent. the fact that a slowing down of growth is first apparent at such low densities argues against this regulation being due to diffusion effects. Confirmation of the role of short-range or contact interactions in growth regulation comes from an experiment using mixed cultures of fibroblasts: this shows that growth inhibition is not carried by medium-borne influences but depends on short-range (<1 mm) interactions. Evidence that cells can escape the effects of such contact interactions and so divide comes from time-lapse studies of dense cultures: there is a burst of motility soon after a fresh-medium change, which is followed by a burst of mitosis × 20 hr later. A medium change to conditioned medium supplemented with 10% foetal calf serum leads to neither the burst of motility nor the subsequent burst of mitosis, although this medium is better able to support the growth of sparse cells than is fresh medium. Data are also presented to show that the amount of collagen deposited in superconfluent cultures affects their growth: the stimulation of collagen production with ascorbic acid leads to an unexpectedly low stationary cell density and rather less movement in the culture. This result suggests that the collagen stabilizes cell contacts that are responsible for growth inhibition. the question of why these cells grow more slowly as density increases cannot be answered directly by these experiments; nevertheless, the results suggest that cell contact affects the permeability of the cell membrane to medium. 相似文献
26.
From middle Pennsylvanian coal balls collected in Kansas and Illinois the ligules and rhizomorph of Paurodendron have been illustrated. Assignment of Paurodendron to the Lycopsida is corroborated. Discovery of the root-bearing portion of this Carboniferous lycopod establishes a diminutive, phyletic line in the Paleozoic with rhizomorphs smaller than many species of Isoeles. Paurodendron resembles the selaginelloid line more closely than other rhizomorphic types. Anatomical comparisons of Paurodendron with roots and root-bearing structures of lycopods are made; the root-bearing and a representative leaf-bearing portion of Paurodendron fraiponti have been reconstructed. The specific diagnosis has been emended, and the three species described by Fry have been reduced to one. 相似文献
27.
Induction of c-fos mRNA Expression by Afterdischarge in the Hippocampus of Naive and Kindled Rats 总被引:5,自引:3,他引:2
Cheolsu Shin‡ James O. McNamara†‡ James I. Morgan§ Tom Curran Donna R. Cohen 《Journal of neurochemistry》1990,55(3):1050-1055
Periodic induction of focal electrical seizure [afterdischarge (AD)] is an absolute prerequisite for the development of kindling, an animal model of complex partial epilepsy. Once established, it is a permanent condition. The mechanism(s) that translate ADs, which last tens of seconds, into life-long alterations in the CNS is unclear. Cellular immediate-early genes have been implicated in the conversion of short-term stimuli to long-term alterations in cellular phenotypes by regulating target gene expression. We have investigated the contribution of one such early gene, c-fos, to this process. The relationship between ADs and expression of c-fos gene in the rat hippocampus, a key structure in kindling development, was studied by analysis of mRNA levels. The low constitutive expression of c-fos mRNA in the hippocampus was not altered by kindling. There was an "all-or-none" relationship between induction of c-fos and the duration of AD. The threshold for induction was approximately 30 s of AD. Above-threshold ADs induced c-fos in both naive and kindled animals to the same extent and with identical temporal profiles. Although the expression of c-fos is unchanged with kindling, c-fos may nonetheless contribute to many long-term changes of kindling, both adaptive and epileptogenic. 相似文献
28.
Chromosomal localization of ARSB,the gene for human N-acetylgalactosamine-4-sulphatase 总被引:2,自引:0,他引:2
Tom Litjens Elizabeth G. Baker Kerri R. Beckmann C. Phillip Morris John J. Hopwood David F. Callen 《Human genetics》1989,82(1):67-68
Summary A deficiency of N-acetylgalactosamine-4-sulphatase (G4S, gene symbol ARSB), results in the accumulation of undegraded substrate and the lysosomal storage disorder, Maroteaux-Lamy syndrome (mucopolysaccharidosis type VI). In situ hybridization using an 3H-labelled human G4S genomic DNA fragment to human metaphase chromosomes localized ARSB to chromosome 5q13–5q14. This location is consistent with, an refines, previous chromosomal assignments based on the expression of human G4S in somatic cell hybrids. 相似文献
29.
María E. Jiménez-Capdeville Tomás A. Reader Eduardo Molina-Holgado Robert W. Dykes 《Neurochemical research》1996,21(1):1-6
This study examined the effects of a nerve transection on monoamine release from primary somatosensory cortex. The technique
of microdialysis was employed to sample extracellular levels of norepinephrine (NE), 3,4-dihydroxyphenylacetic acid (DOPAC),
5-hydroxyindole-3-acetic acid (5-HIAA) and homovanillic acid (HVA) in the barrel field of freely moving rats following the
surgical transection of the contralateral infraorbital nerve. Microdialysates obtained 3, 4, and 5 days after deafferentation
were analyzed using high-performance liquid chromatography with electrochemical detection. We found a significant increase
in the release of the dopamine metabolites, DOPAC and HVA from the deafferented cortex. Three days after deafferentation the
release of DOPAC was three-fold higher in the deafferented than in the control animals, and remained about 100% higher in
the next two days in this group of animals. The release of HVA showed a gradual increase following the deafferentation procedure,
since a 92% larger value on day 3 increased to a 338% difference on day 5. On the other hand, the release rate of NE and the
levels of the serotonin metabolite 5-HIAA were not significantly affected by the deafferentation procedure. These results
are discussed in the context of the possible participation of dopamine in the reorganization of the deafferented somatosensory
cortex. 相似文献
30.
Tom A. Langen 《Ethology : formerly Zeitschrift fur Tierpsychologie》1996,102(1):157-166
The cooperatively breeding white-throated magpie-jay (Calocitta formosa) uses a variety of foraging tactics to find, harvest, and process food. Members of territorial groups forage together and may gain information about how to acquire food by observing each other. A field experiment was performed to determine whether a novel skill, door opening to gain access to food, was more rapidly acquired by members of groups in which a trained individual performed the skill. A higher proportion of jays in groups with behavioural ‘models’ (trained birds that opened doors in the presence of group members) acquired the door-opening skill than those groups without models. Young birds acquired the behaviour more frequently than older individuals. Aggressive behaviour at feeders may have affected the spread of the behaviour by reducing the likelihood that individuals performed the behaviour in the presence of other group members but may also have encouraged subordinate individuals to attempt door opening rather than ‘scrounge’. 相似文献