全文获取类型
收费全文 | 6528篇 |
免费 | 444篇 |
国内免费 | 3篇 |
出版年
2022年 | 35篇 |
2021年 | 68篇 |
2020年 | 34篇 |
2019年 | 59篇 |
2018年 | 62篇 |
2017年 | 55篇 |
2016年 | 97篇 |
2015年 | 193篇 |
2014年 | 224篇 |
2013年 | 302篇 |
2012年 | 337篇 |
2011年 | 364篇 |
2010年 | 295篇 |
2009年 | 250篇 |
2008年 | 338篇 |
2007年 | 367篇 |
2006年 | 363篇 |
2005年 | 339篇 |
2004年 | 356篇 |
2003年 | 348篇 |
2002年 | 356篇 |
2001年 | 92篇 |
2000年 | 77篇 |
1999年 | 103篇 |
1998年 | 128篇 |
1997年 | 85篇 |
1996年 | 83篇 |
1995年 | 96篇 |
1994年 | 77篇 |
1993年 | 89篇 |
1992年 | 68篇 |
1991年 | 62篇 |
1990年 | 75篇 |
1989年 | 54篇 |
1988年 | 54篇 |
1987年 | 38篇 |
1986年 | 52篇 |
1985年 | 34篇 |
1984年 | 64篇 |
1983年 | 53篇 |
1982年 | 66篇 |
1981年 | 73篇 |
1980年 | 60篇 |
1979年 | 54篇 |
1978年 | 45篇 |
1977年 | 41篇 |
1976年 | 34篇 |
1975年 | 34篇 |
1974年 | 36篇 |
1973年 | 18篇 |
排序方式: 共有6975条查询结果,搜索用时 15 毫秒
101.
Vertical structure of seed banks and the impact of depth of burial on recruitment in two temporary marshes 总被引:3,自引:0,他引:3
The structure of the seed bank (including Chara oospores), in relation to depth within the sediment and disturbance, was studied in two Rhône delta temporary marshes for two years. The seeds of all species were concentrated in the top 2 cm of sediment with very low numbers beeing found below 4 cm. When an exclosure eliminated disturbances of the sediment by animals, the vertical repartition of seeds at site 2 was more pronounced than outside the exclosure.In experiment 1, the emergence capacity of seeds from different depths and buried under layers of sterile equivalent to those in the field was measured. Depending of the species, 22 to 98% of the seeds germinated from unburied seeds in the top 2 cm. Only 1% of the oospores of Chara (from site 2) at 2 to 4 cm depth in the sediment emerged.In experiment 2, surface seed bank samples were placed under 0, 2 or 4 cm sterile sediment depth. The samples contained numerous recent seeds and the emergence percentage reached 41% (for Ruppia maritima). Only the seeds of Zannichellia spp failed to germinate from a depth of 2 cm or more. The emergence percentage from 2 cm depth or more was always lower than at the surface. These experiments showed that both burial and ageing of seeds decrease germination capacity.The majority of the active seeds located at the surface germinate when the marsh is flooded. Seeds located between 2 and 4 cm can be brought back to the surface by disturbances and play the role of a reserve involved in maintenance of populations that go without seed production for one or some years. 相似文献
102.
Jacques J. H. Hens Marina De Wit Lodewijk V. Dekker Frans Boomsma A. Beate Oestreicher Frank Margolis† Willem Hendrik Gispen Pierre N. E. De Graan 《Journal of neurochemistry》1993,60(4):1264-1273
Abstract: The involvement of B-50, protein kinase C (PKC), and PKC-mediated B-50 phosphorylation in the mechanism of Ca2+-induced noradrenaline (NA) release was studied in highly purified rat cerebrocortical synaptosomes permeated with streptolysin-O. Under optimal permeation conditions, 12% of the total NA content (8.9 pmol of NA/mg of synaptosomal protein) was released in a largely (>60%) ATP-dependent manner as a result of an elevation of the free Ca2+ concentration from 10?8 to 10?5M Ca2+ The Ca2+ sensitivity in the micromolar range is identical for [3H]NA and endogenous NA release, indicating that Ca2+-induced [3H]NA release originates from vesicular pools in noradrenergic synaptosomes. Ca2+-induced NA release was inhibited by either N- or C-terminal-directed anti-B-50 antibodies, confirming a role of B-50 in the process of exocytosis. In addition, both anti-B-50 antibodies inhibited PKC-mediated B-50 phosphorylation with a similar difference in inhibitory potency as observed for NA release. However, in a number of experiments, evidence was obtained challenging a direct role of PKC and PKC-mediated B-50 phosphorylation in Ca2+-induced NA release. PKC pseudosubstrate PKC19-36, which inhibited B-50 phosphorylation (IC50 value, 10?5M), failed to inhibit Ca2+-induced NA release, even when added before the Ca2+ trigger. Similar results were obtained with PKC inhibitor H-7, whereas polymyxin B inhibited B-50 phosphorylation as well as Ca2+-induced NA release. Concerning the Ca2+ sensitivity, we demonstrate that PKC-mediated B-50 phosphorylation is initiated at a slightly higher Ca2+ concentration than NA release. Moreover, phorbol ester-induced PKC down-regulation was not paralleled by a decrease in Ca2+-induced NA release from streptolysin-O-permeated synaptosomes. Finally, the Ca2+- and phorbol ester-induced NA release was found to be additive, suggesting that they stimulate release through different mechanisms. In summary, we show that B-50 is involved in Ca2+-induced NA release from streptolysin-O-permeated synaptosomes. Evidence is presented challenging a role of PKC-mediated B-50 phosphorylation in the mechanism of NA exocytosis after Ca2+ influx. An involvement of PKC or PKC-mediated B-50 phosphorylation before the Ca2+ trigger is not ruled out. We suggest that the degree of B-50 phosphorylation, rather than its phosphorylation after PKC activation itself, is important in the molecular cascade after the Ca2+ influx resulting in exocytosis of NA. 相似文献
103.
Jeanne-Andrée Boudier Jacques Fantini Corinne Gerard Bernard Verrier 《Cell and tissue research》1993,271(1):23-31
Summary The fine structure of single identified muscle fibers and their nerve terminals in the limb closer muscle of the shore crab Eriphia spinifrons was examined, using a previous classification based on histochemical evidence which recognizes a slow (Type-I) fiber and three fast (Type-II, Type-III, Type-IV) fibers. All four fiber types have a fine structure characteristic of crustacean slow muscle, with 10–12 thin filaments surrounding each thick filament and sarcomere lengths of 6–13 m. Type-IV fibers have sarcomere lengths of 6 m while the other three types have substantially longer sarcomeres (10–13 m). Structural features of nerve terminals revealed excitatory innervation in all four fiber types but inhibitory innervation in Type-I, Type-II, and Type-III fibers only. Thus fibers with longer sarcomeres receive the inhibitor axon but those with shorter sarcomeres do not. Amongst the former, synaptic contact from an inhibitory nerve terminal onto an excitatory one, denoting presynaptic inhibition, was seen in Type-I and Type-II fibers but not in Type-III and Type-IV fibers. Inhibitory innervation of the walking leg closer muscle is therefore highly differentiated: some fibers lack inhibitory nerve terminals, some possess postsynaptic inhibition, and some possess both postsynaptic and presynaptic inhibition. 相似文献
104.
Christine Cans Olivier Cohen Marie-Ange Mermet Jacques Demongeot Pierre Jalbert 《Human genetics》1993,91(3):228-232
Two methods of prediction for the risk of unbalance at birth were tested on a large data base of reciprocal translocation (1376 families): the pachyten diagram predictive method (PDPmethod) and the discriminant method (Dmethod). These method succeeded in correctly predicting the segregation mode in 66% of the data for the PDPmethod and in 80% of the data for the Dmethod. The quality of chromosome material (in particular R bands) must be taken into account for more accurate prediction. Some difficulties still exist in predicting the 31 tertiary segregation mode, which can frequently be incorrectly classified as the adjacent 1 mode. 相似文献
105.
The effects of salinity on the reproduction of coastal submerged macrophyte species were studied on samples of communities from six seasonal marshes in two outdoor experiments performed in autumn and in spring. The submerged macrophyte communities were submitted to five different salinity levels (0, 1, 2, 4 and 6 g/1 Cl?1). In a companion paper (Grillas, van Wijck & Bonis 1993) three groups of species were distinguished on the basis of their biomass production over the salinity range 0 to 6 g/1 Cl?1: (1) glycophytes (non-salt-tolerant species), (2) salt-tolerant species and (3) halo-phytes. This part of the study describes the impact of salinity on the reproduction of the individual species during the two experiments. The species differ in their capacity to reproduce in the autumn; only Zannichelliapedunculata and Tolypella hispánica were able to produce fruits in that season. For all species reproduction was greater in spring and strongly correlated with biomass, except for Chara canescens. Differences in reproductive effort over the salinity range amplified the halophytic nature of Ruppia marítima and Chara canescens and the intolerance of Callitriche truncata and Chara contraria. For the other species, reproductive effort did not differ significantly over the salinity range. Regarding the effect of salinity on biomass and reproductive effort of individual species, there were large differences in the total weight of propagules produced at the community level and in the relative contribution of individual species. The resulting quantitative changes in the species composition of the seed bank could affect the structure of the communities by their effects on the establishment and survival of species populations. 相似文献
106.
René Rezsöhazy Bernard Hallet Jean Delcour Jacques Mahillon 《Molecular microbiology》1993,9(6):1283-1295
The eight IS 231 variants characterized so far (IS 231 A-F, V and W) display similar transposases with an overall 40% identity. Comparison with all the proka-ryotic transposable elements sequenced so far revealed that the IS231 transposases share two conserved regions with those of 35 other insertion sequences of wide origins. These insertion sequences, defining the IS4 family, have a common bipartite organization of their ends and are divided into two similarity groups. Interestingly, the transposase domains conserved within this family display similarities with the well known integrase domain shared by transposases of the IS3 and IS15 families, and integrases of retroelements. This domain is also found in IS30- related elements and Tn7 TnsB protein. Amino acid residues conserved throughout all these prokaryotic and eukaryotic mobile genetic elements define a major transposase/integrase motif, likely to play an important role in the transposition process. 相似文献
107.
108.
Jacques Vasse Françoise de Billy Georges Truchet 《The Plant journal : for cell and molecular biology》1993,4(3):555-566
Nodulation, the organogenetic process resulting from the symbiotic interaction between Rhizobium and legumes, is under the feedback control of the plant. However, the autoregulatory mechanisms controlling root nodule formation are poorly understood. In this paper it is shown that alfalfa can react to infection by its symbiont Rhizobium meliloti by eliciting a defence mechanism similar to the hypersensitive reaction (HR) observed in incompatible plant-pathogen interactions. After the first nodule primordia have been induced, an increasing proportion of infection threads abort in a single or a few root cortical cells in which both symbionts simultaneously undergo necrosis. Autofluorescent, cytochemical and immunolocalization assays revealed that phenolic compounds and proteins associated with defence mechanisms in plants have accumulated in the necrotic cells. These results lead to the proposition that the elicitation of a HR is part of the mechanism by which the plant controls infection and, therefore, regulates nodulation. 相似文献
109.
Phenotypic effects of overexpression of Agrobacterium rhizogenes T-DNA ORF13 in transgenic tobacco plants are mediated by diffusible factor(s) 总被引:1,自引:1,他引:0
Geneviève Hansen Danièle Vaubert Jean Noël Héron Danielle Clérot Jacques Tempe Jean Brevet 《The Plant journal : for cell and molecular biology》1993,4(3):581-585
Nicotiana tabacum cv. Xanthi transgenic plants expressing ORF13 of Agrobacterium rhizogenes 8196 T-DNA under the 35S RNA promoter from the cauliflower mosaic virus displayed developmental abnormalities. They were small, with short and variable internodal lengths, their root systems were poorly developed; leaves were small, asymmetric, rounded, wrinkled and dark green; flowers were short, and irregularly shaped. They exhibited reduced apical dominance and regularly produced offshoots at the base of the plant. This phenotype was also exhibited by offshoots of normal N. tabacum cv. Xanthi stock grafted with a transgenic scion indicating that expression of ORF13 influences plant development via diffusible factor(s). 相似文献
110.
Summary N6-benzyl-adenine (BA) enhanced phyllogenesis and axillary bud development of Paeonia suffruticosa during in vitro culture allowing good propagation while N6-(2isopentenyl)adenine (iP) did not. During the first five days of culture, the mitotic activity of BA-treated explants was higher than in the iP-treated ones. High BA levels were detected in the BA-treated explants, and this was correlated with the absence of or the low indole-3-acetic acid (IAA) content. The low iP levels measured in iP-treated explants were correlated with high endogenous IAA content; the new cytokinin / auxin ratio could explain the lack of axillary buds and the development of only one leaf. Abscisic acid (ABA) was detected neither in the controls nor in the cytokinin-treated explants during the first week. However, intensive restoration of ABA accumulation was observed in controls from the third week onwards. Both BA and iP-treated explants accumulated less ABA than the controls but this hormone appeared later in the BA-treated explants than in the iP-treated ones.Abbreviations ABA
abscisic acid
- BA
N6-benzyl-adenine
- BHT
butyl-hydroxy-toluene
- ELISA
enzyme linked immunosorbent assay
- FM
fresh mass
- HPLC
high performance liquid chromatography
- IAA
indole-3-acetic acid
- iP
N6-(2-isopentenyl)adenine
- MI
mitotic index
- 9RBA
9-ß-D-ribofuranosyl-BA
- 9RiP
9-ß-Dribofuranosyl-iP
- 9RZ
9-ß-D-ribofuranosyl-zeatin
- Z
zeatin 相似文献