全文获取类型
收费全文 | 8469篇 |
免费 | 751篇 |
国内免费 | 1篇 |
专业分类
9221篇 |
出版年
2023年 | 44篇 |
2022年 | 111篇 |
2021年 | 211篇 |
2020年 | 123篇 |
2019年 | 162篇 |
2018年 | 209篇 |
2017年 | 146篇 |
2016年 | 287篇 |
2015年 | 403篇 |
2014年 | 433篇 |
2013年 | 575篇 |
2012年 | 646篇 |
2011年 | 660篇 |
2010年 | 414篇 |
2009年 | 359篇 |
2008年 | 453篇 |
2007年 | 477篇 |
2006年 | 430篇 |
2005年 | 381篇 |
2004年 | 403篇 |
2003年 | 367篇 |
2002年 | 294篇 |
2001年 | 158篇 |
2000年 | 111篇 |
1999年 | 140篇 |
1998年 | 88篇 |
1997年 | 78篇 |
1996年 | 83篇 |
1995年 | 63篇 |
1994年 | 58篇 |
1993年 | 49篇 |
1992年 | 66篇 |
1991年 | 72篇 |
1990年 | 74篇 |
1989年 | 74篇 |
1988年 | 59篇 |
1987年 | 51篇 |
1986年 | 42篇 |
1985年 | 35篇 |
1984年 | 36篇 |
1983年 | 26篇 |
1982年 | 22篇 |
1981年 | 23篇 |
1980年 | 21篇 |
1979年 | 21篇 |
1978年 | 18篇 |
1977年 | 14篇 |
1976年 | 13篇 |
1973年 | 14篇 |
1970年 | 13篇 |
排序方式: 共有9221条查询结果,搜索用时 0 毫秒
1.
Winnie W.Y. Lau Marlene M. Martinez 《Journal of experimental marine biology and ecology》2003,295(1):1-21
Hydrodynamic forces can affect survival as well as limit the movement of motile benthic animals. An animal's danger of dislodgement depends on the hydrodynamic forces it experiences in its microhabitat relative to the force required to dislodge it (tenacity) from the substratum. We measured water flow and substratum characteristics in two different habitats of the shore crab Pachygrapsus crassipes: a wave-swept rocky shore and an intertidal mudflat. The maximum water velocities and accelerations in the microhabitats of the crabs at the wave-swept site were three times and two times greater, respectively, than at the mudflat site. In the laboratory, we measured the tenacity of crabs of various sizes on different substrata, and also measured their drag, lift and added-mass coefficients. Using these data, we calculated the flow conditions under which crabs would be overturned or sheared off the substratum in their two habitats. The net horizontal force (drag plus acceleration reaction) required to dislodge a crab on a rugose rock substratum was an order of magnitude greater than on smooth rock and two orders of magnitude greater than on mud. Our calculations indicate that, under non-storm conditions, crabs will not be dislodged from the substratum in either the mudflat or the wave-swept habitat when grasping the substratum with maximum tenacity. Moving crabs have lower tenacity and our calculations predict that hydrodynamic forces will restrict the mobility of large crabs more than that of small ones on smooth, but not on rugose rock. 相似文献
2.
3.
Dispersal vs. vicariance in the Mediterranean: historical biogeography of the Palearctic Pachydeminae (Coleoptera, Scarabaeoidea) 总被引:4,自引:1,他引:3
Isabel Sanmartín 《Journal of Biogeography》2003,30(12):1883-1897
Aim The geological evolution of the Mediterranean region is largely the result of the Tertiary collision of the African and Eurasian Plates, but also a mosaic of migrating island arcs, fragmenting tectonic belts, and extending back‐arc basins. Such complex paleogeography has resulted in a ‘reticulate’ biogeographical history, in which Mediterranean biotas repeatedly fragmented and merged as dispersal barriers appeared and disappeared through time. In this study, dispersal‐vicariance analysis (DIVA) is used to assess the relative role played by dispersal and vicariance in shaping distribution patterns in the beetle subfamily Pachydeminae Reitter, 1902 (Scarabaeoidea), an example of east–west Mediterranean disjunction. Location The Mediterranean region, including North Africa, the western Mediterranean, Balkans–Anatolia, Middle East, Caucasus, the Iranian Plateau, and Central Asia. Methods A phylogenetic hypothesis of the Palearctic genera of Pachydeminae in conjunction with distributional data was analysed using DIVA. This method reconstructs the ancestral distribution in a given phylogeny based on the vicariance model, while allowing dispersal and extinction to occur. Unlike other methods, DIVA does not enforce area relationships to conform to a hierarchical ‘area cladogram’, so it can be used to reconstruct ‘reticulate’ biogeographical scenarios. Results Optimal reconstructions, requiring 23 dispersal events, suggest that the ancestor of Pachydeminae was originally present in the south‐east Mediterranean region. Basal splitting within the subfamily was caused by vicariance events related to the late Tertiary collision of the African microplates Apulia and Arabia with Eurasia, and the resultant arise of successive dispersal barriers (e.g. the Red Sea, the Zagros Mountains). Subsequent diversification in Pachydeminae involved multiple speciation events within the Middle East and Iran–Afghanistan regions, which gave rise to the least speciose genera of Pachydeminae (e.g. Otoclinius Brenske, 1896). Finally, the presence of Pachydeminae in the western Mediterranean region seems to be the result of a recent dispersal event. The ancestor of the Iberian genera Ceramida Baraud, 1987 and Elaphocera Gené, 1836 probably dispersed from the Middle East to the Iberian Peninsula across North Africa and the Gibraltar Strait during the ‘Messinian salinity crisis’ at the end of the Miocene. Main conclusions Although the basal diversification of Pachydeminae around the Mediterranean appears to be related to vicariance events linked to the geological formation of the Mediterranean Basin, dispersal has also played a very important role. Nearly 38% of the speciation events in the phylogeny resulted from dispersal to a new area followed by allopatric speciation between lineages. Relationships between western and eastern Mediterranean disjuncts are usually explained by dispersal through Central Europe. The biogeographical history of the Pachydeminae corroborates other biogeographical studies that consider North Africa to be an alternative dispersal route by which Mediterranean taxa could have achieved circum‐Mediterranean distributions. 相似文献
4.
Werner E.C. Muller Jürgen Conrad Rudolf K. Zahn Renate Steffen Gerhard Uhlenbruck Isabel Miller 《Differentiation; research in biological diversity》1984,26(1-3):30-35
Abstract. The Hexactinellida sponge Aphrocallistes vastus contains a soluble aggregation factor (AF) whose purification has been described in this communication. It is characterized by a S°20.w value of 37 and a buoyant density of 1.45 g/cm3 . The AF is a glycoporteinaceous particle composed of three major protein species; no core structure could be visualized. In the presence of Ca2+ , the AF causes secondary aggregation of single cells. The aggregation process is temperature, pH, and ionic strength independent within a broad range. Evidence is presented indicating that two (or more) AF molecules are required for the establishment of a stable cell: cell interaction. In contrast to the AFs from demosponges, the hexactinellid AF functions species-unspecifically. 相似文献
5.
Servais P Martinez J Billen G Vives-Rego J 《Applied and environmental microbiology》1987,53(8):1977-1979
Determination of [H] thymidine incorporation into bacterial DNA versus other macromolecules is usually achieved by NaOH and hot trichloroacetic acid hydrolysis. This procedure was found not to be specific enough. An alternative method founded on DNase treatment is proposed. Under the new method, the fraction of thymidine incorporated into DNA ranged from 10 to 83%. 相似文献
6.
The rate of hydrolysis of (+/-)-7 beta,8 alpha-dihydroxy-9 alpha,10 alpha-epoxy-7,8,9,10-tetrahydrobenzo [a]pyrene (BPDE) to tetrahydroxy derivatives (tetrols) in the presence of various subcellular fractions of rat liver was investigated. Microsomes and nuclei increased the half-life of BPDE in a concentration-dependent manner whereas cytosol had no such effect. The presence of 1 mg microsomal protein/ml increased the half-life of BPDE from 4 to 60 min at 22 degrees C and from 1.5 to 20 min at 37 degrees C. Nuclei equivalent of 500 micrograms DNA/ml increased the half-life from 1.9 to 3.6 min at 37 degrees C. Liposomes prepared from microsomal lipids mimicked the effect of microsomes indicating that BPDE is stabilized primarily by interacting with lipids. The significance of these interactions for the stability of BPDE in an intact cell system was evaluated by using isolated hepatocytes. In these cells the half-life of BPDE was substantially shorter (1 min at 5 X 10(6) cells/ml) than in buffer (3 min). However, hydrolysis of BPDE to tetrols was a minor reaction (less than or equal to 3% of added BPDE at a cell density greater than or equal to 5 X 10(6) cells/ml) and the main route of elimination (greater than or equal to 75%) was through conjugation with glutathione. 相似文献
7.
Acetyl-cholinesterase and fluoride-resistant acid phosphatase activities in dorsal root ganglia in the rat 总被引:1,自引:0,他引:1
J A Vega C Rodriguez M Medina A Martinez Telleria M E Bengoechea A Perez-Casas 《Cellular and molecular biology, including cyto-enzymology》1989,35(1):39-46
Acetylcholinesterase and fluoride-resistant acid phosphatase activities were contrasted in alternative serial sections of rat dorsal root ganglia. The morphometric analysis demonstrated no correlation between cellular size and enzymatic activity. Differences with previous works in this area are discussed. 相似文献
8.
Isabel Walls J. J. Sheridean R. W. Welch D. A. McDowell 《Letters in applied microbiology》1990,10(1):23-26
A new method of separating bacteria from beef mince has been developed, in which an alkaline protease, Alcalase 0.6 L, was used to degrade the meat proteins, leaving micro-organisms in suspension. The organisms were then counted, using a membrane filtration-epifluorescent microscopy technique. A correlation coefficient of 0.97 was obtained between this method and the standard plate count, indicating its suitability for use in quality control. 相似文献
9.
Dissociation of fibrinogen and fibronectin binding from transglutaminase-mediated cross-linking at the hepatocyte surface 总被引:2,自引:0,他引:2
C Barsigian F M Fellin A Jain J Martinez 《The Journal of biological chemistry》1988,263(28):14015-14022
The interaction of fibrinogen and fibronectin with hepatocytes has been dissociated into distinct binding and cross-linking steps. Binding and cross-linking of 125I-labeled ligands were both decreased by transglutaminase inhibitors, but not by heparin or hirudin. Transglutaminase activity was manifest by Ca2+-dependent incorporation of [14C]putrescine into cells. Preferential cross-linking of fibrinogen A alpha over gamma chains, and lack of inhibition by heparin or hirudin indicates the involvement of tissue transglutaminase, and not Factor XIIIa. Hepatic transglutaminase activity, as well as binding and cross-linking of fibrinogen and fibronectin, were maximally supported by Ca2+, partially supported by Mn2+ and Sr2+, and markedly decreased by Mg2+ and Ba2+. In contrast, Co2+ supported binding but not cross-linking or transglutaminase activity, indicating that binding and cross-linking are dissociable events. This conclusion was corroborated by the finding that fibrinogen fragments D95 and D78 both inhibited Ca2+-dependent fibrinogen binding without being cross-linked themselves. Ligand binding in the presence of either cation was localized to the cell surface as evidenced by its trypsin sensitivity. Thus, fibrinogen and fibronectin binding to hepatocytes is independent of transglutaminase activity, whereas cross-linking of these adhesive macromolecules requires an enzymatically active cellular transglutaminase. In addition, fibrinogen binding appears to be mediated by molecular determinants present in fragment D78. 相似文献
10.
Culture conditions for arresting and stimulating the proliferation of a rainbow trout fibroblast cell line,RTG-2 总被引:1,自引:0,他引:1
L. E. J. Lee A. Martinez N. C. Bols 《In vitro cellular & developmental biology. Plant》1988,24(8):795-802
Summary Conditions for arresting and stimulating the proliferation of the rainbow trout fibroblast cell line RTG-2 have been examined
and the time course of events after stimulation determined. Quiescent populations were achieved in two ways. Cultures grown
to confluency without a medium change for at least 7 d had fewer than 5% of the cells in S phase and few mitotic figures.
Cultures deprived of serum, which could be done for up to 3 d without a loss in cell number, also achieved quiescence. After
3 d without serum, less than 1% of cells were in S phase and mitotic figures were infrequent. Addition to these cultures of
fresh serum-containing medium brought about the synchronous entry of cells into S phase and mitosis. For cultures in which
either the medium had been changed after 7 d without a change or serum-containing medium had been added after 3 d of serum
deprivation, DNA synthesis increased after a lag period of 20 to 24 h, was pronounced between 30 and 45 h, and then declined.
This was followed by a peak in the mitotic index. These protocols for arresting and subsequently stimulating RTG-2 proliferation
should allow the G1-S transition to be studied in a representative of teleosts.
This research was supported by Natural Sciences and Engineering Research Council of Canada grant to N. C. B. 相似文献