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51.
Olivier Rieppel 《Pal?ontologische Zeitschrift》1983,57(3-4):189-197
The models of punctuated and gradual evolution are put in a historical perspective and contrasted with each other. Mechanisms of saltational change are discussed. A synthesis of the two models might be achieved on the basis ofC. H. Waddington’s theory of developmental canalization as recently discussed byA. Hoffman. 相似文献
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Differential Recovery of Auxotrophs After Penicillin Enrichment in Escherichia coli 总被引:6,自引:3,他引:3
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Various auxotrophs are recovered from a penicillin enrichment cycle with differing efficiencies. Reconstruction experiments indicate that, under starvation conditions in the presence of penicillin, most auxotrophs undergo some death, whereas prolineless mutants are virtually immune to penicillin-induced killing. 相似文献
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Summary Alpha-smooth muscle actin is currently considered a marker of smooth muscle cell differentiation. However, during various
physiologic and pathologic conditions, it can be expressed, sometimes only transiently, in a variety of other cell types,
such as cardiac and skeletal muscle cells, as well as in nonmuscle cells. In this report, the expression of actin mRNAs in
cultured rat capillary endothelial cells (RFCs) and aortic smooth muscle cells (SMCs) has been studied by Northern hybridization
in two-dimensional cultures seeded on individual extracellular matrix proteins and in three-dimensional type I collagen gels.
In two-dimensional cultures, in addition to cytoplasmic actin mRNAs which are normally found in endothelial cell populations,
RFCs expressed α-smooth muscle (SM) actin mRNA at low levels. α-SM actin mRNA expression is dramatically enhanced by TGF-β1. In addition, double immunofluorescence staining with anti-vWF and anti-α-SM-1 (a monoclonal antibody to α-SM actin) shows
that RFCs co-express the two proteins. In three dimensional cultures, RFCs still expressed vWF, but lost staining for α-SM
actin, whereas α-SM actin mRNA became barely detectable. In contrast to two-dimensional cultures, the addition of TGF-β1 to the culture media did not enhance α-SM actin mRNA in three-dimensional cultures, whereas it induced rapid capillary tube
formation. Actin mRNA expression was modulated in SMCs by extracellular matrix components and TGF-β1 with a pattern very different from that of RFCs. Namely, the comparison of RFCs with other cell types such as bovine aortic
endothelial cells shows that co-expression of endothelial and smooth muscle cell markers is very unique to RFCs and occurs
only in particular culture conditions. This could be related to the capacity of these microvascular endothelial cells to modulate
their phenotype in physiologic and pathologic conditions, particularly during angiogenesis, and could reflect different embryologic
origins for endothelial cell populations.
Supported by a Post-Doctoral Fellowship from the Swiss National Science Foundation (OK) and grant HL-RO1-28373 (JAM) from
the Department of Human Services, Public Health Service, Washington, D.C. 相似文献
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Claude Penel Thomas Gaspar Michèle Crèvecoeur Claire Kevers Hubert Greppin 《Physiologia plantarum》1990,79(2):250-254
Ca2+ and Mn2+ activate the conversion of 1-aminocyclopropane-1-carboxylic acid (ACC) by root microsomes of Vicia lens as they do in other similar systems. The preparation of microsomes in the presence of Mn2+ greatly increases their ability to convert ACC into ethylene, without addition of Mn2+ in the reaction mixture. Ca2+ does not have this property. The effect could not be attributed to Mn2+ entrapping into membrane vesicles (sonication followed by repelleting had no effect) but, possibly, in part to Mn2+ -mediated binding to microsomes of a soluble factor favouring the conversion of ACC to C2 H4 . Although no direct correlation could be established in vitro between ethylene-forming-enzyme (EFE) and peroxidase activities, some soluble peroxidases might be this soluble factor. Mn2+ favoured attachment to membranes of some peroxidase activity from the soluble fraction and from commercial HRP and lipoxygenase. This binding effect of Mn2+ cannot be readily distinguished from its role in the generation of a chain of free radicals and in redox mechanisms. 相似文献
60.
The effects of 50 microM of progesterone (P4), estradiol (E2), estrone (E1), estriol (E3), dehydroepiandrosterone (DHIA), androstenedione (delta 4) and testosterone (T) on the bioconversion of [3H]pregnenolone (6 nM) to [3H]P4 were investigated by incubating 200 mg of tissue fragments as well as equivalent aliquots of microsomes from human term placenta during 30 min. All the steroids assayed, except E3, significantly inhibited the [3H]P4 formation in a microsome incubation system with respect to the control assay (P less than 0.001). Conversely in a tissue incubation system. P4, E1 as well as E3 had no effect on [3H]pregnenolone bioconversion while E2 slightly decreased the [3H]P4 formation (P less than 0.05) compared with the control. A significant inhibition was observed in this system with the other steroids (P less than 0.001). To investigate these apparent different results of inhibition-noninhibition of the same steroids irrespective of the system of incubation used, the effects of P4, E2 and T on 3 beta-hydroxysteroid dehydrogenase/isomerase (3 beta-HSD) activity were studied in tissue fragments and microsomes in kinetic terms. The results found indicate that these steroids inhibited in a competitive fashion the 3 beta-HSD activity in both systems. The different Ki values found in tissue fragments and microsomes respectively for P4 (1.8 microM vs 0.5 microM), E2 (2.3 microM vs 0.6 microM) and T (0.25 microM vs 0.3 microM) explain the bioconversion results obtained in presence of 50 microM of the same steroids. These results include inhibition of [3H]P4 formation by T in tissue fragments as well as in microsomes whereas P4 and E2 inhibited the [3H]P4 formation only in microsomes. Furthermore, the comparison of these Ki values with the available data of intraplacental and circulating concentrations of the same steroids in human term pregnancy suggest that only P4 would be expected to cause marked 3 beta-HSD inhibition in physiological conditions. 相似文献