全文获取类型
收费全文 | 599574篇 |
免费 | 68412篇 |
国内免费 | 470篇 |
出版年
2016年 | 8161篇 |
2015年 | 13354篇 |
2014年 | 14601篇 |
2013年 | 18258篇 |
2012年 | 19121篇 |
2011年 | 16723篇 |
2010年 | 11658篇 |
2009年 | 11136篇 |
2008年 | 13771篇 |
2007年 | 14429篇 |
2006年 | 13476篇 |
2005年 | 18743篇 |
2004年 | 17430篇 |
2003年 | 15380篇 |
2002年 | 12666篇 |
2001年 | 23508篇 |
2000年 | 23462篇 |
1999年 | 19675篇 |
1998年 | 6931篇 |
1997年 | 7180篇 |
1996年 | 6925篇 |
1995年 | 6712篇 |
1994年 | 6688篇 |
1993年 | 6566篇 |
1992年 | 17753篇 |
1991年 | 17702篇 |
1990年 | 17436篇 |
1989年 | 16993篇 |
1988年 | 15889篇 |
1987年 | 15260篇 |
1986年 | 14253篇 |
1985年 | 14397篇 |
1984年 | 11856篇 |
1983年 | 10383篇 |
1982年 | 7840篇 |
1981年 | 7332篇 |
1980年 | 6869篇 |
1979年 | 11851篇 |
1978年 | 9141篇 |
1977年 | 8355篇 |
1976年 | 7989篇 |
1975年 | 8982篇 |
1974年 | 9391篇 |
1973年 | 9261篇 |
1972年 | 8769篇 |
1971年 | 7716篇 |
1970年 | 6749篇 |
1969年 | 6427篇 |
1968年 | 5844篇 |
1967年 | 5204篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
We examine conductances for evaporation from both vegetation and soil in response to environmental variables. Data from a vertically-structured pristine forest of Nothofagus are presented as an example of the effects of biodiversity on the scaling of conductances between tiers of plant organisation. Available data sets of maximum leaf stomatal conductances (g
lmax
) and bulk vegetation surface conductances (G
smax
) are compared. Overall, the ratio G
smax
/g
lmax
is consistently close to 3 for seven major vegetation types of diverse structure. An analytical model accounts for this close relationship, and in particular how G
smax
is conservative against changes in leaf area index because of the compensating decrease in plant canopy transpiration and increase in soil evaporation as leaf area index diminishes. The model is also successfully tested by comparison with canopy conductances of emergent trees measured in the Nothofagus forest. The constraint of vegetation surface conductance and evaporation via environmental regulation by irradiance, air saturation deficit and root zone water supply are discussed. 相似文献
992.
How many signals are enough? 总被引:1,自引:0,他引:1
N R Sinclair 《Cellular immunology》1990,130(1):204-12; discussion 213-35
The many signals that control the progress of various immune responses to both foreign and self antigens can be divided into no less than three major groups. The first group is the initial positive stimulus, associated with activation events through antigen receptors and their associated proteins. These signals launch lymphocytes in their response to antigen, either foreign or self. The second group of signals is negative and involves various end products and interactions between cells, all recognizing antigen. These signals are endogenous to the reacting cell, or nearly so (two interacting cells from the same clone, daughter cells, which are in the same locale and bind to the same ligand). The third group (the prevention of end product feedback, involving various forms of antigen presentation, T cell contributions, rheumatoid factor activity, and other mechanisms) is more likely to occur with nonself antigens, which are temporally and spatially more restricted than self antigens. Experimental evidence for this immunological schema is summarized and clarified in its relationship to the Bretscher-Cohn theory of self-nonself recognition and to suppressor cell and idiotype-antiidiotypic theories. 相似文献
993.
Reuben S.E. Young Andrew P. Bowman Kaylyn D. Tousignant Berwyck L.J. Poad Jennifer H. Gunter Lisa K. Philp Colleen C. Nelson Shane R. Ellis Ron M.A. Heeren Martin C. Sadowski Stephen J. Blanksby 《Journal of lipid research》2022,63(6):100223
The cellular energy and biomass demands of cancer drive a complex dynamic between uptake of extracellular FAs and their de novo synthesis. Given that oxidation of de novo synthesized FAs for energy would result in net-energy loss, there is an implication that FAs from these two sources must have distinct metabolic fates; however, hitherto, all FAs have been considered part of a common pool. To probe potential metabolic partitioning of cellular FAs, cancer cells were supplemented with stable isotope-labeled FAs. Structural analysis of the resulting glycerophospholipids revealed that labeled FAs from uptake were largely incorporated to canonical (sn-) positions on the glycerol backbone. Surprisingly, labeled FA uptake also disrupted canonical isomer patterns of the unlabeled lipidome and induced repartitioning of n-3 and n-6 PUFAs into glycerophospholipid classes. These structural changes support the existence of differences in the metabolic fates of FAs derived from uptake or de novo sources and demonstrate unique signaling and remodeling behaviors usually hidden from conventional lipidomics. 相似文献
994.
Marine phytoplankton can evolve rapidly when confronted with aspects of climate change because of their large population sizes and fast generation times. Despite this, the importance of environment fluctuations, a key feature of climate change, has received little attention—selection experiments with marine phytoplankton are usually carried out in stable environments and use single or few representatives of a species, genus or functional group. Here we investigate whether and by how much environmental fluctuations contribute to changes in ecologically important phytoplankton traits such as C:N ratios and cell size, and test the variability of changes in these traits within the globally distributed species Ostreococcus. We have evolved 16 physiologically distinct lineages of Ostreococcus at stable high CO2 (1031±87 μatm CO2, SH) and fluctuating high CO2 (1012±244 μatm CO2, FH) for 400 generations. We find that although both fluctuation and high CO2 drive evolution, FH-evolved lineages are smaller, have reduced C:N ratios and respond more strongly to further increases in CO2 than do SH-evolved lineages. This indicates that environmental fluctuations are an important factor to consider when predicting how the characteristics of future phytoplankton populations will have an impact on biogeochemical cycles and higher trophic levels in marine food webs. 相似文献
995.
996.
A study was made of the dose-time relationship during chromatin degradation in white blood cells of non-irradiated and irradiated rats. There was a linear increase in the release of PDN from leukocytes 1,2 and 3 days after irradiation (1-3 Gy) followed by the deceleration of the chromatin degradation at doses exceeding 3 Gy. 相似文献
997.
Dealing with patients'' sex-related complaints is uncomfortable for many physicians. It can become more comfortable if the physician learns to apply his or her clinical knowledge of sexuality in combination with specific interpersonal interviewing skills. The level of comfort can be increased if the physician accepts his or her professional limitations in providing therapy and knows what other sources of care are available. 相似文献
998.
J C Lambert G Vallette G E Seralini R Vranckx E Nunez C Stora 《Comptes rendus de l'Académie des sciences. Série III, Sciences de la vie》1986,302(9):353-358
Data are presented which indicate a possible action of alpha-fetoprotein (AFP) on female germinal cells. The in vitro maturation of mature mice oocytes was significantly inhibited when mouse AFP replaced albumin in the culture medium. In addition, the degenerative aspect of oocytes cultured with AFP seemed to indicate that this me?otic inhibition was caused by a premature degeneration of oocytes rather than by a blockage at a specific stage of maturation. Thus AFP, perhaps through its ligands, may play a role in the reduction of germinal cells during fetal and immediate post-natal life rather than in the arrest of me?osis at the diplotene stage. 相似文献
999.
1000.
F. F. Snyder A. E. Chudley P. M. MacLeod R. J. Carter E. Fung J. K. Lowe 《Human genetics》1984,67(1):18-22
Summary A family is described in which four affected males, spanning two generations, have hyperuricemia and gout accompanied by hematuria but are without severe neurologic involvement. The affected males were found to have markedly reduced levels of erythrocytic hypoxanthine-guanine phosphoribosyltransferase (HGPRT) activity; these were 5–12% with hypoxanthine and 0.5–3% with guanine as compared to controls. Erythrocytic adenine phosphoribosyltransferase (APRT) was approximately three-fold elevated in the affected individuals.The residual HGPRT activity in affected males enabled characterization of some of the properties of this mutation. The apparent Michaelis constants (km) for both hypoxanthine and guanine were essentially unchanged, whereas the km for PP-ribose-P was approximately 10–20-fold elevated for all four affected males. The enzyme was more sensitive to product inhibition by IMP and GMP than controls, and exhibited greater thermal lability at 65°C than found with control lysates. 相似文献