首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1313705篇
  免费   117080篇
  国内免费   1412篇
  2021年   17812篇
  2020年   12606篇
  2019年   16162篇
  2018年   17516篇
  2017年   16323篇
  2016年   27977篇
  2015年   42384篇
  2014年   50384篇
  2013年   76690篇
  2012年   35604篇
  2011年   23630篇
  2010年   43231篇
  2009年   44642篇
  2008年   22662篇
  2007年   20645篇
  2006年   26093篇
  2005年   27236篇
  2004年   26619篇
  2003年   24222篇
  2002年   22474篇
  2001年   31566篇
  2000年   28793篇
  1999年   29606篇
  1998年   25206篇
  1997年   24992篇
  1996年   24634篇
  1995年   22785篇
  1994年   22626篇
  1993年   21681篇
  1992年   26133篇
  1991年   24353篇
  1990年   23028篇
  1989年   23919篇
  1988年   21956篇
  1987年   20851篇
  1986年   19728篇
  1985年   21692篇
  1984年   21853篇
  1983年   19388篇
  1982年   20233篇
  1981年   19440篇
  1980年   18200篇
  1979年   18227篇
  1978年   17167篇
  1977年   16622篇
  1976年   15735篇
  1975年   15114篇
  1974年   15547篇
  1973年   16002篇
  1972年   13575篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
891.
892.
Cord-forming fungi form extensive networks that continuously adapt to maintain an efficient transport system. As osmotically driven water uptake is often distal from the tips, and aqueous fluids are incompressible, we propose that growth induces mass flows across the mycelium, whether or not there are intrahyphal concentration gradients. We imaged the temporal evolution of networks formed by Phanerochaete velutina, and at each stage calculated the unique set of currents that account for the observed changes in cord volume, while minimizing the work required to overcome viscous drag. Predicted speeds were in reasonable agreement with experimental data, and the pressure gradients needed to produce these flows are small. Furthermore, cords that were predicted to carry fast-moving or large currents were significantly more likely to increase in size than cords with slow-moving or small currents. The incompressibility of the fluids within fungi means there is a rapid global response to local fluid movements. Hence velocity of fluid flow is a local signal that conveys quasi-global information about the role of a cord within the mycelium. We suggest that fluid incompressibility and the coupling of growth and mass flow are critical physical features that enable the development of efficient, adaptive biological transport networks.  相似文献   
893.
894.
Summary The conditions required for the production of a polylysine-coated gold (PL-G) complex, which shows optimal sensitivity for the demonstration of tissue anionic sites, expressed under different conditions of pH have been investigated. Problems encountered with this complex have been compared with those found with other methods of conjugation of polylysine to colloidal gold. The performance of a bovine serum albumin (BSA)-stabilized PL-G complex was examined against other PL-G conjugates, including complexes that are commercially available, for the detection of heterogeneous glomerular anionic site populations, expressed at pH 2.5 and pH 7.0.  相似文献   
895.
896.
Cyclic GMP causes the release of endogenous Ca2+ from rod outer segments, whose plasma membrane has been made permeable, or from isolated discs. Approximately 11,000 Ca2+ ions are released per disc at saturating concentrations of cyclic GMP. The velocity and the amplitude of the release of Ca2+ are dependent on the concentration of cyclic GMP. The maximal rate of the Ca2+ efflux is approximately 7 X 10(4) Ca2+ ions s-1 rod-1. The Ca2+ release by cyclic GMP is independent of light. The activation of the efflux occurred within a narrow range of the cyclic GMP concentration (30-80 microM) and does not obey a simple Michaelis-Menten scheme. Instead, the kinetic analysis of the Ca2+ efflux suggests that a minimum number of 2 molecules of cyclic GMP activates the ion conductance in a cooperative fashion. The release of Ca2+ by cyclic GMP requires a gradient of Ca2+ ions across the disc membrane. If the endogenous Ca2+ gradient is dissipated by means of the ionophore A23187, the release of Ca2+ by cyclic GMP is abolished. Ca2+ is released by analogues of cyclic GMP which are either modified at the 8-carbon position of the imidazole ring or by the deaza-analogue of cyclic GMP. Congeners of cyclic GMP which are modified at the ribose, phosphodiester, or pyrimidine portion of the molecule are ineffective. The hydrolysis of cyclic GMP by the light-regulated phosphodiesterase of rod outer segments is not a necessary condition for the Ca2+ release because 8-bromo-cyclic GMP, a congener resistant to hydrolysis, is a more powerful activator of the release than cyclic GMP itself. Ca2+ release by cyclic GMP is inhibited by organic and inorganic blockers of Ca2+ channels. The l-stereoisomer of cis-diltiazem blocks the release of Ca2+ at micromolar concentrations, whereas the d-form is much less effective. These results suggest that disc membranes contain a cationic conductance which is permeable to Ca2+ ions and which is regulated through the cooperative binding of at least 2 molecules of cyclic GMP to regulatory sites of the transport protein. By this mechanism, subtle changes in the concentration of cyclic GMP could promote large changes in the flux of Ca2+ ions across the disc membrane.  相似文献   
897.
898.
899.
Enumeration of denitrifying microbial populations in turf   总被引:2,自引:0,他引:2  
Summary Denitrifer populations of a silt and silt loam soil under a Kentucky bluegrass turf were enumerated using the most probable number (MPN) procedure. The influence of soil texture, soil depth, soil moisture, and additions of nitrate fertilizer on denitrifier populations were determined. Saturated soil conditions increased denitrifier populations 87-fold in the silt soil and 121-fold in the silt loam soil. Denitrifier populations did not differ significantly between soil depths and additions of fertilizer nitrate did not influence populations.  相似文献   
900.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号