全文获取类型
收费全文 | 48171篇 |
免费 | 4041篇 |
国内免费 | 4篇 |
出版年
2023年 | 229篇 |
2021年 | 386篇 |
2020年 | 322篇 |
2019年 | 323篇 |
2018年 | 927篇 |
2017年 | 954篇 |
2016年 | 1023篇 |
2015年 | 927篇 |
2014年 | 1096篇 |
2013年 | 1978篇 |
2012年 | 3269篇 |
2011年 | 3671篇 |
2010年 | 1882篇 |
2009年 | 1259篇 |
2008年 | 3000篇 |
2007年 | 3080篇 |
2006年 | 2888篇 |
2005年 | 2590篇 |
2004年 | 2460篇 |
2003年 | 2301篇 |
2002年 | 2294篇 |
2001年 | 1522篇 |
2000年 | 1767篇 |
1999年 | 946篇 |
1998年 | 466篇 |
1997年 | 381篇 |
1996年 | 440篇 |
1995年 | 396篇 |
1994年 | 415篇 |
1993年 | 380篇 |
1992年 | 397篇 |
1991年 | 327篇 |
1990年 | 316篇 |
1989年 | 295篇 |
1988年 | 269篇 |
1987年 | 288篇 |
1986年 | 233篇 |
1985年 | 321篇 |
1984年 | 416篇 |
1983年 | 371篇 |
1982年 | 357篇 |
1981年 | 330篇 |
1980年 | 297篇 |
1979年 | 278篇 |
1978年 | 293篇 |
1977年 | 261篇 |
1976年 | 273篇 |
1975年 | 303篇 |
1974年 | 247篇 |
1973年 | 229篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
Paola A Gilardoni Stefan Schuck Ruth Jüngling Björn Rotter Ian T Baldwin Gustavo Bonaventure 《BMC plant biology》2010,10(1):66
Background
Plants trigger and tailor defense responses after perception of the oral secretions (OS) of attacking specialist lepidopteran larvae. Fatty acid-amino acid conjugates (FACs) in the OS of the Manduca sexta larvae are necessary and sufficient to elicit the herbivory-specific responses in Nicotiana attenuata, an annual wild tobacco species. How FACs are perceived and activate signal transduction mechanisms is unknown. 相似文献72.
Walter Döpp 《Planta》1932,17(1):86-152
Ohne ZusammenfassungMit 2 Textabbildungen und Tafel I–X. 相似文献
73.
Suzanne M. Lohmann Ralph P. Miech Fred R. Butcher 《Biochimica et Biophysica Acta (BBA)/General Subjects》1977,499(2):238-250
The effect of theophylline and isoproterenol on bovine tracheal smooth muscle tension and cyclic AMP levels was investigated. Concentrations of isoproterenol (4 × 10?6 M) and theophylline (10 mM) that relaxed carbachol-contracted tracheal muscle by 85–95% did not significantly elevate control levels of cyclic AMP. In the absence of carbachol, several-fold increases in cyclic AMP were caused by isoproterenol although no elevations by theophylline were measurable. However, when isoproterenol and theophylline were administered together, theophylline potentiated the rise in cyclic AMP caused by isoproterenol. Phosphodiesterase studies in tracheal muscle showed the presence of a high and a low Km enzyme which were inhibited by theophylline. Cyclic GMP levels were elevated in muscles contracted by carbachol as well as in carbachol-contracted muscles that had been relaxed by theophylline. In non-tension studies, in which the tracheal muscle was not under isometric tension, carbachol or theophylline alone increased cyclic GMP and together they synergistically elevated cyclic GMP. Atropine blocked the elevation caused by carbachol but not that caused by theophylline. In contrast to theophylline, isoproterenol did not elevate cyclic GMP, and in carbachol-contracted muscles that had been relaxed by isoproterenol, cyclic GMP levels were no different from control. Also, in non-tension studies, isoproterenol decreased basal cyclic GMP and antagonized the increase in cyclic GMP due to carbachol.The results indicate that whole-tissue levels of cyclic AMP and cyclic GMP do not correlate with the state of tracheal smooth muscle tension. Cyclic GMP levels do not clearly correlate with either contraction or relaxation. The inhibition by carbachol of increases in cyclic AMP due to isoproterenol and the inhibition by isoproterenol of increases in cyclic GMP due to carbachol provide evidence for a reciprocal cholinergic-adrenergic antagonism of cyclic AMP and cyclic GMP levels. The antagonism did not appear to be due to either cyclic nucleotide affecting the elevation of the other since the levels of both cyclic nucleotides were depressed. 相似文献
74.
Tor Strömgren 《Coral reefs (Online)》1987,6(1):43-47
Series of apices of Acropora pulchra from an intertidal reef at Phuket, Thailand, were grown at different depths in the sea, and the length growth was monitored at 12–24 h intervals with laser diffraction. The growth rates of actively growing apices were normally distributed and showed a high variability, with an average coefficient of variation of 58%. There was a highly significant difference in average growth rate between adjacent colonies. A significant linear relationship was found between irradiance and length growth, with a saturating level at 300–400 Wm-2. At 1000 Wm-2 length growth was significantly reduced. Under normal daylight conditions, day and night growth rates were equal. 相似文献
75.
76.
77.
78.
Summary The replication model for sister chromatid exchange (SCE), when introduced in 1980 by Painter, was claimed to be consistent with the one hit property of SCE. However, the argument offered in favour of the one hit property was based on a defective dose-response function, as shown in this paper, since dose as the independent parameter of any dose-response function was not included in the considerations. This missing part of the model's dose-response function is added and, by using Bessel functions, a formula for the complete dose-response function is presented. A re-examination of the newly derived function shows that, in the model, linearity holds only under certain restricted circumstances. 相似文献
79.
80.
Bernd Höffken Reinhard Ködding Alexander von zur Mühlen Tainer Hehrmann Rolf-Dieter Hesch 《Biochimica et Biophysica Acta (BBA)/General Subjects》1978,539(1):114-124
The nature of the conversion of thyroxine (T4) to triiodothyronine (T3) and reverse triiodothyronine (rT3) was investigated in rat liver homogenate and microsomes. A 6-fold rise of T3 and 2.5-fold rise of rT3 levels determined by specific radioimmunoassays was observed over 6 h after the addition of T4. An enzymic process is suggested that converts T4 to T3 and rT3. For T3 the optimal pH is 6 and for rT3, 9.5. The converting activity for both T3 and rT3 is temperature dependent and can be suppressed by heat, H2O2, merthiolate and by 5-propyl-2-thiouracil. rT3 and to a lesser degree iodide, were able to inhibit the production of T3 in a dose related fashion. Therefore the pH dependendy, rT3 and iodide may regulate the availability of T3 or rT3 depending on the metabolic requirements of thyroid hormones. 相似文献