全文获取类型
收费全文 | 6231篇 |
免费 | 591篇 |
国内免费 | 134篇 |
出版年
2023年 | 44篇 |
2022年 | 75篇 |
2021年 | 128篇 |
2020年 | 93篇 |
2019年 | 121篇 |
2018年 | 197篇 |
2017年 | 153篇 |
2016年 | 205篇 |
2015年 | 312篇 |
2014年 | 308篇 |
2013年 | 442篇 |
2012年 | 464篇 |
2011年 | 410篇 |
2010年 | 273篇 |
2009年 | 206篇 |
2008年 | 270篇 |
2007年 | 273篇 |
2006年 | 258篇 |
2005年 | 214篇 |
2004年 | 227篇 |
2003年 | 205篇 |
2002年 | 205篇 |
2001年 | 210篇 |
2000年 | 224篇 |
1999年 | 130篇 |
1998年 | 62篇 |
1997年 | 71篇 |
1996年 | 53篇 |
1995年 | 60篇 |
1994年 | 63篇 |
1993年 | 45篇 |
1992年 | 81篇 |
1991年 | 72篇 |
1990年 | 69篇 |
1989年 | 66篇 |
1988年 | 46篇 |
1987年 | 46篇 |
1986年 | 41篇 |
1985年 | 33篇 |
1984年 | 40篇 |
1983年 | 42篇 |
1982年 | 27篇 |
1981年 | 26篇 |
1980年 | 30篇 |
1979年 | 24篇 |
1978年 | 29篇 |
1977年 | 25篇 |
1976年 | 26篇 |
1975年 | 38篇 |
1974年 | 31篇 |
排序方式: 共有6956条查询结果,搜索用时 15 毫秒
21.
Luc C. Duchesne R. S. Jeng M. Hubbes M. B. Sticklen 《Trees - Structure and Function》1990,4(4):187-190
Summary The accumulation of mansonones E and F was investigated in Ulmus americana L. seedlings 5 weeks after inoculation with three aggressive and three non-aggressive isolates of Ophiostoma ulmi (Buism.) Nannf. The three non-aggressive isolates stimulated significantly more mansonone E and F accumulation than the three aggressive isolates of O. ulmi. Mansonone induction also varied within both the aggressive and the non-aggressive groups. Aggressive and non-aggressive isolates were recovered in equal frequencies from the inoculation wounds, whereas the aggressive isolates were recovered more frequently than the non-aggressive isolates 15 cm and 25 cm up the seedlings' stem. Vascular browning in the outer xylem of the seedlings correlated with mansonone E and F accumulation. Mansonone accumulation in U. americana seedlings is therefore associated with vascular browning and a reduction in fungal spread. 相似文献
22.
Henk Gude Aart Beekman Paul H. B. Van't Padje Linus H. W. Van Der Plas 《Plant Growth Regulation》1988,7(2):101-116
Incubation of potato tuber tissue discs on B5 medium supplemented with 1-naphtyl-acetic acid (NAA) led to callus formation, irrespective of the presence of kinetin; without NAA no callus formation occurred. Incubation in the presence of abscisic acid (ABA) reduced the increases in fresh weight and dry weight both in callus-forming and in non-callus-forming tissue. Mitochondrial respiration was lowered by ABA as well. The induction of the alternative, CN-resistant pathway was inhibited by the presence of ABA, especially in mitochondria from non-callus-forming tissue.The in vivo respiration of the callus-forming tissue was higher than that of the non-callus-forming tissue. Total respiration, cytochrome pathway activity and the capacity of the alternative pathway were all lowered in callus-forming tissue by treatment with ABA. The in vivo activity of the alternative pathway was low in all tissue types, especially after ABA-treatment. The slight stimulation by hydroxamates of the oxygen uptake of callus-forming tissue incubated on medium with NAA and ABA indicates the involvement of a hydroxamate-activated peroxidase in the oxygen uptake of this tissue; this peroxidase seemed not to participate in the oxygen uptake of the other tissues types.In non-callus-forming tissue the oxygen uptake of ABA-treated tissue was very low and almost completely resistant to the combined addition of inhibitors of both the cytochrome and the alternative pathway, indicating that the in vivo activity of the mitochondria in the oxygen uptake of the tissue was very low. The possible causes for this ABA-effect are discussed. In non-callus-forming tissue the treatment with ABA creates a situation which is comparable with that observed in intact potato tubers. This situation is characterized by a tissue respiration lower than that of the isolated mitochondria and an alternative pathway capacity that is low or absent. 相似文献
23.
Air-grown cells of a marine, small-celled (2 m diameter) strain of Stichococcus bacillaris contained appreciable carbonic-anhydrase activity but this was repressed when cells were grown on air enriched with 5% (v/v) CO2. Assay of carbonic-anhydrase activity using intact cells and cell extracts showed all activity was intracellular in this Stichococcus strain. Measurement of inorganic-carbon-dependent photosynthetic O2 evolution at pH 5.0, where CO2 is the predominant form of inorganic carbon, showed that the concentration of inorganic carbon required for half-maximal rate of photosynthetic O2 evolution [K0.5(CO2)] was 4.0 M for both air- and CO2-grown cells. At pH 8.3 the K0.5(CO2) was 0.3 mM for air-grown and 0.6 mM for CO2-grown cells. Sodium ions did not enhance bicarbonate utilization. Measurement of the internal inorganic-carbon pool (HCO
3
–
+CO2) by the silicone-oil-layer centrifugal filtering technique showed that air- and CO2-grown cells were able to concentrate inorganic carbon up to 20-fold in relation to the external medium at pH 5.0 but not at pH 8.3. In this alga the high affinity for CO2 and inorganic-carbon accumulation in CO2- and air-grown cells results from active CO2 transport that is not dependent on carbonic-anhydrase activity.Abbreviation Hepes
4-(2-hydroxyethyl)-1-piperazine ethanesulfonic acid 相似文献
24.
25.
一、前言哈巴德布鲁克实验林(Hubbard Brook Experimental Forest)位于美国东北部新罕布什尔州中部的白山国家森林中。该地区位于典型温带湿润气候区内,年平均降水量为129.5cm,全年月平均降水量变化不大,冬雪夏雨。蒸发蒸腾量以每年6—9月为最大(Likens等,1977;Bormann等,1979)。该实验林为北美温带落叶阔叶林,属红果云杉(Picea rubens)-阔叶林。Hubbard Brook Ecosystem Study(HBES)是开始最 相似文献
26.
Infraciliature, morphogenesis and life cycle of Endosphaera terebrans (Suctoria, Tokophridae) 总被引:1,自引:0,他引:1
The morphology, infraciliature, and life cycle of Endosphaera terebrans, a suctorian endocommensal of peritrichs, have been studied with the aid of silver impregnation. The life cycle of Endosphaera terebrans begins with infection of the host cell by a small larva. The swarmer has a pointed needle-like cellular projection and two rings of cilia. The swarmer penetrates the the peritrich, loses the cilia, and then matures into an adult. The infraciliature of the adult form has four rows of barren kinetosomes that lack kinetodesmal fibers. By endogenous budding, a migratory larva is produced that leaves the host cell through the peristomial disc and that can infect other peritrichs. 相似文献
27.
The effect of copper on the uptake of nitrogen and the tissue contents of inorganic nitrogen, amino acids and proteins were studied in cooper-sensitive Silene vulgaris (Moench) Garcke, grown at different nitrogen sources (NH4
+ or NO3
-). All the toxic copper levels tested, i.e. 4, 8, 16 M Cu2+, strongly inhibited the uptake of nitrogen, especially of NO3
-, and decreased the content of NO3
-, amino acids and proteins. Especially at 4 and 8 M Cu2+, NH4
+ accumulated in the plants, suggesting that the conversion of NH4
- into amino acids was inhibited. 相似文献
28.
R Mu?oz R Iglesias J M Ferreras F J Arias M A Rojo T Girbés 《Cellular and molecular biology, including cyto-enzymology》1991,37(7):671-677
Long-term ingestion of sublethal n-butanol doses by rats led to a noteworthy increase in the resistance of in vitro brain ribosomal function to the acute inhibitory action of ethanol and isopropanol. Withdrawal of n-butanol did not change this adaptation process immediately. The step affected seems to be the elongation of polypeptide chains. The dependence of in vitro translation on incubation temperature was affected by the adaptation process, the translation system of chronic animals being less stimulatable than that of control animals at low temperature. 相似文献
29.
Cytosolic diacylglycerol kinase was irreversibly inactivated by 5'-AMP since the enzyme remained less active after the removal of 5'-AMP by P-10 gel chromatography. The inactivation was time-dependent, suggesting the involvement of a covalent bond modification. A reconstitution experiment detected a rat brain cytosolic mediator for the effect of 5'-AMP. A protein kinase rich fraction prepared from rat liver was also capable of restoring the sensitivity of diacylglycerol kinase-II to 5'-AMP. We propose that 5'-AMP-activated protein kinase is the mediator which inactivates diacylglycerol kinase-II, possibly by phosphorylation. 相似文献
30.
The AXB and BXA set of recombinant inbred mouse strains 总被引:1,自引:1,他引:0
Jan D. Marshall Jian-Long Mu Yin-Chai Cheah Muriel N. Nesbitt Wayne N. Frankel Beverly Paigen 《Mammalian genome》1992,3(12):669-680
The recombinant inbred (RI) set of strains, AXB and BXA, derived from C57BL/6J and A/J, originally constructed and maintained at the University of California/San Diego, have been imported into The Jackson Laboratory and are now in the 29th to 59th generation of brother-sister matings. Genetic quality control testing with 45 proviral and 11 biochemical markers previously typed in this RI set indicated that five strains had been genetically contaminated sometime in the past, so these strains have been discarded. The correct and complete strain distribution patterns for 56 genetic markers are reported for the remaining RI strain set, which consists of 31 living strains and 8 extinct strains for which DNA is available. Two additional strains, AXB 12 and BXA 17, are living and may be added to the set pending further tests of genetic purity. The progenitors of this RI set differ in susceptibility to 27 infectious diseases as well as atherosclerosis, obesity, diabetes, cancer, cleft palate, and hydrocephalus. Thus, the AXB and BXA set of RI strains will be useful in the genetic analysis of several complex diseases. 相似文献