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   56719篇
  免费   4746篇
  国内免费   37篇
  61502篇
  2023年   286篇
  2022年   603篇
  2021年   1095篇
  2020年   807篇
  2019年   877篇
  2018年   1276篇
  2017年   1073篇
  2016年   1711篇
  2015年   2446篇
  2014年   2432篇
  2013年   3030篇
  2012年   3562篇
  2011年   3279篇
  2010年   2013篇
  2009年   1936篇
  2008年   2377篇
  2007年   2346篇
  2006年   2151篇
  2005年   2441篇
  2004年   2421篇
  2003年   2024篇
  2002年   1634篇
  2001年   1506篇
  2000年   1413篇
  1999年   1276篇
  1998年   580篇
  1997年   547篇
  1996年   606篇
  1995年   463篇
  1994年   473篇
  1993年   443篇
  1992年   944篇
  1991年   843篇
  1990年   779篇
  1989年   773篇
  1988年   766篇
  1987年   698篇
  1986年   655篇
  1985年   638篇
  1984年   597篇
  1983年   451篇
  1982年   340篇
  1981年   339篇
  1980年   319篇
  1979年   443篇
  1978年   364篇
  1977年   300篇
  1975年   301篇
  1974年   304篇
  1973年   303篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
Zusammenfassung Mit Bodenfallen wurde das Auftreten der Laufk?fer vorwiegend in Winterweizenfeldern in der Zeit von Ende Mai bis Anfang August 1973 und 1974 in Belgien, Deutschland, den Niederlanden und Schweden festgestellt. Unter den h?ufigsten Arten tratenPterostichus vulgaris undBembidion lampros in allen,Agonum dorsale in fast allen Untersuchungsgebieten gemeinsam auf. Sechzehn weitere Arten konnten als allgemein für die meisten Weizenfelder der Untersuchungsgebiete genannt werden. In fast allen Gebieten waren (gemessen an der Artenzahl) die Frühlingstiere mit Herbstbestand in der überzahl Die durchschnittliche Anazahl der pro Falle und Tag gefangenen Laufk?fer variierte von 1 bis 13 und lag im Mittel aller Gebiete bei 6 K?fern. Mit der Quadratmenthode wurden in einem norddeutschen Gebiet 6000 bis 11000 Individuen der ArtAgonum dorsale pro ha festgestellt. Es wird darauf hingewiesen, dass es lohnenswert ist, die Laufk?fer bei der Erstellung integrierter Pflanzenschutzprogramme zu berücksichtigen.
Summary The occurrence ofCarabidae was investigated by pitfall trapping mainly on fields of winter wheat from the end of May to the beginning of August 1973 and 1974 in Belgium, Germany, the Netherlands, and Sweden. Among the most common speciesPterostichus vulgaris andBembidion lampros were found in all,Agonum dorsale in nearly all regions. Furthermore the following 16 species could be regarded as common for most of the wheat fields in the regions of investigation:Trechus quadristriatus, T. discus, Pterostichus cupreus, P. niger, Asaphidion flavipes, Clivina fossor, Loricera pilicornis, Nebria brevicollis, Harpalus pubescens, H. aeneus, Agonum mülleri, Bembidion ustulatum, Amara familiaris, Calathus fuscipes, Carabus cancellatus ardC. granulatus. In most of the regions, if numbers of species were regarded, spring breeders with an autumn generation were more numerous than spring breedens without an autumn generation, and autumn breeders. The average number of ground beetles per pitfall trap and day varied from 1 to 13 and averaged 6, if all regions were regarded. By using the square method in a Northern German region there was stated a number of 6,000 up to 11,000 individuals ofAgonum dorsale per hectare. Where a comparison was possible, no distinct difference could be stated concerning the number of carabid species between the present investigations and those having taken place ca. 20 years ago. It is emphasized, that it is worth while taking into account the ground beetles, too, when establishing integrated control programs in cereal growing.


West-Pal?arktisch Regionale Sektion der O.I.L.B., Arbeitsgruppe Integrierte Bek?mpfung im Getreidebau

Dem Andenken unseres Freundes und KollegenThyge Thygesen (1923–1974) gewidmet, deran der Gründung der Arbeitsgruppe beteiligt vor Abschluss der Untersuchungen verstorben ist.  相似文献   
993.
The effect of epinephrine on the oxidative desaturation of fatty acids by liver microsomal preparations of rats has been studied. Administration of epinephrine (1 mg/kg body weight) produced a significant decrease in desaturation of [l-14C]=linoleic acid to gamma-linolenic acid and of [L-14C]alpha-linolenic acid to actadeca-6,9,12,15-tetraenoic acid 12 hr after the infection. Lower doses produced a lesser effect on the delta6-desaturation activity. Epinephrine administration modified the V max of linoleic acid desaturation but not the K m. There was also a slight increase in palmityl desaturation activity. The effect of epinephrine on delta6-desaturation activity was postulated to be mediated through an enhancement of the intracellular cyclic AMP levels that lead to an increase of a glucose metabolite. This metabolite would inhibit delta6-desaturation activity.  相似文献   
994.
995.
Summary In man congenital lack of an enzyme of the purine salvage system, hypoxanthineguanine phosphoribosyl transferase (HG-PRT E.C. 2.4.2.8), is mostly accompanied by a picture known as the Lesch-Nyhan syndrome. The degree of deficiency may vary from zero to a few percent of normal activity but a correlation between the severity of HG-PRT deficiency and the clinical picture has not been observed, no more than a correlation between HG-PRT deficiency and neurological dysfunction. But individuals with undetectable HG-PRT activity but without the Lesch-Nyhan syndrome have been described. Patients with partial HG-PRT deficiency have clinically distinctive findings. Sometimes mild neurological abnormalities are observed. Because of marked overproduction of uric acid severe gouty arthritis and renal dysfunction are often encountered in both complete and partial deficiency.There is considerable molecular heterogeneity in HG-PRT deficiency in man. Mutant enzymes may exhibit different kinetic and electrophoretic properties, indicating that there might be a mutation on the structural gene coding for HG-PRT.Lack of HG-PRT disturbs purine interconversions profoundly. In addition to an important function of HG-PRT in the uptake of the purine bases hypoxanthine and guanine into the cell, the effective uptake of inosine, guanosine and adenosine also seems to be dependent on HG-PRT. Uptake of purine bases into intact red blood cells occurs according to a two component mechanism, one component probably involving a phosphoribosyl transferase system.The inheritance of HG-PRT deficiency is X-linked recessive and it is transmitted by asymptomatic carrier females. Several methods have been introduced for carrier detection. As a consequence of X chromosome inactivation, in these females a mosaicism of HG-PRT positive and HG-PRT negative fibroblasts can be demonstrated after cloning or after selection of HG-PRT negative cells in a selective medium. A more rapid method involves direct measurements of HG-PRT activities in single hair roots from the scalp. Because hair roots develop more or less clonally, in heterozygote females HG-PRT positive and negative hair roots are encountered. HG-PRT deficiency can be detected antenatally by demonstrating the presence or absence of enzyme activity in ammiotic fluid derived fibroblasts qualitatively by autoradiography and quantitatively by ultramicrochemical measurements of enzyme activities in single or small numbers of cells.In studies with isolated cells the metabolic defect can be corrected in several ways. Metabolic cooperation between HG-PRT positive and HG-PRT negative cells leads to apparently normal phenotype of all cells, provided there is cell to cell contact. There is evidence that a missing enzyme product or a derivative might be transferred from the normal to the mutant cells. Apparent correction of the enzyme defect is also observed when HG-PRT deficient lymphocytes are stimulated with phytohaemagglutinin.The first data suggestive of genetic complementation between two human HG-PRT deficient cell strains by which hybrid cells can synthesize a functionally active HG-PRT, are consistent with the view that HG-PRT deficiency in man is due to a structural gene mutation. Recent results show that other interesting findings might come from experiments in which HG-PRT deficient cells are treated with exogenous genetic material (isolated DNA or metaphase chromosomes) to reactivate or induce HG-PRT activity.Supported by grants from FUNGO (Foundation for Medical Scientific Research in the Netherlands) and the Medical Prevention Fund.  相似文献   
996.
997.
998.
The chain stiffness of linear native DNA is represented by a generalized bead and spring model recently proposed. It incorporates molecular rigidity by means of springs between beads, which are second neighbors along the contour of the chain. These springs are equivalent to elastic forces having longitudinal and transversal contributions. The model is compared with existing experimental data of sedimentation and low-angle light scattering to obtain the statistical parameters of DNA. The value of the statistical length obtained with this model is 1300 Å. The same value is obtained with the wormlike chain. Throughout this analysis, excluded volume is left out as a simplifying assumption.  相似文献   
999.
The effect of catecholamine-depleting pretreatments, reserpine, and 6-hydroxydopamine (6-OH-DA) on left ventricular pressure (LVP) and the inotropic response to graded doses of ouabain (up to 300 mug/0.05 ml) was studied in isolated perfused rat and guinea-pig hearts. In rats, reserpine and 6-OH-DA depleted the cardiac content of catecholamine, but did not increase initial LVP and did not reduce the inotropic response to the highest dose of ouabain. It is concluded that in isolated rat hearts, these catecholamine-depleting pretreatments nearly abolish the inotropic response to ouabain, and this effect appears to be mediated mainly through an increase in initial LVP. The reason why catecholamine depletion failed to increase initial LVP in guinea pigs remains unexplained.  相似文献   
1000.
Silver and mercury ions are known to react with the bases of nucleic acids in solution. At low cation/base ratios Ag+ has an affinity for GC pairs in DNA, whereas Hg++ is preferentially bound to AT-rich nucleic acids. We have used fluorometry to measure the effect of these cations on the fluorescence intensity of preformed complexes of acranil and DNA in solution. The results are: 1) Ag+ enhances the fluorescence intensity presumably by affecting the dye intercalated in the vicinity of GC-pairs. 2) The addition of Hg++ leads to a quenching of the fluorescence intensity of the complex at low ion/base ratios, suggesting an effect on the dye molecules bound to AT pairs. At high GC-content of the nucleic acid, slight enhancement of the fluorescence intensity occurs with Hg++. 3) With both metals there is a correlation between base content of DNA and effect on the intensity of fluorescence indicating base specificity of the dye-polymer interaction.  相似文献   
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