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   10330篇
  免费   418篇
  国内免费   467篇
  2023年   81篇
  2022年   88篇
  2021年   138篇
  2020年   160篇
  2019年   238篇
  2018年   343篇
  2017年   294篇
  2016年   175篇
  2015年   179篇
  2014年   420篇
  2013年   483篇
  2012年   343篇
  2011年   378篇
  2010年   268篇
  2009年   238篇
  2008年   326篇
  2007年   330篇
  2006年   287篇
  2005年   247篇
  2004年   183篇
  2003年   194篇
  2002年   169篇
  2001年   121篇
  2000年   120篇
  1999年   111篇
  1998年   103篇
  1997年   96篇
  1996年   78篇
  1995年   82篇
  1994年   80篇
  1993年   71篇
  1992年   52篇
  1991年   52篇
  1990年   62篇
  1989年   48篇
  1988年   42篇
  1987年   42篇
  1985年   500篇
  1984年   600篇
  1983年   334篇
  1982年   519篇
  1981年   395篇
  1980年   415篇
  1979年   339篇
  1978年   295篇
  1977年   242篇
  1976年   218篇
  1975年   222篇
  1974年   201篇
  1973年   154篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
91.
In an attempt to show that the open field can still be used as a valid measure of fear, Jones (1983) has reported a failure to replicate some of our findings. The present studies show that this was due to procedural and methodological differences. For instance, we found that birds tested in a novel environment behaved quite differently from those, as in Jones' case, which were placed in one resembling the home cage. Moreover, birds housed in isolation for two days prior to testing reacted differently than those, as again in Jones' case, which were reared in isolation from hatching to the time of testing. The results were interpreted as being consistent with our view that open-field behaviour reflects a conflict between the need to reinstate contact with conspecifics on the one hand, and evade predation on the other.  相似文献   
92.
cDNA clones coding for rat liver ribosomal proteins S17 and L30 have been isolated by positive hybridization-translation assay from a cDNA library prepared from 8-9S poly(A)+RNA from free polysomes of regenerating rat liver. The cDNA clone specific for S17 protein (pRS17-2) has a 466-bp insert with the poly(A) tail. The complete amino acid (aa) sequence of S17 protein was deduced from the nucleotide sequence of the cDNA. S17 protein consists of 134 aa residues with an Mr of 15 377. The N-terminal aa sequence of S17 protein determined by automatic Edman degradation is consistent with the sequence data. The aa sequence of S17 shows strong homology (76.9%) to that of yeast ribosomal protein 51 [Teem and Rosbash, Proc. Natl. Acad. Sci. USA 80 (1983) 4403-4407] in the two-thirds N-terminal region. The cDNA clone specific for L30 protein (pRL30) has a 394-bp insert. The aa sequence of L30 protein was deduced from the nucleotide sequence of the cDNA. The protein consists of 114 aa residues with an Mr of 12 652. When compared with the N-terminal aa sequence of rat liver L30 protein [Wool, Annu. Rev. Biochem. 48 (1979) 719-754], pRL30 was found not to contain the initiation codon and 5'-noncoding region. The cDNA showed twelve silent changes in the coding region, one point mutation and one base deletion in the 3'-noncoding region, compared with mouse genomic DNA for L30 protein [Wiedemann and Perry, Mol. Cell Biol. 4 (1984) 2518-2528].  相似文献   
93.
Synopsis Scaphirhynchus albus and S. platorynchus were studied in Missouri during 1978–1979 to assess their distribution and abundance, to obtain information on their life histories, and to identify existing or potential threats to their survival. S. platorynchus was collected in substantial numbers (4355 specimens) at all 12 sampling stations in the Missouri and Mississippi rivers, while only 11 S. albus were captured from 6 stations. Twelve specimens identified in the field as hybrids between the two species were captured from 4 stations. Morphometric and meristic comparisons of presumed hybrids with the parent species, using cluster and principal components analyses, demonstrated intermediacy of most specimens identified in the field as hybrids. Aquatic insects comprised most of the diet of S. platorynchus and S. albus, but S. albus and the hybrids had consumed considerable quantities of fish. S. albus grew more rapidly than S. platorynchus, while the growth of hybrids was intermediate. Hybridization appears to be a recent phenomenon, resulting from man-caused changes in the big-river environment. Hybridization may be a threat to survival of S. albus in the study streams.  相似文献   
94.
Human erythrocytes were incubated in isotonic solutions of different monovalent cations. The apparent size of the red cells measured on scanning electron microscopic pictures decreases in the order Li+>Na+=K+>Rb+. These differences in size are abolished after pretreatment with trypsin, which removes a large part of the charges associated with membrane glycoproteins. Shape alterations are also observed. Normal biconcave shapes are visible after Na+ or K+ incubation, whereas Li+ leads to flabby, flattened cells with a certain tendency to crenation, and Rb+ causes more pronounced biconcavity with a certain tendency to cupping. The overall effects of pretreatment with trypsin are similar to those of Li+. Our results provide evidence that the electrostatic repulsion of glycoproteins and other charged membrane components may play an essential role in maintaining red cell shape.  相似文献   
95.
We evaluated in a double-blind study the bronchodilatory properties of 2-decarboxy-2-hydroxymethyl prostaglandin E1 (PGE1-carbinol), described recently as a nonirritant bronchodilator in animals. Fifteen asthmatic patients received by inhalation single doses of 1, 10, and 30 μg PGE1-carbinol, 55 μg PGE2, and placebo (10% ethanol in normal saline, which was also used as diluent for the PGs). Such pulmonary function tests as forced expiratory volume in 1 second, forced vital capacity, and maximal expiratory flow were monitored during 2 hours following inhalation of each compound. 10 and 30 μg PGE1-carbinol produced significant but short-acting bronchodilation, similar to that caused by 55 μg PGE2. One-third of the patients reported mild cough and throat irritation during and shortly after inhalation of 30 μg PGE1-carbinol or 55 μg PGE2. Placebo and 1 μg PGE1-carbinol produced minimal side effects, but neither agent caused bronchodilation. In an adjunctive, unblinded trial, the same patients received 400 μg fenoterol. Fenoterol caused greater bronchodilation 15 and 30 minutes after inhalation than did the PGs in the double-blind study.  相似文献   
96.
1. Die anscheinend in Vergessenheit geratenen älteren Angaben von HÄRDTL (1927 und spärer) und PRINGSHEIM (1931), nach denen einseitig belastete Blätter Gleichgewichtsbewegungen ausführen können (Isoklinotropismus nach HÄRDTL), wurden an mehreren Arten (Chelidonium majus, Aegopodium podagra ria, Ranunculus repens, Sambucus nigra, Coleus blumei und Hibiscus rosa sinensis) bestätigt,— Die einseitige Belastung erfolgte durch Einstecken von Nadeln in die eine Hälfte der Blattspreite bzw. in eine Blattfieder. 2. Bei Blättern, die in Spreite oder Stiel epinastisch gekrtimmt sind (die überwiegende Mehrzahl) oder auch durch die Belastung abwärts gebogen sind, kommt es dabei zu zwei verschiedenen Reaktionen: a) zu einer sofort eintretenden rein physikalischen reversiblen Reaktion, da die einseitige Last eine Torsion der Spreite bewirkt, die die Spreitenspitze nach der der Last gegenüberliegenden Seite [führt, und b) zu einer spärer eintretenden physiologischen Reaktion durch eine Wachstumskrümmung nach derselben Seite, - Beide Komponenten der Bewegung ftihren zu einer Verringerung oder einem Ausgleich des Ungleichgewichtes. 3. Die physiologische Reaktion war meist nach einem oder einigen Tagen erkennbar, bei Chelidonium mitunter schon nach einigen Stunden, bei Hibiscus gelegentlich erst nach etwa einer Woche. Mitunter blieb die Reaktion auch aus. Bei den gefiederten Blattern erfolgte sie in der Rhachis, ober- und unterhalb der belasteten Fieder, mitunter auch im Blattstiel, bei Coleus im Spreitengrund und im Blattstiel, bei Hibiscus im oberen Gelenkpolster (“Sekundargelenk”) des Blattstiels. Auch ältere Blätter reagierten oft noch überraschend gut. Vielfach führte die physiologische Reaktion zu einer vollständigen Ausbalancierung der einseitig belasteten Blattspreite. Auf nachtragliche Entfernung der eingesteckten Nadeln gingen auch die Krtimmungen in 2–3 Tagen wieder weitgehend zurück. 4. Als Ursache ftir die Gleichgewichtsbewegungen der Blätter kommen zwei verschiedene Mechanismen in Betracht: a) Infolge der durch die einseitige Belastung hervorgerufenen Schräglage der Blattspreite sammelt sich das Auxin auf der tiefer liegenden Flanke von Mittelnerv bzw. Rhachis und Blattstiel an, was zu einem stärkeren Wachstum dieser Seite und einer Gleichgewichtskrümmung führen muß. Nach dieser Auffassung ordnet sich der Isoklinotropismus dem Gravitropismus ein. b) Die nach der Belastung sofort eintretende auf der Torsionsspannung beruhende rein physikalische und zunächst reversible Gleichgewichtsreaktion des Blattes wird nach einigen Stunden oder Tagen teilweise irreversibel. (Für die spannungsfreien Abschnitte der Rhachis eines gefiederten Blattes oberhalb der Belastung kann diese Erklärung natürlich nicht gelten). Vermutlich sind beide Mechanismen, vor allern wohl der erstgenannte, bei den einzelnen Arten in verschiedenem Maße, als Ursache der Gleichgewichtsbewegungen wirksam. 5. Der biologische Sinn der Ausbalancierung eines (größeren) Blattes (PRINGSHELM 1931, HÄRDTL 1927, 1937 a) liegt darin, daß a) ein ausbalanciertes Blatt den geringsten Aufwand an mechanischen Elementen erfordert, und b) Photo- und Gravitropismus nur ein ausbalanciertes Blatt ohne Komplikationen in die angestrebte Lage fuhren konnen.  相似文献   
97.
Using hapten-reversible inhibition of plaque formation as an assay for auto-anti-idiotype antibody (anti-Id) and as a means for following idiotype (Id) expression, we have obtained evidence that following immunization with trinitrophenyl (TNP) conjugates (a) there are differences in Id expression in the anti-TNP antibody response to different TNP conjugates although there is some overlap; (b) different strains, although showing some differences in Id expression, tend to produce cross-reactive Ids, thus no obvious allotype linked inheritance of Id expression is observed in this heterogeneous immune response; (c) the auto-anti-Id produced following immunization with TNP-Brucella abortus or TNP-Ficoll tends to be of the IgG2a and IgG2b isotypes.  相似文献   
98.
This investigation examines the immunologic basis for specific antigen-induced tube leukocyte adherence inhibition (LAI) reactivity of draining lymph node cells (LNC) from dogs with canine transmissible venereal sarcoma (CTVS). CTVS regressor LNC, macrophage-depleted LNC, and enriched T lymphocyte fractions, but not enriched B lymphocyte fractions, were specifically reactive to CTVS antigen extract in direct tube LAI. In addition, regressor LNC amplified tube LAI responses by generating supernatants with leukocyte adherence inhibition factor (LAIF) activity for normal dog indicator LNC and enriched peripheral blood mononuclear cells (PBMC) in an indirect tube LAI assay. However, macrophage-depleted LNC and enriched T lymphocyte fractions failed to generate supernatants with LAIF activity, suggesting that macrophage accessory cells play a central role in the amplification of tube LAI. Interestingly, CTVS regressor peripheral blood leukocytes (PBL) and PBMC, which were specifically reactive in direct tube LAI, also failed to generate supernatants with LAIF activity. These findings demonstrate a distinction between LAIF-mediated amplification and direct tube LAI reactivity, and suggest that leukocyte populations with differing cellular proportions and from different immunologic compartments may participate in tube LAI via different mechanisms.  相似文献   
99.
Induction and expression of interleukin 2 (IL 2) receptor have been studied using a poly( Glu60 Ala30 Tyr10 ) (GAT)-specific T cell clone of mouse origin. This clone (52-3) has been characterized and it exhibits functional properties of T helper (TH) cells: it leads to a specific anti-DNP response in the presence of DNP-GAT and DNP-primed B cells and it secretes biological activities which can induce polyclonal B cell proliferation and IgM secretion. In vitro this clone mimics the activation stages of normal T lymphocytes and can be obtained under two states of differentiation. depending on the antigen-specific signal provided by antigen-presenting cells (APC). The expression of IL 2 receptor by this clone has been studied by (i) its response to IL 2, (ii) its capacity to absorb IL 2 bioactivity, and (iii) its reactivity with monoclonal antibody 7D4 specific for mouse IL 2 receptor. All the results indicate that the unstimulated state does not express the IL 2 receptor while the activated state does. Clone 52-3 has been compared with clone 14-1.6 that derives from a TH cell line and expresses the IL 2 receptor constitutively. 52-3 offers a good experimental model for studying in vitro, in a clonal TH cell population, the detailed mechanism of IL 2 receptor induction.  相似文献   
100.
The effects of glucagon on the respiratory function of mitochondria in situ were investigated in isolated perfused rat liver. Glucagon at the concentrations higher than 20 pM and cyclic AMP (75 microM) stimulated hepatic respiration, and shifted the redox state of pyridine nucleotide (NADH/NAD) in mitochondria in situ to a more reduced state as judged by organ fluorometry and beta-hydroxybutyrate/acetoacetate ratio. The organ spectrophotometric study revealed that glucagon and cyclic AMP induced the reduction of redox states of cytochromes a(a3), b and c+c1. Atractyloside (4 micrograms/ml) abolished the effects of glucagon on these parameters and gluconeogenesis from lactate. These observations suggest that glucagon increases the availability of substrates for mitochondrial respiration, and this alteration in mitochondrial function is crucial in enhancing gluconeogenesis.  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