全文获取类型
收费全文 | 38953篇 |
免费 | 238篇 |
国内免费 | 289篇 |
专业分类
系统科学 | 699篇 |
丛书文集 | 833篇 |
教育与普及 | 117篇 |
理论与方法论 | 142篇 |
现状及发展 | 15138篇 |
研究方法 | 1320篇 |
综合类 | 20408篇 |
自然研究 | 823篇 |
出版年
2013年 | 421篇 |
2012年 | 797篇 |
2011年 | 1666篇 |
2010年 | 439篇 |
2009年 | 415篇 |
2008年 | 772篇 |
2007年 | 908篇 |
2006年 | 926篇 |
2005年 | 1030篇 |
2004年 | 1168篇 |
2003年 | 1075篇 |
2002年 | 696篇 |
2001年 | 1097篇 |
2000年 | 1217篇 |
1999年 | 755篇 |
1992年 | 602篇 |
1991年 | 538篇 |
1990年 | 570篇 |
1989年 | 507篇 |
1988年 | 529篇 |
1987年 | 508篇 |
1986年 | 499篇 |
1985年 | 648篇 |
1984年 | 532篇 |
1983年 | 430篇 |
1982年 | 373篇 |
1981年 | 374篇 |
1980年 | 491篇 |
1979年 | 1069篇 |
1978年 | 850篇 |
1977年 | 840篇 |
1976年 | 690篇 |
1975年 | 790篇 |
1974年 | 1031篇 |
1973年 | 901篇 |
1972年 | 896篇 |
1971年 | 1063篇 |
1970年 | 1339篇 |
1969年 | 1092篇 |
1968年 | 986篇 |
1967年 | 1009篇 |
1966年 | 928篇 |
1965年 | 693篇 |
1959年 | 385篇 |
1958年 | 578篇 |
1957年 | 443篇 |
1956年 | 377篇 |
1955年 | 372篇 |
1954年 | 372篇 |
1948年 | 244篇 |
排序方式: 共有10000条查询结果,搜索用时 734 毫秒
221.
W. F. Osswald J. P. Shapiro R. E. McDonald R. P. Niedz R. T. Mayer 《Cellular and molecular life sciences : CMLS》1993,49(10):888-892
Several acidic chitinase and chitosanase isoforms were found in 4-week-old nonembryogenic sweet orange (Valencia [Citrus sinensis (L.) Osbeck]) callus tissue. Two isoforms (designated A1-CF1 and A1-CF2) were purified to homogeneity using HPLC size exclusion, anion exchange, and chromatofocusing techniques. Both hydrolase isoforms exhibited activity with either colloidal chitin or solubilized shrimp shell chitosan. Specific activities for the purified isoforms could not be calculated because of the lack of protein and contamination of ampholytes. However, the specific activities for chitinase and chitosanase after anion exchange were respectively 404 nmol GlcNAc per min per mg protein and 2,475 nmol GlcN per min per mg protein. The Mr for both enzymes was 30,500. The homogeneous proteins cross-reacted in western blots with antiserum against a basic class I potato leaf chitinase. 相似文献
222.
M. Rizzardini A. Ferraroli D. Dal Fiume L. Cantoni 《Cellular and molecular life sciences : CMLS》1984,40(12):1390-1392
Summary Im male rats hepatic cytochromes b5 and P-450 were reduced at different times after treatment with cyclophosphamide (CP) (200 mg/kg i.p. for 3 days). In contrast, microsomal heme did not change until 48 h after the last dose of CP, leading to accumulation of heme in a non-cytochromal form. Parallel to the above changes the heme metabolism showed derangement: -aminolaevulinate synthase, the rate-limiting enzyme in heme synthesis, was depressed and heme oxygenase, the enzyme which catalyzes the oxidative degradation of heme, was increased. 相似文献
223.
224.
Summary Behavioral arousal and fright responses ofHalichoeres bivittatus occurred in aquaria to a moving shadow and a tap stimulus. Arousal was characterized by changes in the beat of pectoral fins, dorsal fin erection and eye movements, whereas in fright, adduction of pectoral and dorsal fins and rapid forward movement occurred. Serial stimulus presentation caused the fright response to be replaced by arousal which habituated in that the proportion of behavioral components exhibited decreased during the process.P.R. Laming was in receipt of a Royal Society travel grant for assistance in the expenses of this work, for which he is duly grateful. We are grateful to Nick Zinkowski, Culebra for the loan of minnow traps. 相似文献
225.
Summary In the rat, prolonged activation of 1-adrenoceptors causes a gain in salivary gland weight, whereas prolonged blockade of these receptors causes a reduction in weight.This work was supported by grants to J.E. from the Medical Faculty in Lund and the Swedish Medical Research Council (05927). 相似文献
226.
Summary The level of the acid hydrolases -glucuronidase (EC 3.2.3.11), acid phosphatase (EC 3.1.3.2) and acid protease (EC 3.4.4) was studied during larval growth and molting inP. ricini. The level of activity of these enzymes remained low during larval growth; however, the level increased sharply at the time of molting and declined sharply thereafter in the newly ecdysed insect. Interestingly, the diminished activity of these enzymes was almost quantitatively recovered in the cast-off cuticle. The excretion of acid hydrolases through the cast-off cuticle has hitherto not been reported in insects during molting. 相似文献
227.
228.
D. Lewinson M. Rosenberg M. R. Warburg 《Cellular and molecular life sciences : CMLS》1984,40(9):956-958
Summary Two types of mitochondria-rich cells (MRC) are described ultrastructurally in the gill epithelium of salamander larva. They resemble MRC found in larval ventral epidermis. Histochemical localization of carbonic anhydrase indicated numerous positive reacting cells, most of them flask-shaped. Morphological and functional similarities to fish chloride cells are discussed. 相似文献
229.
The cold dark interstellar Taurus Molecular Cloud One (TMC-1) is a rich source of acetylenic and polyacetylenic molecular species. As well as linear closed-shell molecules (H(C triple bond C)nCN) and symmetric rotors (CH3C triple bond CH, CH3C triple bond CCN), several radicals (C triple bond CH, C triple bond CCN, (C triple bond C2H) have also been identified, many of which had not been studied previously in the laboratory. Whether the observed abundances can be understood in terms of purely gas-phase ion-molecule chemical schemes, which produce reasonable agreement for the simplest polyatomic species, is unclear; alternative models involving the particulate interstellar grains as catalysts or sources have also been suggested. We now report the detection in TMC-1 of a new molecule, tricarbon monoxide (C3O), whose pure rotational spectrum has only very recently been studied in the laboratory. As C3O is the first known interstellar carbon chain molecule to contain oxygen, its existence places an important new constraint on chemical schemes for cold interstellar clouds. In fact, the observed abundance of tricarbon monoxide fits quite well into our model of galactochemistry. 相似文献
230.
Summary Resting oxygen consumption of the vascularly isolated gracilis anticus muscle of the rat perfused by natural circulation via the femoral artery with diluted or undiluted blood depends on oxygen delivery (the product of flow rate and arterial oxygen concentration-DO2) only when DO2 falls below 16 l/min/g. 相似文献