全文获取类型
收费全文 | 30687篇 |
免费 | 69篇 |
国内免费 | 107篇 |
专业分类
系统科学 | 117篇 |
丛书文集 | 419篇 |
教育与普及 | 86篇 |
理论与方法论 | 115篇 |
现状及发展 | 14540篇 |
研究方法 | 1102篇 |
综合类 | 14171篇 |
自然研究 | 313篇 |
出版年
2013年 | 148篇 |
2012年 | 335篇 |
2011年 | 631篇 |
2008年 | 428篇 |
2007年 | 468篇 |
2006年 | 483篇 |
2005年 | 491篇 |
2004年 | 607篇 |
2003年 | 493篇 |
2002年 | 478篇 |
2001年 | 802篇 |
2000年 | 820篇 |
1999年 | 551篇 |
1994年 | 366篇 |
1992年 | 493篇 |
1991年 | 401篇 |
1990年 | 460篇 |
1989年 | 427篇 |
1988年 | 415篇 |
1987年 | 463篇 |
1986年 | 512篇 |
1985年 | 627篇 |
1984年 | 420篇 |
1983年 | 434篇 |
1982年 | 376篇 |
1981年 | 391篇 |
1980年 | 413篇 |
1979年 | 983篇 |
1978年 | 820篇 |
1977年 | 759篇 |
1976年 | 597篇 |
1975年 | 717篇 |
1974年 | 953篇 |
1973年 | 804篇 |
1972年 | 838篇 |
1971年 | 1046篇 |
1970年 | 1231篇 |
1969年 | 992篇 |
1968年 | 1005篇 |
1967年 | 927篇 |
1966年 | 786篇 |
1965年 | 613篇 |
1964年 | 229篇 |
1959年 | 340篇 |
1958年 | 600篇 |
1957年 | 454篇 |
1956年 | 371篇 |
1955年 | 362篇 |
1954年 | 366篇 |
1948年 | 273篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
New phytotoxic butenolides produced bySeiridium cardinale,the pathogen of cypress canker disease 总被引:1,自引:0,他引:1
L. Sparapano A. Evidente A. Ballio A. Graniti G. Randazzo 《Cellular and molecular life sciences : CMLS》1986,42(6):627-628
Summary Two new butenolides, seiridin andiso-seiridin, were isolated from culture filtrates ofSeiridium cardinale, the pathogen of cypress canker, a destructive disease ofCupressus and relatedConiferae These metabolites were characterized as 3-methyl-4-(2-hydroxyheptyl)-2(5H)-furanone and its 4-(3-hydroxyheptyl) isomer, respectively. Chlorotic, and necrotic symptoms were produced on leaves of either host or non-host test plants by absorption of 0.3 mg/ml solutions of either compound. These also showed antibacterial activity.This study was supported in part by the National Research Council, Rome (special ad hoc project Chimica fine e secondaria) and in part by the Ministry of Education, Rome. 相似文献
102.
R. S. Verma S. Thomas M. Coleman R. T. Silver H. Dosik 《Cellular and molecular life sciences : CMLS》1986,42(4):440-441
Summary A random distribution of the Y-chromosome at somatic metaphase was found in 50 patients with Ph' positive chronic myelogenous leukemia (CML). Thus, it is concluded that the positive of the Y-chromosome at somatic metaphase does not appear to influence the loss from bone marrow cells. 相似文献
103.
Summary Glycosidases like sialidase,-galactosidase, -L-fucosidase, N-acetyl hexosaminidase and proteases were detected in toad testis. Neuraminic acid aldolase activity was also detected. The enzyme activities were found to vary as production of spermatozoa varied. All enzymes, except N-acetyl glucosaminidase, were shown to decrease after injection of toad pituitary extract and they were also found to be absent from testis containing no spermatozoa. The glycosidases were found to act on toad oviduct jelly and they may therefore be involved in the degradation of the jelly after fertilization, into smaller bits, which may be utilized as nutrients by the fertilized zygote.Acknowledgment. We thank Prof. T.R. Ramaiah, Head of the Department of Biochemistry, University of Mysore, for his help. We also acknowledge the financial assistance of University Grants Commission to one of us (MS) and CSIR through a grant No. 9 (165)83/EMR-II to HSS. Please address all correspondence to H.S. Seshadri. 相似文献
104.
Antidiuretic effects of oxytocin in the Brattleboro rat 总被引:1,自引:0,他引:1
J. Lyness A. G. Robinson M. N. Sheridan D. M. Gash 《Cellular and molecular life sciences : CMLS》1985,41(11):1444-1446
Summary The antidiuretic activity of oxytocin (OT) was measured in Brattleboro rats with congenital diabetes insipidus. A dose dependent antidiuretic response was found in animals receiving chronic infusions of 0.1 g/h, 1.0 g/h, and 5 g/h of OT. OT infused at the rate of 5 g/h over a 7-day period completely reversed the symptoms of diabetes insipidus. The results support the concept that OT serves as a weak agonist of vasopressin at the level of the kidney and at pharmacological levels exhibits antidiuretic activity. 相似文献
105.
J. Lukanov E. Milieva G. Russev B. Trifonov 《Cellular and molecular life sciences : CMLS》1985,41(1):68-70
Summary Follow-up investigation of the blood sera from preparturient women and women with habitual, abortions showed the presence of a factor which has an activating effect on smooth muscle preparations because it causes the release of prostaglandins. Gel-chromatographic counter flow separation and microelectrophoresis of the blood sera have shown that the isolated serum factor is a water soluble glycopeptide with a molecular weight of about 2000. 相似文献
106.
On some significance tests in cluster analysis 总被引:1,自引:1,他引:0
Bock H.H. 《Journal of Classification》1985,2(1):77-108
We investigate the properties of several significance tests for distinguishing between the hypothesisH of a homogeneous population and an alternativeA involving clustering or heterogeneity, with emphasis on the case of multidimensional observationsx
1, ...,x
n
p
. Four types of test statistics are considered: the (s-th) largest gap between observations, their mean distance (or similarity), the minimum within-cluster sum of squares resulting from a k-means algorithm, and the resulting maximum F statistic. The asymptotic distributions underH are given forn and the asymptotic power of the tests is derived for neighboring alternatives. 相似文献
107.
R. K. A. Giger H. R. Loosli M. D. Walkinshaw B. J. Clark J. M. Vigouret 《Cellular and molecular life sciences : CMLS》1987,43(10):1125-1130
Summary We report the synthesis, stereochemistry and preliminary pharmacological evaluation of DCN 203-922, a novel ergot alkaloid of the cyclol type, which contains in its peptide moiety the uncommon amino acid L-allo-isoleucine.Part of this paper was reported by this author at the Herbstversammlung der Schweizerischen Chemischen Gesellschaft, Bern, in October 1986. 相似文献
108.
The dry valleys of southern Victoria Land in Antarctica contain several closed basins in which perennially ice-covered lakes are found. One of the most unusual features of these lakes is the occurrence of high O2 concentrations in the water column; values ranging from slightly more than saturation to more than four times saturation have been reported. Recently, we considered a bulk O2 budget for Lake Hoare, Antarctica, which led us to suggest that biological processes alone were not sufficient to explain the observed elevated oxygen levels. Consequently, there must be a non-biological source of O2. We suggested that this source results from the exclusion of O2 during the freezing of aerated meltstream water at the bottom of the ice cover, and predicted that this physical mechanism should also enhance the other atmospheric gases. Here we report the results of a study which, for the first time, documents the supersaturation of N2 in a lake. Dissolved N2 levels of 145% and 163% were determined from samples taken just below the ice cover and at a depth of 12 m, respectively. The relatively importance of biological and abiological sources is reflected in the ratio of N2 concentration to O2 concentration. In Lake Hoare this ratio was 1.20 at ice/water interface and 1.05 at 12 m; considerably different from the ratio in equilibrium with air (approximately 1.8). Based on these results, we have determined that about half of the net O2 production in the lake is the result of biological processes. 相似文献
109.
Ross P Weinhouse H Aloni Y Michaeli D Weinberger-Ohana P Mayer R Braun S de Vroom E van der Marel GA van Boom JH Benziman M 《Nature》1987,325(6101):279-281
Cellulose is the most abundant renewable carbon resource on earth and is an indispensable raw material for the wood, paper, and textile industries. A model system to study the mechanism of cellulose biogenesis is the bacterium Acetobacter xylinum which produces pure cellulose as an extracellular product. It was from this organism that in vitro preparations which possessed high levels of cellulose synthase activity were first obtained in both membranous and soluble forms. We recently demonstrated that this activity is subject to a complex multi-component regulatory system, in which the synthase is directly affected by an unusual cyclic nucleotide activator enzymatically formed from GTP, and indirectly by a Ca (2+) -sensitive phosphodiesterase which degrades the activator. The cellulose synthase activator (CSA) has now been identified as bis-(3' 5')-cyclic diguanylic acid (5'G3'p5'G3'p) on the basis of mass spectroscopic data, nuclear magnetic resonance analysis and comparison with chemically synthesized material. We also report here on intermediary steps in the synthesis and degradation of this novel circular dinucleotide, which have been integrated into a model for the regulation of cellulose synthesis. 相似文献
110.
Chromosomal localization of human haemoglobin structural genes 总被引:15,自引:0,他引:15