全文获取类型
收费全文 | 26706篇 |
免费 | 46篇 |
国内免费 | 103篇 |
专业分类
系统科学 | 152篇 |
丛书文集 | 363篇 |
教育与普及 | 66篇 |
理论与方法论 | 103篇 |
现状及发展 | 11767篇 |
研究方法 | 1211篇 |
综合类 | 12790篇 |
自然研究 | 403篇 |
出版年
2013年 | 196篇 |
2012年 | 421篇 |
2011年 | 821篇 |
2010年 | 167篇 |
2009年 | 146篇 |
2008年 | 456篇 |
2007年 | 533篇 |
2006年 | 519篇 |
2005年 | 531篇 |
2004年 | 523篇 |
2003年 | 487篇 |
2002年 | 470篇 |
2001年 | 831篇 |
2000年 | 785篇 |
1999年 | 563篇 |
1992年 | 485篇 |
1991年 | 398篇 |
1990年 | 434篇 |
1989年 | 401篇 |
1988年 | 403篇 |
1987年 | 438篇 |
1986年 | 404篇 |
1985年 | 513篇 |
1984年 | 407篇 |
1983年 | 347篇 |
1982年 | 316篇 |
1981年 | 308篇 |
1980年 | 390篇 |
1979年 | 874篇 |
1978年 | 669篇 |
1977年 | 663篇 |
1976年 | 525篇 |
1975年 | 599篇 |
1974年 | 772篇 |
1973年 | 704篇 |
1972年 | 721篇 |
1971年 | 820篇 |
1970年 | 1051篇 |
1969年 | 797篇 |
1968年 | 727篇 |
1967年 | 787篇 |
1966年 | 671篇 |
1965年 | 454篇 |
1959年 | 253篇 |
1958年 | 450篇 |
1957年 | 330篇 |
1956年 | 284篇 |
1955年 | 237篇 |
1954年 | 219篇 |
1948年 | 196篇 |
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
121.
J. L. Lacuara S. R. de Barioglio P. P. de Oliva A. S. Bernacchi A. F. de Culasso J. A. Castro B. M. Franke de Cazzulo J. J. Cazzulo 《Cellular and molecular life sciences : CMLS》1991,47(6):612-616
Summary The tricyclic anti-calmodulin drug trifluoperazine (TFP) inhibited growth and motility of epimastigotes ofTrypanosoma cruzi, at concentrations lower than 100 M, and motility and infectivity of the bloodstream trypomastigote form at 200 M. Electron microscopy of TFP-treated epimastigotes showed that the major effect was at the mitochondrial level, with gross swelling and disorganization. The oligomycin-sensitive, mitochondrial ATPase was completely inhibited by 20 M TFP, and the same drug concentration caused a 60% decrease in intracellular ATP content. The results suggest that the trypanocidal effect of TFP may be related more to mitochondrial damage than to the well-known anticalmodulin effect of the drug. 相似文献
122.
The endangered Kemp's ridley sea turtle (Lepidochelys kempi) nests almost exclusively at a single locality in the western Gulf of Mexico, whereas the olive ridley (L. olivacea) nests globally in warm oceans. Morphological similarities between kempi and olivacea, and a geographical distribution that "...makes no sense at all under modern conditions of climate and geography", raise questions about the degree of evolutionary divergence between these taxa. Analysis of mitochondrial (mt) DNA restriction sites shows that Kemp's ridley is distinct from the olive ridley in matriarchal phylogeny, and that the two are sister taxa with respect to other marine turtles. Separation of olive and the Kemp's ridley lineages may date to formation of the Isthmus of Panama, whereas the global spread of the olive ridley lineage occurred recently. In contrast to recent examples in which molecular genetic assessments challenged systematic assignments underlying conservation programmes, our mtDNA data corroborate the taxonomy of an endangered form. 相似文献
123.
Effects of a change in the level of inbreeding on the genetic load 总被引:10,自引:0,他引:10
"The effects of inbreeding may not be as noticeable in the first generation as the invigoration immediately apparent after crossing". This statement, published in 1919, has received little attention, and has apparently never been tested empirically, although the reduction of the genetic load of populations by inbreeding is well known in theoretical terms. Because inbreeding increases homozygosity, and hence the effectiveness of selection against recessive or partially recessive detrimental alleles, changes in levels of inbreeding can lead to a reduction in the frequencies of such mutant alleles. This results in equilibration at higher population mean fitness and is referred to as 'purging' populations of their genetic load. Severe inbreeding can also reduce genetic load due to overdominant alleles, provided selection coefficients are not symmetrical at all loci, because alleles giving lower fitness will be reduced in frequency at equilibrium. With either fitness model, however, reduction in genetic load takes time, and the initial effect of an increase in inbreeding is reduced fitness due to homozygosity. There are few data relating to the extent to which fitness is reduced during inbreeding in a set of lines and to how long the reduction lasts before increasing again to the initial level, or higher. Inbreeding experiments involving sib mating in mice and Drosophila subobscura, and successive bottlenecks in house flies have yielded some evidence consistent with the purging hypothesis. Here, we report results of an experiment demonstrating a prolonged time-course of recovery of mean fitness under self-fertilization of a naturally outcrossing plant, and also compare our results with expectations derived by computer calculations. Our results show that the genetic load present in an outcrossing population can be explained only with a high mutation rate to partially recessive deleterious alleles, and that inbreeding purges the population of mutant alleles. 相似文献
124.
125.
126.
Summary Normal human spermatozoa were demonstrated by dot immunoblot analysis and immunohistochemistry to possess transglutaminase (TGase). The immunological identification of spermatozoal TGase is consistent with reports by others of its biochemical identification and suggested role in sperm motility,and provides, in view of the immunoregulatory properties of seminal plasma TGase, presumptive identification of a means whereby spermatozoa, under normal physiological conditions, may possibly be protected from immunological attack within the female reproductive tract. 相似文献
127.
Summary The in vitro absorption of calcium from the duodenum was significantly less in a group of rats fed on a wheat diet than in a group fed a wheat and Bengal gram (7030) diet. 相似文献
128.
129.
A. Alkofahi J. K. Rupprecht Y. -M. Liu C. -J. Chang D. L. Smith J. L. McLaughlin 《Cellular and molecular life sciences : CMLS》1990,46(5):539-541
Summary Gigantecin (I), a novel tetrahydroxy-di-tetrahydrofuran fatty acid -lactone (acetogenin), was isolated from an ethanolic extract of the stem bark ofGoniothalamus giganteus Hook. f., Thomas (Annonaceae), by means of activity-directed fractionation (brine shrimp lethality test). This new compound is extremely cytotoxic to human tumor cells, inhibits crown gall tumors on potato discs, and is active in an assay designed to detect antimitotic agents (9 ASK). 相似文献
130.
Summary Metabolism of 26-hydroxycholesterol to 3-hydroxychol-5-en-24-oic acid and other C24-bile acids has been expected to occur by way of 3-hydroxycholest-5-en-26-oic acid in studies in vitro. 3-Hydroxycholest-5-en-26-oic acid was infused intravenously into bile fistula hamsters and the following C24-bile acids were identified: 3-hydroxychol-5-en-24-oic acid, lithocholic acid, chenodeoxycholic acid, and a small amount of cholic acid. 相似文献