全文获取类型
收费全文 | 30276篇 |
免费 | 1894篇 |
国内免费 | 3057篇 |
出版年
2023年 | 435篇 |
2022年 | 564篇 |
2021年 | 681篇 |
2020年 | 770篇 |
2019年 | 1006篇 |
2018年 | 921篇 |
2017年 | 743篇 |
2016年 | 701篇 |
2015年 | 789篇 |
2014年 | 1414篇 |
2013年 | 2010篇 |
2012年 | 1107篇 |
2011年 | 1338篇 |
2010年 | 1070篇 |
2009年 | 1329篇 |
2008年 | 1383篇 |
2007年 | 1490篇 |
2006年 | 1364篇 |
2005年 | 1187篇 |
2004年 | 1068篇 |
2003年 | 1015篇 |
2002年 | 972篇 |
2001年 | 804篇 |
2000年 | 725篇 |
1999年 | 698篇 |
1998年 | 657篇 |
1997年 | 573篇 |
1996年 | 593篇 |
1995年 | 548篇 |
1994年 | 506篇 |
1993年 | 464篇 |
1992年 | 464篇 |
1991年 | 491篇 |
1990年 | 432篇 |
1989年 | 326篇 |
1988年 | 375篇 |
1987年 | 302篇 |
1986年 | 299篇 |
1985年 | 392篇 |
1984年 | 456篇 |
1983年 | 302篇 |
1982年 | 388篇 |
1981年 | 324篇 |
1980年 | 323篇 |
1979年 | 280篇 |
1978年 | 229篇 |
1977年 | 207篇 |
1976年 | 152篇 |
1974年 | 116篇 |
1973年 | 119篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
A review of in vitro mutagenesis assessment of metal compounds in mammalian and nonmammalian test systems has been compiled.
Prokaryotic assays are ineffective or inconsistent in their detection of most metals as mutagens, with the notable exception
of hexavalent chromium. Mammalian assay systems appear to be similarly inappropriate for the screening of metal compounds
based upon the limited number of studies that have employed those compounds having known carcinogenic activity. Although of
limited value as screening tests for the detection of potentially carcinogenic metal compounds, the well-characterized in
vitro mutagenesis systems may prove to be of significant value as a means to elucidate mechanisms of metal genotoxicity. 相似文献
22.
Ice crystal formation temperature was determined in the region of the crown in one group of 7-day-old intact unhardened high-salt plants of winter wheat (Triticum aestivum L. cv. Weibulls Starke II) with TA (Thermal Analysis) and DTA (Differential Thermal Analysis) methods. After exposure of another group of plants, grown for the first 7 days in the same way as the first group, to various sub-zero temperatures (-1 to 5°C), influx in roots of Rb+(86Rb+) and Ca2+(45Ca2+) and contents of K+ and Ca2+ were determined at intervals during 7 days of recovery. Ice crystal formation in the crown tissue was probably extracellular and took place at about -4°C. There was a large loss of K+ from the roots after treatment at sub-zero temperatures. This loss increased as the temperature of the sub-zero treatment decreased. During recovery, roots of plants exposed to -1, -2 and -3°C gradually reabsorbed K+. Reabsorption of K+ in roots of plants exposed to -4°C was greatly impaired. Rb+ influx decreased and Ca2+ influx increased after sub-zero temperature treatments of the plants. Active Rb+ influx mechanisms and active extrusion of Ca2+ were impaired or irreversibly damaged by the exposure. While Rb+ influx mechanisms were apparently repaired during recovery in plants exposed to temperatures down to -3°C, Ca2+ extrusion mechanisms were not. The temperature for ice crystal formation in the region of the crown tissue coincides with the temperature at which the plants lost the ability to reabsorb K+ and to repair Rb+ influx mechanisms during the recovery period. Plants were lethally damaged at temperatures below ?4°C. 相似文献
23.
Alain Dejean Arthur Compin Jacques H. C. Delabie Frdric Azmar Bruno Corbara Maurice Leponce 《Ecological Entomology》2019,44(4):560-570
1. Ants are widespread in tropical rainforests, including in the canopy where territorially dominant arboreal species represent the main part of the arthropod biomass. 2. By mapping the territories of dominant arboreal ant species and using a null model analysis and a pairwise approach this study was able to show the presence of an ant mosaic on the upper canopy of a primary Neotropical rainforest (c. 1 ha sampled; 157 tall trees from 28 families). Although Neotropical rainforest canopies are frequently irregular, with tree crowns at different heights breaking the continuity of the territories of dominant ants, the latter are preserved via underground galleries or trails laid on the ground. 3. The distribution of the trees influences the structure of the ant mosaic, something related to the attractiveness of tree taxa for certain arboreal ant species rather than others. 4. Small‐scale natural disturbances, most likely strong winds in the area studied (presence of canopy gaps), play a role by favouring the presence of two ant species typical of secondary formations: Camponotus femoratus and Crematogaster levior, which live in parabiosis (i.e. share territories and nests but lodge in different cavities) and build conspicuous ant gardens. In addition, pioneer Cecropia myrmecophytic trees were recorded. 相似文献
24.
25.
26.
27.
28.
F. H. Bronson 《Philosophical transactions of the Royal Society of London. Series B, Biological sciences》2009,364(1534):3331-3340
Seasonal reproduction is common among mammals at all latitudes, even in the deep tropics. This paper (i) discusses the neuroendocrine pathways via which foraging conditions and predictive cues such as photoperiod enforce seasonality, (ii) considers the kinds of seasonal challenges mammals actually face in natural habitats, and (iii) uses the information thus generated to suggest how seasonal reproduction might be influenced by global climate change. Food availability and ambient temperature determine energy balance, and variation in energy balance is the ultimate cause of seasonal breeding in all mammals and the proximate cause in many. Photoperiodic cueing is common among long-lived mammals from the highest latitudes down to the mid-tropics. It is much less common in shorter lived mammals at all latitudes. An unknown predictive cue triggers reproduction in some desert and dry grassland species when it rains. The available information suggests that as our climate changes the small rodents of the world may adapt rather easily but the longer lived mammals whose reproduction is regulated by photoperiod may not do so well. A major gap in our knowledge concerns the tropics; that is where most species live and where we have the least understanding of how reproduction is regulated by environmental factors. 相似文献
29.
D. S. Coffman Ben H. Leichtling H. V. Rickenberg 《Journal of cellular biochemistry》1981,15(4):369-385
The phosphoproteins of Dictyostelium discoideum were compared at different stages of development by polyacrylamide gel electrophoresis. Certain phosphoproteins of vegetative amoebae were conserved while others appeared and disappeared during development. Four major phosphoproteins with apparent subunit molecular weights of 50,000, 47,000, 38,000, and 34,000 disappeared precociously in response to exogenous cAMP. Two membranal phosphoproteins, with apparent subunit molecular weights of 80,000 and 81,000, appeared precociously in response to added cAMP. One of these phosphoproteins, molecular weight of 80,000, has been identified tentatively as the “contact site A” glycoprotein. Another membranal protein, with apparent subunit molecular weight of 42,000, unaffected in its appearance by cAMP, has been identified tentatively as phosphoactin. 相似文献
30.