全文获取类型
收费全文 | 967660篇 |
免费 | 102954篇 |
国内免费 | 800篇 |
专业分类
1071414篇 |
出版年
2018年 | 9252篇 |
2017年 | 8739篇 |
2016年 | 12559篇 |
2015年 | 16687篇 |
2014年 | 19647篇 |
2013年 | 28000篇 |
2012年 | 31143篇 |
2011年 | 31858篇 |
2010年 | 21705篇 |
2009年 | 19766篇 |
2008年 | 28232篇 |
2007年 | 28901篇 |
2006年 | 27288篇 |
2005年 | 26291篇 |
2004年 | 26021篇 |
2003年 | 25108篇 |
2002年 | 24323篇 |
2001年 | 44824篇 |
2000年 | 45113篇 |
1999年 | 35672篇 |
1998年 | 12379篇 |
1997年 | 12922篇 |
1996年 | 12168篇 |
1995年 | 11368篇 |
1994年 | 11079篇 |
1993年 | 10787篇 |
1992年 | 29058篇 |
1991年 | 28241篇 |
1990年 | 27705篇 |
1989年 | 27069篇 |
1988年 | 25114篇 |
1987年 | 23609篇 |
1986年 | 21736篇 |
1985年 | 21870篇 |
1984年 | 18057篇 |
1983年 | 15158篇 |
1982年 | 11340篇 |
1981年 | 10254篇 |
1980年 | 9594篇 |
1979年 | 16372篇 |
1978年 | 12680篇 |
1977年 | 11501篇 |
1976年 | 10581篇 |
1975年 | 11894篇 |
1974年 | 12652篇 |
1973年 | 12613篇 |
1972年 | 11189篇 |
1971年 | 10326篇 |
1970年 | 8906篇 |
1969年 | 8452篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
Studies on larval population densities and adult emergence rates of the Brugian filariasis vectors Mansonia bonneae Edwards and Ma. dives Schiner were conducted in freshwater swamp forest bordering the Sadong River, Serian District, Sarawak, East Malaysia, during 1984-85. Three species of aquatic host-plants in the Family Araceae were identified as supporting immature stages of the Ma. bonneae/dives complex. Proportions of positive plants were 4.7%, 6.5% and 3.4% with 6.4 +/- 2.6, 7.3 +/- 2.8 and 10.1 +/- 1.1 larvae per positive plant, respectively, for the plant species Homalomena cordata Schott, H. rostrata Griffiths and Hydrostemma motleyi (Hook. f.) Mabberley. These data indicate no significant preferences between the three types of host-plant. Detailed monitoring of the host-plant H. cordata revealed no significant monthly fluctuations in larval density per plant nor the proportion of positive plants. 11.6% of larvae were Ma. dives and 88.4% were Ma. bonneae. Mean daily yields of Ma. bonneae/dives adults per square metre of H. cordata vegetated water surface were 0.45 males plus 0.57 females during the wet season (December-February) compared with 0.2 males plus 0.31 females during the dry season (June-August). Thus output of adults per plant was approximately halved, and suitable breeding areas were further reduced, during the dry season. By extrapolation from these rates, a crude mean estimate for productivity of Ma. bonneae/dives females is 1.6 million per hectare per annum in swamp forest habitats vegetated with any of the host-plants studied. 相似文献
982.
MATING FREQUENCY AND FECUNDITY IN INSECTS 总被引:8,自引:0,他引:8
M. RIDLEY 《Biological reviews of the Cambridge Philosophical Society》1988,63(4):509-549
1. The paper summarizes the published evidence on the relation between mating frequency and fecundity in insects. There is experimental evidence of varying quality for 63 species and non-experimental evidence for about 60.
2. Repeated mating may be universally necessary for full fecundity and fertility in female insects (in species in which the females normally mate more than once).
3. The evidence is remarkably poor. We need more properly designed experiments (and not just observations of natural variation), with sufficient sample sizes and statistics, and measurements of the fecundities and fertilities of singly and multiply mated females, when the multiple matings are separated by many days or weeks. Most of the existing experiments of this sort are defective in some way.
4. In species with greater total fecundity and longevity, multiple mating may be more likely to enhance fertility than in species with small fecundity and short life span.
5. Females in naturally monandrous species do not show increased fecundity or fertility with repeated mating, whereas females of polyandrous species do.
6. There is no obvious connexion between paternal investment, in so far as we know about it, and the increase of fecundity by repeated mating.
7. There is a small tendency for females to breed more quickly and be shorter lived if they mate repeatedly. 相似文献
2. Repeated mating may be universally necessary for full fecundity and fertility in female insects (in species in which the females normally mate more than once).
3. The evidence is remarkably poor. We need more properly designed experiments (and not just observations of natural variation), with sufficient sample sizes and statistics, and measurements of the fecundities and fertilities of singly and multiply mated females, when the multiple matings are separated by many days or weeks. Most of the existing experiments of this sort are defective in some way.
4. In species with greater total fecundity and longevity, multiple mating may be more likely to enhance fertility than in species with small fecundity and short life span.
5. Females in naturally monandrous species do not show increased fecundity or fertility with repeated mating, whereas females of polyandrous species do.
6. There is no obvious connexion between paternal investment, in so far as we know about it, and the increase of fecundity by repeated mating.
7. There is a small tendency for females to breed more quickly and be shorter lived if they mate repeatedly. 相似文献
983.
984.
Changes in the heat resistance of Salmonella typhimurium during heating at rising temperatures 总被引:12,自引:8,他引:4
The heat resistance of Salmonella typhimurium , measured as survival following a standard heat challenge at 55°C for 25 min, increased progressively as cells were heated up at linearly rising temperatures. The amount by which heat resistance increased depended on the rate of temperature rise; the slower the temperature rise, the greater the increase in resistance. 相似文献
985.
The visible and proton NMR spectral responses of imidazole methemoglobin by the binding of inositol hexaphosphate were examined in the 2-40 degrees C range. The magnitude of the +/- (inositol hexaphosphate) visible difference spectrum increased and the intensity of the 33 ppm NMR peak decreased with lowering of the temperature. The NMR results were quantitatively analyzed with a simple two-state allosteric model. The results show that the T conformer fraction is 0.6 at 20 degrees C and that the equilibrium shifts toward the T state at lower temperature. The large changes in delta H and delta S associated with the equilibrium suggest participation of numerous factors in the determination of the equilibrium position. The increase in the T conformer population of imidazole methemoglobin, which is pure low-spin, suggests that the appearance of the T state with decreasing temperature is not directly coupled to an increase in spin of the heme iron. 相似文献
986.
B M Austen C J Evans D G Smyth 《Biochemical and biophysical research communications》1979,91(4):1211-1217
Peptide fragments derived from the NH2-terminus of corticotropin were found to exhibit widely differing degrees of stability to degradation by aminopeptidase M. Corticotropin itself was 135 times more stable than its NH2-terminal pentapeptide, and similar differences in stability were observed with peptides derived from the B-chain of bovine insulin. Enkephalin linked covalently to the A-chain of bovine insulin was at least 100 times more stable than the pentapeptide. The results demonstrate that the molecular size of a peptide is one factor that determines its NH2-terminal stability. 相似文献
987.
988.
Biochemical, biological, and immunological properties of chemically deglycosylated human choriogonadotropin 总被引:5,自引:0,他引:5
A chemical method of deglycosylation of human choriogonadotropin (hCG) was used to assess the role of carbohydrate moiety in the maintenance of quaternary structure and functional parameters such as receptor binding, immunological activity, and in vitro biological response. Treatment of purified hCG with anhydrous HF at 0 degrees C for 60 min was effective in removing more than 75% of the carbohydrate moiety. This extent of deglycosylation altered its chromatographic characteristics as revealed by retarded behavior on Sephadex G-100 and failure to be retained on concanavalin A-Sepharose. The electrophoretic heterogeneity present in native hCG was markedly reduced by deglycosylation. The deglycosylated hCG was stable in the lyophilized form and retained its quaternary structure as revealed by the fluorescence probe 8-anilino 1-naphthalene sulfonic acid, receptor binding, and immunological activities. Unlike receptor binding and immunological activities, which were fully retained, the ability of the hormone to stimulate cyclic AMP accumulation in vitro in rat interstitial cells was completely abolished. 相似文献
989.
990.