全文获取类型
收费全文 | 3729篇 |
免费 | 77篇 |
国内免费 | 4篇 |
出版年
2013年 | 81篇 |
2012年 | 75篇 |
2011年 | 126篇 |
2010年 | 187篇 |
2009年 | 173篇 |
2008年 | 165篇 |
2007年 | 202篇 |
2006年 | 116篇 |
2005年 | 123篇 |
2004年 | 63篇 |
2003年 | 38篇 |
2002年 | 34篇 |
1999年 | 35篇 |
1998年 | 71篇 |
1997年 | 93篇 |
1996年 | 76篇 |
1995年 | 66篇 |
1994年 | 65篇 |
1993年 | 74篇 |
1992年 | 62篇 |
1991年 | 68篇 |
1990年 | 72篇 |
1989年 | 83篇 |
1988年 | 67篇 |
1987年 | 76篇 |
1986年 | 66篇 |
1985年 | 55篇 |
1984年 | 45篇 |
1983年 | 53篇 |
1982年 | 34篇 |
1981年 | 51篇 |
1980年 | 50篇 |
1979年 | 68篇 |
1978年 | 41篇 |
1977年 | 41篇 |
1976年 | 30篇 |
1975年 | 43篇 |
1974年 | 47篇 |
1973年 | 59篇 |
1972年 | 77篇 |
1971年 | 50篇 |
1959年 | 38篇 |
1958年 | 47篇 |
1957年 | 47篇 |
1956年 | 43篇 |
1955年 | 41篇 |
1954年 | 46篇 |
1953年 | 42篇 |
1952年 | 39篇 |
1951年 | 33篇 |
排序方式: 共有3810条查询结果,搜索用时 156 毫秒
1.
Wei CHEN Liqing PENG Lichun JIANG David A.PIKE Christopher R.FRIESEN Gregory BROWN 《亚洲两栖爬行动物研究(英文版)》2018,(1)
Environmental unpredictability can influence strategies of maternal investment among eggs within a clutch. Models predict that breeding females should adopt a diversified bet-hedging strategy in unpredictable environments, but empirical field evidence from Asia is scarce. Here we tested this hypothesis by exploring spatial patterns in egg size along an altitudinal gradient in a frog species(Rana kukunoris) inhabiting the Tibetan Plateau. Within-clutch variability in egg size increased as the environment became variable(e.g., lower mean monthly temperature and mean monthly rainfall at higher altitudes), and populations in environments with more unpredictable rainfall produced eggs that were smaller and more variable in size. We provide support for a diversified bet-hedging strategy in high-altitude environments, which experience dynamic weather patterns and therefore are of unpredictable environmental quality. This strategy may be an adaptive response to lower environmental quality and higher unpredictable environmental variance. Such a strategy should increase the likelihood of breeding success and maximize maternal lifetime fitness by producing offspring that are adapted to current environmental conditions. We speculate that in high-altitude environments prone to physical disturbance, breeding females are unable to consistently produce the optimal egg size due to physiological constraints imposed by environmental conditions(e.g., duration of the active season, food availability). Species and populations whose breeding strategies are adapted to cope with uncertain environmental conditions by adjusting offspring size and therefore quality show a remarkable degree of ability to cope with future climatic changes. 相似文献
2.
Drought-stressed flatpea (Lathyrus sylvestris L.) plants from8 to 22 weeks old were analysed for nitrogen, soluble proteinand free amino acids. An increase in nitrogen and free aminoacid concentrations and a decrease in soluble protein levelwere observed in roots of plants up to 16 weeks old. The cumulativeconcentration of free amino acids increased with drought stress.Tissue concentrations of 2, 4-diaminobutyric acid (1.62.6%of the dry weight) were highest in leaves. Levels increasedsteadily, nearly doubling, in leaves and stems between weeks10 and 16. Levels in drought-stressed leaves were, on average,11.9% higher than those of controls. Estimated concentrationsof a mixture of 4-aminobutyric acid and an unknown amino acidwere highest in stems, increased in this tissue with age andtended to increase in stems and leaves and decrease in rootsin response to water deficit. Levels of the mixture of homoserineand another unidentified amino acid were not influenced by ageor water status of the plants. Root concentrations of asparagine,arginine, glutamine, aspartate, and another prominent, unidentifiedamino acid increased with plant age and reached a peak at thetime of flowering (14 to 18 weeks). Only the concentration ofthe unknown compound was elevated following drought stress.Concentrations of valine, isoleucine, leucine, phenylalanine,and methionine also increased during this period and were elevatedin drought-stressed plants. Proline levels increased with plantage and drought stress, but proline accounted for only about10% of the total free amino acids in the drought-stressed plants. Key words: 2, 4-Diaminobutyric acid, drought, flatpea 相似文献
3.
4.
The presence of the internal parasite (Aphidius matricariae) of the aphid Myzus persicae can be identified by electrophoresis, and staining of several enzyme systems, of which malate dehydrogenase is recommended as the most reliable. It is suggested that the technique could be extended to other small insects, and that pest populations can be screened for percentage parasitism as an adjunct to insecticide and integrated control field trials. 相似文献
5.
6.
7.
T. R. FRASIER T. RASTOGI M. W. BROWN P. K. HAMILTON S. D. KRAUS B. N. WHITE 《Molecular ecology resources》2006,6(4):1025-1029
A North Atlantic right whale (Eubalaena glacialis) genomic library was developed and screened with a (GATA)8 probe to identify tetranucleotide microsatellite loci. Sixteen characterized loci were polymorphic in North Atlantic and/or South Atlantic (Eubalaena australis) right whales, 12 being polymorphic in E. glacialis, and 15 in E. australis. Fourteen of these were combined with 21 other previously identified loci for a suite of 35 loci which can be used to increase resolution of genetic analyses of these species. Multiplex reactions were developed for genotyping samples at these loci, providing a method that is rapid, reliable and cost‐effective. 相似文献
8.
9.
10.
Nicotine and nicotine salts are taken up by the roots of plants from solutions, and when 0.01–0.001 % nicotine is used the plants become toxic to Aphis fabae and to Pieris brassicae larvae and can be shown to contain nicotine. The results with Phaedon cochleariae adults and larvae are less satisfactory. No systemic action is observed when the nicotine is watered on to soil in which plants are growing and no nicotine can be detected in the plants. Apparently the nicotine is decomposed in the soil.
When applied several times to the upper surface of a bean leaf nicotine kills aphids on the underside. There is some evidence that nicotine can be translocated further through the plant following leaf applications, but the toxic action at any distance is very weak in the plants used in the present experiments and can only be produced by frequent applications of rather concentrated nicotine solutions. Leaf absorption and subsequent translocation has not been observed with nicotine salts.
The various organic bases, including some piperidine phosphonites and allied compounds tested, are of very little interest as contact or systemic insecticides against aphids. 相似文献
When applied several times to the upper surface of a bean leaf nicotine kills aphids on the underside. There is some evidence that nicotine can be translocated further through the plant following leaf applications, but the toxic action at any distance is very weak in the plants used in the present experiments and can only be produced by frequent applications of rather concentrated nicotine solutions. Leaf absorption and subsequent translocation has not been observed with nicotine salts.
The various organic bases, including some piperidine phosphonites and allied compounds tested, are of very little interest as contact or systemic insecticides against aphids. 相似文献