全文获取类型
收费全文 | 1149篇 |
免费 | 53篇 |
国内免费 | 3篇 |
出版年
2013年 | 15篇 |
2012年 | 16篇 |
2011年 | 23篇 |
2010年 | 51篇 |
2009年 | 38篇 |
2008年 | 31篇 |
2007年 | 43篇 |
2006年 | 33篇 |
2005年 | 28篇 |
2004年 | 12篇 |
1999年 | 12篇 |
1998年 | 13篇 |
1997年 | 23篇 |
1996年 | 18篇 |
1995年 | 11篇 |
1994年 | 17篇 |
1993年 | 18篇 |
1992年 | 14篇 |
1991年 | 22篇 |
1990年 | 22篇 |
1989年 | 25篇 |
1988年 | 17篇 |
1987年 | 15篇 |
1985年 | 13篇 |
1984年 | 11篇 |
1983年 | 10篇 |
1982年 | 13篇 |
1980年 | 16篇 |
1979年 | 12篇 |
1978年 | 18篇 |
1977年 | 18篇 |
1976年 | 16篇 |
1975年 | 11篇 |
1974年 | 10篇 |
1972年 | 16篇 |
1971年 | 17篇 |
1969年 | 9篇 |
1968年 | 9篇 |
1959年 | 24篇 |
1958年 | 24篇 |
1957年 | 31篇 |
1956年 | 27篇 |
1955年 | 33篇 |
1954年 | 37篇 |
1953年 | 40篇 |
1952年 | 30篇 |
1951年 | 26篇 |
1950年 | 26篇 |
1949年 | 23篇 |
1948年 | 18篇 |
排序方式: 共有1205条查询结果,搜索用时 343 毫秒
1.
NITROGEN METABOLISM IN POTERIUM SANGUISORBA DURING WATER STRESS 总被引:1,自引:1,他引:0
2.
DRIES KNAPEN MARTIN I. TAYLOR RONNY BLUST ERIK VERHEYEN 《Molecular ecology resources》2006,6(2):387-389
A total of seven polymorphic microsatellite loci from Gobio gobio were isolated and characterized. A preliminary population survey of 82 specimens from four populations, located in a downstream pollution gradient of cadmium and zinc, demonstrated the usefulness of these primers both in population genetic studies in general, as well as in evaluating the effects of anthropogenic pollution on genetic structure. Overall locus polymorphism ranged from two to 13 alleles. Observed heterozygosity per locus ranged from 0.39 to 0.73. 相似文献
3.
4.
SUMMARY. 1. Periphyton. measured as particulate phosphorus (PP) and expressed as periphyton PP, growing on vertically oriented substrata (polyvinyl impregnated nylon) under different nutrient loadings, light intensities (exposures), and grazer communities was examined in eight large enclosures (750 m3) where nutrients (N and P) and planktivorous fish (1+yellow perch) were added in a 2x2 factorial design.
2. During the first 3 weeks of the experiment (25 June to 15 July), there was a significantly higher accumulation of phosphorus into periphyton (periphyton PP) with fertilization, but fish addition had no effect. During the fourth to seventh weeks (16 July to 12 August), addition of fish was associated with lower abundance of amphipods and chironomids and higher concentration of periphyton PP. In the enclosures without fish, these invertebrates were over 25 times more abundant, and periphyton PP decreased substantially compared to the June-July period. Fertilization increased periphyton PP only at high exposures in the enclosures with fish.
3. Exposure had a significant effect on periphyton PP. In the enclosures with fish, high abundance of nanoplankton reduced water transparency, and periphyton PP was lower in the deeper waters which may have been due to limitation by low light. Lower periphyton PP was also observed at the surface on sunny sides of enclosures without fish, and therefore with high water transparency. This pattern may have been due to inhibitory effects of high light intensity.
4. Periphyton communities in the enclosures with fish had higher uptake rates for planktonic phosphorus, and lower rates of phosphorus release, suggesting that periphyton with high phosphorus demand may have high internal cycling of assimilated phosphorus. 相似文献
2. During the first 3 weeks of the experiment (25 June to 15 July), there was a significantly higher accumulation of phosphorus into periphyton (periphyton PP) with fertilization, but fish addition had no effect. During the fourth to seventh weeks (16 July to 12 August), addition of fish was associated with lower abundance of amphipods and chironomids and higher concentration of periphyton PP. In the enclosures without fish, these invertebrates were over 25 times more abundant, and periphyton PP decreased substantially compared to the June-July period. Fertilization increased periphyton PP only at high exposures in the enclosures with fish.
3. Exposure had a significant effect on periphyton PP. In the enclosures with fish, high abundance of nanoplankton reduced water transparency, and periphyton PP was lower in the deeper waters which may have been due to limitation by low light. Lower periphyton PP was also observed at the surface on sunny sides of enclosures without fish, and therefore with high water transparency. This pattern may have been due to inhibitory effects of high light intensity.
4. Periphyton communities in the enclosures with fish had higher uptake rates for planktonic phosphorus, and lower rates of phosphorus release, suggesting that periphyton with high phosphorus demand may have high internal cycling of assimilated phosphorus. 相似文献
5.
MAAS FRANK M.; DE KOK LUIT J.; PETERS JANNY L.; KUIPER PIETER J. C. 《Journal of experimental botany》1987,38(9):1459-1469
Maas, F. M., De Kok, L. J., Peters, J. L. and Kuiper, P. J.C. 1987. A comparative study on the effects of H2S and SO2 fumigationon the growth and accumulation of sulphate and sulphydryl compoundsin Trifolium pratense L., Glycine max Merr. and Phaseolus vulgarisL.J. exp. Bot. 38: 1459-1469. The effects of 025 mm3 dm3 H2S and SO2 on growth andsulphur content of shoots of Trifolium pratense, Glycine maxand Phaseolus vulgaris were studied. After 2 weeks of fumigationthe yield of T. pralense was reduced by 32% by H2S, but notaffected by SO2. Yield of G. max was not affected by H2S, butreduced by 20% by SO2, whereas that of P. vulgaris was increasedby 11% by H2S and not affected by SO2. Increases in sulphydrylcontent were already observed after 24 h of exposure to H2Sand SO2 in all plants. The increase was greatest in T. pratenseand smallest in P. vulgaris and, except for T. pratense, alwaysgreater in the H2S-exposed than the SO2-exposed plants. Oneday of exposure resulted in an increase in sulphate contentonly in the SO2-fumigated plants, with the highest accumulationin T. pratense and the lowest in P. vulgaris. After 2 weeksan increase in sulphate content was also observed in the H2S-exposedplants. This increase was also highest in T. pratense and lowestin P. vulgaris. Transpiration rate was not affected by a 24 h exposure to H2Sor SO2 and was highest in T. pratense, intermediate in G. maxand lowest in P. vulgaris. The order of theoretical rates of deposition of H2S and SO2correlated with the observed increases in sulphydryl contentduring the first 24 h of exposure in both H2S and SO2-fumigatedplants and with the increase in sulphate content in the SO2-exposedplants. The increases in sulphydryl content were only 8% ofthe theoretical H2S and SO2-deposition fluxes, whereas sulphateaccumulation accounted for at least 57% of the theoretical SO2-depositionflux. Key words: Air pollution, clover, French bean, Glutathione, Soybean, sulphur metabolism. 相似文献
6.
Akaike's information criterion and the histogram 总被引:1,自引:0,他引:1
7.
Major histocompatibility (B) complex and sex effects on the phytohaemagglutinin wattle response 总被引:2,自引:0,他引:2
The influence of the major histocompatibility (B) complex and sex on the phytohaemagglutinin (PHA) wattle response was studied in 136 segregants (B2/B2, B2/B5 and B5/B5) of a fourth generation cross between inbred lines 6(1) and 15(1). At 6 weeks of age, chickens were injected with 100 micrograms purified PHA-P. Wattle thickness measurements were taken 4, 24, 48, 72 and 96 h after injection. Analysis of variance showed that 4 h after injection, males had a significantly higher response than females but the sex-genotype interaction was also significant. Females had higher responses than males 24 and 48 h after injection as a consequence of more rapid development and earlier resolution of the reaction in males. B2/B2 chickens had significantly lower responses than B5/B5 chickens 72 and 96 h after injection, signifying a faster late resolution phase in the B2/B2 genotype. The developmental and early resolution phases of the PHA wattle response were influenced by sex while the late resolution phase was influenced by B genotype. 相似文献
8.
9.
Recent advances in palaeomycology 总被引:1,自引:1,他引:0
10.
Proplastids of both tapetal cells and microsporocytes were presentearly in anther development. Tapetal proplastids differentiatedprobablyinto elaioplastsat late microspore stage. The tapetalcytoplasm was completely resorbed by early tricellular pollenstage. Microspore proplastids differentiated into amyloplastsat early bicellular stage, and were present in both vegetativeand generative cells. In the generative cell, the amyloplastswere ephemeral and apparently degenerated within autophagicvacuoles. Plastids were absent from sperm cells. Vegetativecell amyloplasts increased in number apparently by fission suchthat one amyloplast produced one amyloplast and one proplastidper division. Mature pollen grains were estimated to containbetween 550 and 820 amyloplasts with only one starch granuleper plastid. Elaioplasts, amyloplasts, plastid division, plastid differentiation, starch granules, autophagy, Lolium perenne, Poaceae, rye-grass 相似文献