全文获取类型
收费全文 | 6396篇 |
免费 | 626篇 |
国内免费 | 2篇 |
出版年
2022年 | 48篇 |
2021年 | 85篇 |
2020年 | 59篇 |
2019年 | 69篇 |
2018年 | 87篇 |
2017年 | 77篇 |
2016年 | 142篇 |
2015年 | 235篇 |
2014年 | 213篇 |
2013年 | 330篇 |
2012年 | 394篇 |
2011年 | 380篇 |
2010年 | 217篇 |
2009年 | 199篇 |
2008年 | 324篇 |
2007年 | 376篇 |
2006年 | 315篇 |
2005年 | 320篇 |
2004年 | 302篇 |
2003年 | 287篇 |
2002年 | 263篇 |
2001年 | 69篇 |
2000年 | 72篇 |
1999年 | 92篇 |
1998年 | 88篇 |
1997年 | 69篇 |
1996年 | 68篇 |
1995年 | 83篇 |
1994年 | 43篇 |
1993年 | 58篇 |
1992年 | 67篇 |
1991年 | 68篇 |
1990年 | 50篇 |
1989年 | 51篇 |
1988年 | 36篇 |
1987年 | 52篇 |
1986年 | 44篇 |
1985年 | 57篇 |
1984年 | 53篇 |
1983年 | 49篇 |
1982年 | 62篇 |
1981年 | 64篇 |
1980年 | 64篇 |
1979年 | 36篇 |
1978年 | 46篇 |
1977年 | 45篇 |
1976年 | 42篇 |
1975年 | 34篇 |
1971年 | 32篇 |
1969年 | 32篇 |
排序方式: 共有7024条查询结果,搜索用时 15 毫秒
21.
The presence of chlorophyll b and the estimation of phaeopigments in marine phytoplankton 总被引:1,自引:0,他引:1
A reverse-phase h.p.l.c. technique was used to estimate theconcentration of chlorophyll b in phytoplankton cultures, fecalpellets of Calanus pacificus, and suspended paniculate matterfrom the Central North Pacific, Oregon coastal waters, and DabobBay (a temperate fjord in Puget Sound, WA, USA). The purposewas to assess the distribution of this pigment in the euphoticzone and its effect on the fluorometnc estimation of phaeopigments.Analyses of natural waters confirm high chlorophyll b concentrations(median mass ratio of b:a > 0.3) at the depth of the chlorophylla maximum in tropical waters while values for temperate planktonare relatively low (median mass ratio of chl b:a = 0.05) andpatchy. Zooplankton fecal pellets showed a significant enrichmentin chlorophyll b, suggesting grazing as a mechanism to explainhigh concentrations of this pigment at the bottom of the euphoticzone. It is estimated that the presence of chlorophyll b couldcause an average overestimation of phaeopigment concentrationby the fluorometnc technique of 38% between 0 and 200 m in theCentral North Pacific. This effect is more pronounced at thelayer of chlorophyll b maximum (120140 m).
1Present address: Marine Biology Research Division, A-002, ScrippsInstitution of Oceanography, La Jolla, CA 92093, USA 相似文献
22.
23.
A study was initiated to establish an in vitro culture protocol for tea (Camellia sinensis). Explant sources, disinfestation methods and culture media were examined. Segments (divots) were dissected from greenwood stem (current year growth) internodes of field grown plants. Disinfestation was achieved by separate treatments of 3.75% sodium hypochlorite and 7.5% CaCl2. MS medium with sucrose (30 g/L), inositol (100 mg/L) and thiamine-HCl (1.3 mg/L) and kinetin was used with combinations of the auxins: (2,4-dichlorophenoxy) acetic acid (2,4-D), (2,4,5-trichlorophenoxy) acetic acid (2,4,5-T), (naphthalene) acetic acid (NAA) and 4-amino-3,5,6-trichloro-2-pyridinecarboxylic acid (Picloram). Picloram (10-7M) induced the most callus proliferation without kinetin. At a constant level of kinetin (10-5M), the concentrations inducing the most callus growth were 10-7M for 2,4-D, 10-6M for 2,4,5-T, 10-7M for Picloram and 10-8M for NAA. A factorial test of 2,4,5-T and kinetin concentrations showed the optimum for callus growth was 10-7M and 10-5M, respectively.Technical Contribution No. 2532 of the South Carolina Agricultural Experiment Station, Clemson University.Graduate Research Assistant and Professor, respectively. 相似文献
24.
Robert A. Goldstein Steven A. Gherini Charles T. Driscoll Richard April Carl L. Schofield Carl W. Chen 《Biogeochemistry》1987,3(1-3):5-20
An integrated analysis of a terrestrial-aquatic ecosystem, the North Branch of the Moose River in the Adirondack region of New York, was conducted. This basin contains a large number of interconnected surface waters that exhibit marked gradients in pH and acid neutralizing capacity (ANC). As a result, the basin has been the focus of research activity, including the Regional Integrated Lake-Watershed Acidification Study (RILWAS). The objective of the current analysis was to use the North Branch of the Moose River as a case study to:
- Evaluate processes regulating the acid-base chemistry of surface waters.
- To assess the effects of surface water acidification on fish populations.
25.
Big Moose Basin: simulation of response to acidic deposition 总被引:2,自引:1,他引:1
George F. Davis John J. Whipple Steven A. Gherini Carl W. Chen Robert A. Goldstein Arland H. Johannes Peter W.H Chan Ronald K. Munson 《Biogeochemistry》1987,3(1-3):141-161
The ILWAS model has been enhanced for application to multiple-lake hydrologic basins. This version of the model has been applied to the Big Moose basin, which includes Big Moose Lake and its tributary streams, lakes, and watersheds. The basin, as defined, includes an area of 96 km2, with over 20 lakes and ponds, and 70 km of streams. Hydrologic and chemical calibrations have been made using data from seven sampling stations. When total atmospheric sulfur loading to the basin is halved, the model predicts, after four years of simulation, a decreasing sulfate concentration and to a lesser extent a rising alkalinity at Big Moose Lake outlet. At the end of four years, the results show an increase in pH of 0.1 to 0.5 pH units depending upon season. 相似文献
26.
The physical status of water in seeds has a pivotal role in determining the physiological reactions that can take place in the dry state. Using water sorption isotherms from cotyledon and axis tissue of five leguminous seeds, the strength of water binding and the numbers of binding sites have been estimated using van't Hoff analyses and the D'Arcy/Watt equation. These parameters of water sorption are calculated for each of the three regions of water binding and for a range of temperatures. Water sorption characteristics are reflective of the chemical composition of the biological materials as well as the temperature at which hydration takes place. Changes in the sorption characteristics with temperature and hydration level may suggest hydration-induced structural changes in cellular components. 相似文献
27.
Floral determination in the terminal bud of the short-day plant Nicotiana tabacum cv. Maryland Mammoth has been investigated. Plants grown continuously in short days flowered after producing 31.4±1.6 (SD) nodes while plants grown continuously in long days did not flower and produced 172.5±9.5 nodes after one year. At various ages, expressed as number of leaves that were at least 1.0 cm in length above the most basal 10-cm leaf, one of three treatments was performed on plants grown from seed in short days: 1) whole plants were shifted from short days to long days, 2) the terminal bud was removed and then rooted and grown in long days, and 3) the terminal bud was removed and then rooted and grown in short days. Whole plants flowered only when shifted from short days to long days at age 15 or later. Only rooted terminal buds from plants at age 15 or older produced plants that flowered when grown in long days. Only terminal buds from plants at age 15 or older that were rooted and grown in short days produced the same number of nodes as they would have produced in their original locations while buds from younger plants produced more nodes than they would have in their original locations. Thus, determination for floral development in the terminal bud, as assayed by rooting, is simultaneous with the commitment to flowering as assayed by shifting whole plants to non-inductive conditions.Abbreviations LD
long day(s)
- SD
short day(s)
- DN
dayneutral 相似文献
28.
Ciliary ganglion neurons and nonneurons can be separated from each other, based on the greater adhesivity of the nonneurons to untreated tissue culture plastic in the presence of serum. When the separation was carried out in the presence of Serum Plus (a commercially available supplemented serum), two populations of neurons were distinguished. Neurons in the first class (50–60% of total) adhered to plastic within 15 min, tended to aggregate into clumps, and were not well supported in long term culture by brain extracts. Neuronal adhesion to plastic was inhibited by heparin but not by chondroitin sulfate. Neurons in the second class did not attach to plastic for up to 90 min (and could thus be purified), were not as prone to aggregation, and were quantitatively supported for long periods (>2 weeks) by the neurotrophic factor(s) present in extracts of injured brain. Although no direct evidence is provided, these populations may correspond to the well characterized ciliary and choroid neurons.Special Issue dedicated to Dr. E. M. Shooter and Dr. S. Varon. 相似文献
29.
Auto anti-A1 and auto anti-NA1 after bone marrow transplantation 总被引:1,自引:0,他引:1
P Herzog P Korínková M Stambergová M Lukásová 《Folia haematologica (Leipzig, Germany : 1928)》1987,114(6):874-880
The production of auto anti-A1 and auto anti-NA1 antibodies in patient with aplastic anemia has been described. The patient of group A1 received bone marrow from his brother of group A2. For immunosuppression cyclosporine A was administered. 相似文献
30.
Carl D. Schlichting 《Oecologia》1989,78(4):496-501
Summary Three species of Phlox (Polemoniaceae) were grown in 6 greenhouse treatments. A variety of traits were recorded and the correlations among them were computed for each treatment. The phenotypic correlations between characters are significantly altered when plants are grown under different environmental conditions. These changes in correlation structure result from the differential phenotypic plasticity of traits. Partial correlations between flower production and other traits are also environment-dependent. Such changes can alter the intensity of, and possibly the response to, selection on traits correlated with fitness in natural plant populations. 相似文献