全文获取类型
收费全文 | 3970篇 |
免费 | 419篇 |
国内免费 | 506篇 |
出版年
2023年 | 105篇 |
2022年 | 74篇 |
2021年 | 109篇 |
2020年 | 157篇 |
2019年 | 197篇 |
2018年 | 140篇 |
2017年 | 151篇 |
2016年 | 186篇 |
2015年 | 149篇 |
2014年 | 165篇 |
2013年 | 249篇 |
2012年 | 158篇 |
2011年 | 199篇 |
2010年 | 170篇 |
2009年 | 221篇 |
2008年 | 210篇 |
2007年 | 213篇 |
2006年 | 189篇 |
2005年 | 191篇 |
2004年 | 148篇 |
2003年 | 148篇 |
2002年 | 126篇 |
2001年 | 125篇 |
2000年 | 84篇 |
1999年 | 64篇 |
1998年 | 85篇 |
1997年 | 59篇 |
1996年 | 50篇 |
1995年 | 46篇 |
1994年 | 55篇 |
1993年 | 58篇 |
1992年 | 55篇 |
1991年 | 45篇 |
1990年 | 39篇 |
1989年 | 36篇 |
1988年 | 28篇 |
1987年 | 45篇 |
1986年 | 25篇 |
1985年 | 29篇 |
1984年 | 42篇 |
1983年 | 21篇 |
1982年 | 50篇 |
1981年 | 35篇 |
1980年 | 27篇 |
1979年 | 26篇 |
1978年 | 22篇 |
1977年 | 17篇 |
1976年 | 17篇 |
1974年 | 13篇 |
1973年 | 11篇 |
排序方式: 共有4895条查询结果,搜索用时 547 毫秒
51.
An exhaustive analysis of registers of births, deaths, and marriages is used to reconstitute genealogies, both ascending and
descending, for all individuals who lived in two mountain valleys: the Valserine valley (in the French Jura), which is studied
from the end of the seventeenth century to the present, and the Vallouise valley (in the French Alps) studied from the seventeenth
to the nieteenth centuries.
This genealogical approach makes it possible to trace the reproductive process in the populations whose members lived in the
valley for several generations. The Vallouise valley forms an important demographic isolate; nearly 98 per cent of those born
between 1820 and 1849 were descendants of ancestors who had lived in the valley two centuries earlier. By contrast, there
has been continuous immigration into the Valserine valley, which has resulted in a constant renewal of the gene pool. Only
18 per cent of the gene pool of individuals living in this valley two centuries later was contributed by those who lived in
the valley and who were born before 1750. 相似文献
52.
S. G. Nugroho J. Lumbanraja H. Suprapto Sunyoto H. Haraguchi M. Kimura 《Plant and Soil》1996,181(2):287-293
Yearly and seasonal (rainy and dry seasons) variations of CH4 emission from a Sumatra paddy field were measured for 3 years. The mean CH4 emission rates during the growth period were in the range of 16.0–26.1 mg CH4 m-2 h-1 for the chemical fertilizer plots and 23.3–34.9 mg CH4 m-2 h-1 for the plots with rice straw application, respectively. The increase in the amounts of CH4 emission by rice straw application were from 1.3 to 1.6 times. There was no significant difference in the mean CH4 emission rates between rainy and dry seasons.Total amounts of CH4 emitted during the period of rice growth were in the ranges of 29.5–48.2 and 43.0–64.6 g CH4 m-2 for the plots applied with chemical fertilizer and those with rice straw application, respectively. Nearly the same amounts of CH4 were emitted in the first and second half of the growth period, irrespective of rice straw application. 相似文献
53.
Edwin M. Everham III Randall W. Myster Eric VanDeGenachte 《American journal of botany》1996,83(8):1063-1068
Field experiments were conducted to investigate the effects of light, moisture, temperature, and litter on the regeneration of two early-, one mid-, and two late-successional tropical tree species. High light and litter seem to be universally good cues for regeneration, increasing seed/seedling survival for all species except for Cecropia (an early-successional species) whose small seeds may not be able to penetrate the litter layer. In addition, the high temperature environment in both artificially shaded and nonshaded areas of a natural gap exhibits less seed loss, an increase in the percent and rate of germination, and an increase in seedling survival for Dacryodes (a late-successional species), than the lower temperature environment under an intact canopy. Low soil water is also a good cue for Dacryodes germination as it is for Prestoea and Cecropia. Finally, the lower temperature environment found under the forest canopy (compared to the natural gap) leads to less seed loss and more germination for Guarea (a mid-successional species). Our results suggest that a good patch for regeneration of many species in this forest, early- as well as late-successional species, would have high light and a litter layer that moderates temperature and moisture extremes. The substantial variation in suitability among regeneration filters and species could: (1) contribute to low establishment success, i.e., most dispersed propagules do not become trees, (2) make it difficult to group species into germination strategies, and (3) make it hard to generalize about a net effect of any specific environmental variable on establishment. We suggest that tropical disturbances should be viewed in terms of their impact on a variety of environmental cues, which may signal germination and impact subsequent growth and survival. 相似文献
54.
55.
E. Molina Grima A. Robles Medina A. Giménez Giménez M. J. Ibáñez González 《Journal of applied phycology》1996,8(4-5):359-367
Eicosapentaenoic acid (EPA, 20:5n-3) was obtained from the microalgaPhaeodactylum tricornutum following a three-step process: fatty acid extraction by direct saponification of wet biomass, polyunsaturated fatty acid (PUFA) concentration by formation of urea inclusion compounds and EPA isolation by preparative HPLC. Direct saponification of wet biomass was carried out with KOH-ethanol (96% v:v) (1 h, 60 °C), extracting 91% of the EPA. PUFAs were concentrated by the urea method with an urea/fatty acid ratio of 4:1 at a crystallization temperature of 28 °C using methanol as the urea solvent. An EPA concentration ratio of 1.5 (55.2/36.3) and recovery of 79% were obtained. This PUFA concentrate was used to obtain 95.8% pure EPA by preparative HPLC, using a reverse-phase column (C18, 4.7 cm i.d. × 30 cm) and methanol-water (1% AcH) 80:20 w/w as the mobile phase. Ninety-seven per cent of EPA loaded was recovered and 70% EPA present in theP. tricornutum biomass was recovered in a highly pure form by means of this three-step downstream processing. In each of the HPLC preparative runs, 635 mg PUFA concentrate were loaded, obtaining 326 mg of a highly concentrated EPA fraction (2.46 g d–1). Finally, a preliminary cost statement has been calculated. 相似文献
56.
Engineering challenges in high density cell culture systems 总被引:2,自引:0,他引:2
Ozturk SS 《Cytotechnology》1996,22(1-3):3-16
High density cell culture systems offer the advantage of production of bio-pharmaceuticals in compact bioreactors with high volumetric production rates; however, these systems are difficult to design and operate. First of all, the cells have to be retained in the bioreactor by physical means during perfusion. The design of the cell retention is the key to performance of high density cell culture systems. Oxygenation and media design are also important for maximizing the cell number. In high density perfusion reactors, variable cell density, and hence the metabolic demand, require constant adjustment of perfusion rates. The use of cell specific perfusion rate (CSPR) control provides a constant environment to the cells resulting in consistent production. On-line measurement of cell density and metabolic activities can be used for the estimation of cell densities and the control of CSPR. Issues related to mass transfer and mixing become more important at high cell densities. Due to the difference in mass transfer coefficients for oxygen and CO2, a significant accumulation of dissolved CO2 is experienced with silicone tubing aeration. Also, mixing is observed to decrease at high densities. Base addition, if not properly done, could result in localized cell lysis and poor culture performance. Non-uniform mixing in reactors promotes the heterogeneity of the culture. Cell aggregation results in segregation of the cells within different mixing zones. This paper discusses these issues and makes recommendations for further development of high density cell culture bioreactors. 相似文献
57.
Guo-Ling Nan Adelheid R. Kuehnle 《In vitro cellular & developmental biology. Plant》1995,31(3):131-136
Summary Five parameters were examined for their effect on transformation ofDendrobium tissues by microprojectile bombardment. The superpromoter in pBI426 produced at least 1.5 times as many transient transformants
as the single cauliflower mosaic virus 35S promoter in pBI121 (37 to 69% vs. 0 to 44%) with dark and frequent GUS (β-glucuronidase) staining. Tissue, genotype, and type of microparticle significantly affected transient GUS activity. Higher
expression was seen in protocormlike bodies and in hybrid UH44 compared to etiolated shoots and protocorms and to hybrids
M61 and K1329-39. Microparticles of 1.6-μm Bio-Rad gold were more effective than 1.0-μm ASI gold. Transient GUS activity did
not differ among protocormlike bodies bombarded using helium propellant pressures of 650, 900, or 1100 psi. Transgenic plants
were recovered fromDendrobium UH800 protocormlike bodies bombarded with pBI426-coated, 1.1-μm tungsten particles using an early-model gunpowder-driven
apparatus with an estimated stable transformation rate of 11.7%. One transgenic plant ofDendrobium UH44 was recovered from etiolated shoot explants bombarded with pBI121-coated, 1.1-μm tungsten particles using the Dupont
PDS-1000 with a stable transformation rate of 0.17%. Positive selection results showed 100 to 200 mg·liter−1 kanamycin to be appropriate for regeneration of transgenic plants from protocormlike bodies, protocorms, and etiolated shoot
explants over a 3- to 9.5-mo. period. 相似文献
58.
On semiparametric inference for modulated renewal processes 总被引:1,自引:0,他引:1
59.
60.
John H. Gillespie 《Evolution; international journal of organic evolution》1994,48(4):1101-1113
Substitution processes are of two sorts: origination processes record the times at which nucleotide mutations that ultimately fix in the population first appear, and fixation processes record the times at which they actually fix. Substitution processes may be generated by combining models of population genetics—here the symmetrical-neutral, overdominance, underdominance, TIM, and SAS-CFF models—with the infinite-sites, no-recombination model of the gene. This paper is mainly concerned with a computer simulation study of these substitution processes. The rate of substitution is shown to be remarkably insensitive to the strength of selection for models with strong balancing selection caused by the genealogical drift of mutations through alleles held in the population by selection. The origination process is shown to be more regular than Poisson for the overdominance, TIM, and SAS-CFF models but more clustered for the underdominance model. A class of point processes called Sawyer processes is introduced to help explain these observations as well as the observation that the times between successive originations are nearly uncorrelated. Fixation processes are shown to be more complex than origination processes, with regularly spaced bursts of multiple fixations. An approximation to the fixation process is described. One important conclusion is that protein evolution is not easily reconciled with any of these models without adding perturbations that recur on a time scale that is commensurate with that of molecular evolution. 相似文献