全文获取类型
收费全文 | 3251篇 |
免费 | 293篇 |
出版年
2023年 | 16篇 |
2022年 | 36篇 |
2021年 | 64篇 |
2020年 | 53篇 |
2019年 | 53篇 |
2018年 | 49篇 |
2017年 | 70篇 |
2016年 | 89篇 |
2015年 | 150篇 |
2014年 | 165篇 |
2013年 | 194篇 |
2012年 | 207篇 |
2011年 | 190篇 |
2010年 | 133篇 |
2009年 | 131篇 |
2008年 | 152篇 |
2007年 | 147篇 |
2006年 | 130篇 |
2005年 | 119篇 |
2004年 | 106篇 |
2003年 | 126篇 |
2002年 | 111篇 |
2001年 | 89篇 |
2000年 | 62篇 |
1999年 | 66篇 |
1998年 | 48篇 |
1997年 | 25篇 |
1996年 | 37篇 |
1995年 | 32篇 |
1994年 | 42篇 |
1993年 | 35篇 |
1992年 | 55篇 |
1991年 | 38篇 |
1990年 | 33篇 |
1989年 | 33篇 |
1988年 | 29篇 |
1987年 | 32篇 |
1986年 | 25篇 |
1985年 | 37篇 |
1984年 | 16篇 |
1983年 | 21篇 |
1982年 | 17篇 |
1981年 | 20篇 |
1980年 | 15篇 |
1979年 | 21篇 |
1978年 | 17篇 |
1975年 | 16篇 |
1973年 | 20篇 |
1972年 | 17篇 |
1971年 | 18篇 |
排序方式: 共有3544条查询结果,搜索用时 18 毫秒
61.
Callus derived from Lemhi Russet and Russet Burbank tuber tissue was incubated at 20°C and 30°C on a high sucrose medium for starch-formation and subjected to simulated storage and reconditioning treatments at 5°C and 25°C after transfer of the callus to a medium without a carbon source. Percent dry weight of callus from both cultivars averaged about 21% after starch formation and 5% after storage and reconditioning treatments. Total sugars were higher in callus incubated on the starch forming treatment. Lemhi Russet callus contained predominantly reducing sugars, while Russet Burbank callus contained mostly non-reducing sugars. Total sugar content was generally lower for both cultivars after the storage and reconditioning treatment in callus after starch formation at 20°C. Starch content was generally higher in Lemhi Russet tissue. After starch formation at 20°C Lemhi Russet had higher starch after the storage and reconditioning treatment than tissue from 30°C, while the opposite trend was found in Russet Burbank tissue. Total protein declined in Russet Burbank tissue during the storage and reconditioning treatment regardless of prior incubation conditions, while this decline only occurred in Lemhi Russet tissue initially incubated at 30°C during the starch formation treatment. In tissue of both cultivars, ATP- and PPi-dependent phosphofructokinase activities were inversely proportional to total sugar concentrations, while in RB callus ADP glucose pyrophosphorylase activities were proportional to starch content.Research Paper 91B1 of the Idaho Agricultural Experiment Station. 相似文献
62.
Mang C. Yu Amos G. Gona Ophelia Gona Suad Al-Rabiai Stanley Von Hagen Edwin Cohen 《Bioelectromagnetics》1993,14(5):449-458
This study was undertaken to determine the effects of extremely low frequency (ELF; 60 Hz) electromagnetic (EM) fields on somatic growth and cortical development, as well as biochemical and morphological maturation, of the rat neopallium. On the fifth day of pregnancy, female rats were put in pairs into plastic cages that were housed in a specially constructed apparatus for irradiation under three separate sets of combination and intensity: 1) 1 kV/m and 10 gauss; 2) 100 kV/m and 1 gauss; and 3) 100 kV/m and 10 gauss. The dams were exposed for 23 h daily, from days 5 through 19 postconception after which they were returned to cages outside the exposure apparatus until they littered. The neonates were culled to eight pups per litter. At 0 (birth), 5, 12, and 19 days postnatally, they were killed for biochemical and morphological studies. Another group of pregnant rats was sham-exposed in an identical apparatus, which was not energized, and the pups were used as controls. The irradiated rats exhibited no physical abnormalities, nor did they show brain deformities such as swelling or herniation following exposure to ELF-EM fields. There was no difference in somatic growth between control and exposed rats, but a small reduction in cortical weight was observed in rats exposed at 1 kV/m and 10 gauss, and 100 kV/m and 1 gauss, respectively. Biochemical measurements of DNA. RNA, protein, and cerebroside concentrations indicated that among the three separate exposures, only the neopallium of rats exposed at 1 kV/m and 10 gauss showed a small reduction in DNA level, as well as small reductions in RNA and protein levels. No changes were noticed in cerebroside levels in any exposed animals, and there were no differences in protein/DNA and cerebroside/DNA ratios between control and exposed rats. Morphological observations did not reveal any detectable alterations in the irradiated rats. These results indicate that exposure to ELF-EM fields caused minimal or no changes in somatic growth and cerebral development of the rat. © 1993 Wiley-Liss, Inc. 相似文献
63.
Eutrophication and recovery of Lake Vesijärvi (south Finland): Diatom frustules in varved sediments over a 30-year period 总被引:2,自引:2,他引:0
Lake Vesijärvi was loaded by sewage from the City of Lahti for 60 years until 1976 when the discharge was diverted. Paleolimnological analyses of the varved bottom sediment indicate that the sedimentation rate within the Enonselka basin, the most eutrophic part of the lake, has been as high as 2 cm yr–1, and total phosphorus accumulation was 20–40 g P m–2 yr–1, during the last 20 years. Within the less eutrophic Laitialanselkä basin, the sedimentation rate did not exceed 1 cm yr–1, and the formation of varved sediment only began at the end of the 1960's, i.e. about 10 years later than in Enonselkä.Planktonic diatom production was highest in the Enonselka basin. The most abundant diatoms in the sediment between 1970–1985 were Asterionella formosa, Aulacoseira islandica and Stephanodiscus spp. Fragilaria crotonensis and Tabellaria fenestrata had low abundances in the middle of the 1970's but increased again at the end of the 1970's. Asterionella formosa and Diatoma elongatum reached their maxima between 1979–1984 when the hypolimnion of the Enonselk/:a basin was aerated artificially. In the Laitialanselkä basin, the production of planktonic diatoms has been lower and the species composition of the diatom community differed from that in Enonselkä. However, at the end of 1980's the total accumulation of diatoms in Laitialanselkä approached levels which were observed at the end of 1950's in Enonselkä, prior to the rapid eutrophication period.The production and thereby the sedimentation of diatoms has decreased towards the end of the 1980's in Enonselkä, indicating reduced nutrient availability in the lake water. This reduction was due to the decreased external loading of phosphorus as well as to the decreased release of phosphorus from the sediment as a result of improved oxygen balance in the hypolimnion. 相似文献
64.
Biosynthesis of alginate in algae may be studied by following the cell wall regeneration of brown seaweed protoplasts in culture. The enzyme mannuronan C-5 epimerase will control the composition of the alginate being synthetized.Freshly isolated protoplasts from the thallus of young Laminaria digitata plants showed only low expression of this enzyme. However, after prolonged periods in culture, this activity increased 15-fold. The synthesis of C-5 epimerase by the protoplasts is probably essential for the formation of a new cell wall.After cellular disruption by osmotic shock and centrifugation, most of the epimerase activity resided in the pellet fraction. This may indicate that the enzyme is membrane associated. 相似文献
65.
Keli Li G. F. Veen Freddy C. ten Hooven Jeffrey A. Harvey Wim H. van der Putten 《Ecology letters》2023,26(1):37-52
Soils contain biotic and abiotic legacies of previous conditions that may influence plant community biomass and associated aboveground biodiversity. However, little is known about the relative strengths and interactions of the various belowground legacies on aboveground plant–insect interactions. We used an outdoor mesocosm experiment to investigate the belowground legacy effects of range-expanding versus native plants, extreme drought and their interactions on plants, aphids and pollinators. We show that plant biomass was influenced more strongly by the previous plant community than by the previous summer drought. Plant communities consisted of four congeneric pairs of natives and range expanders, and their responses were not unanimous. Legacy effects affected the abundance of aphids more strongly than pollinators. We conclude that legacies can be contained as soil ‘memories’ that influence aboveground plant community interactions in the next growing season. These soil-borne ‘memories’ can be altered by climate warming-induced plant range shifts and extreme drought. 相似文献
66.
Yaoyu Ren Timo Danner Alexandra Moy Martin Finsterbusch Tanner Hamann Jan Dippell Till Fuchs Marius Müller Ricky Hoft André Weber Larry A. Curtiss Peter Zapol Matthew Klenk Anh T. Ngo Pallab Barai Brandon C. Wood Rongpei Shi Liwen F. Wan Tae Wook Heo Martin Engels Jagjit Nanda Felix H. Richter Arnulf Latz Venkat Srinivasan Jürgen Janek Jeff Sakamoto Eric D. Wachsman Dina Fattakhova-Rohlfing 《Liver Transplantation》2023,13(1):2201939
The garnet-type phase Li7La3Zr2O12 (LLZO) attracts significant attention as an oxide solid electrolyte to enable safe and robust solid-state batteries (SSBs) with potentially high energy density. However, while significant progress has been made in demonstrating compatibility with Li metal, integrating LLZO into composite cathodes remains a challenge. The current perspective focuses on the critical issues that need to be addressed to achieve the ultimate goal of an all-solid-state LLZO-based battery that delivers safety, durability, and pack-level performance characteristics that are unobtainable with state-of-the-art Li-ion batteries. This perspective complements existing reviews of solid/solid interfaces with more emphasis on understanding numerous homo- and heteroionic interfaces in a pure oxide-based SSB and the various phenomena that accompany the evolution of the chemical, electrochemical, structural, morphological, and mechanical properties of those interfaces during processing and operation. Finally, the insights gained from a comprehensive literature survey of LLZO–cathode interfaces are used to guide efforts for the development of LLZO-based SSBs. 相似文献
67.
Sven Rossel Patricia Kaiser Maya Bode-Dalby Jasmin Renz Silke Laakmann Holger Auel Wilhelm Hagen Pedro Martínez Arbizu Janna Peters 《Molecular ecology resources》2023,23(2):382-395
Species identification is pivotal in biodiversity assessments and proteomic fingerprinting by MALDI-TOF mass spectrometry has already been shown to reliably identify calanoid copepods to species level. However, MALDI-TOF data may contain more information beyond mere species identification. In this study, we investigated different ontogenetic stages (copepodids C1–C6 females) of three co-occurring Calanus species from the Arctic Fram Strait, which cannot be identified to species level based on morphological characters alone. Differentiation of the three species based on mass spectrometry data was without any error. In addition, a clear stage-specific signal was detected in all species, supported by clustering approaches as well as machine learning using Random Forest. More complex mass spectra in later ontogenetic stages as well as relative intensities of certain mass peaks were found as the main drivers of stage distinction in these species. Through a dilution series, we were able to show that this did not result from the higher amount of biomass that was used in tissue processing of the larger stages. Finally, the data were tested in a simulation for application in a real biodiversity assessment by using Random Forest for stage classification of specimens absent from the training data. This resulted in a successful stage-identification rate of almost 90%, making proteomic fingerprinting a promising tool to investigate polewards shifts of Atlantic Calanus species and, in general, to assess stage compositions in biodiversity assessments of Calanoida, which can be notoriously difficult using conventional identification methods. 相似文献
68.
Margreet ten Have Marlous Tuithof Saskia van Dorsselaer Frederiek Schouten Annemarie I. Luik Ron de Graaf 《World psychiatry》2023,22(2):275-285
Up-to-date information on the prevalence and trends of common mental disorders is relevant to health care policy and planning, owing to the high burden associated with these disorders. In the first wave of the third Netherlands Mental Health Survey and Incidence Study (NEMESIS-3), a nationally representative sample was interviewed face-to-face from November 2019 to March 2022 (6,194 subjects; 1,576 interviewed before and 4,618 during the COVID-19 pandemic; age range: 18-75 years). A slightly modified version of the Composite International Diagnostic Interview 3.0 was used to assess DSM-IV and DSM-5 diagnoses. Trends in 12-month prevalence rates of DSM-IV mental disorders were examined by comparing these rates between NEMESIS-3 and NEMESIS-2 (6,646 subjects; age range: 18-64 years; interviewed from November 2007 to July 2009). Lifetime DSM-5 prevalence estimates in NEMESIS-3 were 28.6% for anxiety disorders, 27.6% for mood disorders, 16.7% for substance use disorders, and 3.6% for attention-deficit/hyperactivity disorder. Over the last 12 months, prevalence rates were 15.2%, 9.8%, 7.1%, and 3.2%, respectively. No differences in 12-month prevalence rates before vs. during the COVID-19 pandemic were found (26.7% pre-pandemic vs. 25.7% during the pandemic), even after controlling for differences in socio-demographic characteristics of the respondents interviewed in these two periods. This was the case for all four disorder categories. From 2007-2009 to 2019-2022, the 12-month prevalence rate of any DSM-IV disorder significantly increased from 17.4% to 26.1%. A stronger increase in prevalence was found for students, younger adults (18-34 years) and city dwellers. These data suggest that the prevalence of mental disorders has increased in the past decade, but this is not explained by the COVID-19 pandemic. The already high mental disorder risk of young adults has particularly further increased in recent years. 相似文献
69.
Hagen Preik-Steinhoff Malte Kelm 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》1996,685(2):4563
All photometric or HPLC methods described to date have been unable to detect nitrite, a reliable marker of NO synthase activity, in human blood because of its rapid metabolism within the erythrocytes. We now elaborate on method to prevent nitrite degradation during sample preparation which in combination with high-performance anion-exchange chromatography and electrochemical detection allows a sensitive measurement of nitrite. A linear current response in the concentration range of 10–1000 nmol/l nitrite was observed yielding a correlation coefficient of 0.99. In addition, the combination of the electrochemical with a UV detector allowed us to simultaneously quantify nitrate one analytical run, which is the end product of NO/nitrite metabolism. Basal levels for nitrate and nitrite in human blood were determined with 25±4 μmol/l and 578±116 nmol/l (n=8), respectively and thus were in the same concentration range as expected from NO measurement in saline perfused isolated organs or cultured endothelial cells. Therefore, the presented method may be used to assess activity of endothelial constitutive NO synthase in humans under physiological and pathophysiological conditions. 相似文献
70.