全文获取类型
收费全文 | 729篇 |
免费 | 54篇 |
专业分类
林业 | 32篇 |
农学 | 9篇 |
94篇 | |
综合类 | 142篇 |
农作物 | 21篇 |
水产渔业 | 18篇 |
畜牧兽医 | 401篇 |
园艺 | 12篇 |
植物保护 | 54篇 |
出版年
2023年 | 8篇 |
2022年 | 9篇 |
2021年 | 7篇 |
2020年 | 13篇 |
2019年 | 7篇 |
2018年 | 9篇 |
2017年 | 13篇 |
2016年 | 10篇 |
2015年 | 8篇 |
2014年 | 9篇 |
2013年 | 40篇 |
2012年 | 24篇 |
2011年 | 27篇 |
2010年 | 19篇 |
2009年 | 15篇 |
2008年 | 25篇 |
2007年 | 30篇 |
2006年 | 21篇 |
2005年 | 18篇 |
2004年 | 29篇 |
2003年 | 21篇 |
2002年 | 28篇 |
2001年 | 19篇 |
2000年 | 23篇 |
1999年 | 13篇 |
1997年 | 7篇 |
1996年 | 6篇 |
1993年 | 7篇 |
1992年 | 10篇 |
1991年 | 12篇 |
1990年 | 14篇 |
1989年 | 9篇 |
1988年 | 15篇 |
1987年 | 13篇 |
1986年 | 14篇 |
1985年 | 11篇 |
1984年 | 7篇 |
1983年 | 11篇 |
1982年 | 8篇 |
1981年 | 6篇 |
1980年 | 8篇 |
1979年 | 7篇 |
1976年 | 9篇 |
1975年 | 8篇 |
1974年 | 14篇 |
1973年 | 21篇 |
1972年 | 9篇 |
1969年 | 8篇 |
1968年 | 6篇 |
1966年 | 9篇 |
排序方式: 共有783条查询结果,搜索用时 15 毫秒
81.
Murphy WJ Larkin DM Everts-van der Wind A Bourque G Tesler G Auvil L Beever JE Chowdhary BP Galibert F Gatzke L Hitte C Meyers SN Milan D Ostrander EA Pape G Parker HG Raudsepp T Rogatcheva MB Schook LB Skow LC Welge M Womack JE O'brien SJ Pevzner PA Lewin HA 《Science (New York, N.Y.)》2005,309(5734):613-617
The genome organizations of eight phylogenetically distinct species from five mammalian orders were compared in order to address fundamental questions relating to mammalian chromosomal evolution. Rates of chromosome evolution within mammalian orders were found to increase since the Cretaceous-Tertiary boundary. Nearly 20% of chromosome breakpoint regions were reused during mammalian evolution; these reuse sites are also enriched for centromeres. Analysis of gene content in and around evolutionary breakpoint regions revealed increased gene density relative to the genome-wide average. We found that segmental duplications populate the majority of primate-specific breakpoints and often flank inverted chromosome segments, implicating their role in chromosomal rearrangement. 相似文献
82.
Squyres SW Aharonson O Clark BC Cohen BA Crumpler L de Souza PA Farrand WH Gellert R Grant J Grotzinger JP Haldemann AF Johnson JR Klingelhöfer G Lewis KW Li R McCoy T McEwen AS McSween HY Ming DW Moore JM Morris RV Parker TJ Rice JW Ruff S Schmidt M Schröder C Soderblom LA Yen A 《Science (New York, N.Y.)》2007,316(5825):738-742
Home Plate is a layered plateau in Gusev crater on Mars. It is composed of clastic rocks of moderately altered alkali basalt composition, enriched in some highly volatile elements. A coarsegrained lower unit lies under a finer-grained upper unit. Textural observations indicate that the lower strata were emplaced in an explosive event, and geochemical considerations favor an explosive volcanic origin over an impact origin. The lower unit likely represents accumulation of pyroclastic materials, whereas the upper unit may represent eolian reworking of the same pyroclastic materials. 相似文献
83.
Davis ST Benson BG Bramson HN Chapman DE Dickerson SH Dold KM Eberwein DJ Edelstein M Frye SV Gampe RT Grifffen RJ Harris PA Hassell AM Holmes WD Hunter RN Knick VB Lackey K Lovejoy B Luzzio MJ Murray D Parker P Rocque WJ Shewchuk-Chapman L Veal JM Walker DH Kuyper LF 《Science (New York, N.Y.)》2002,298(5602):2327
84.
85.
Armitage SJ Jasim SA Marks AE Parker AG Usik VI Uerpmann HP 《Science (New York, N.Y.)》2011,331(6016):453-456
The timing of the dispersal of anatomically modern humans (AMH) out of Africa is a fundamental question in human evolutionary studies. Existing data suggest a rapid coastal exodus via the Indian Ocean rim around 60,000 years ago. We present evidence from Jebel Faya, United Arab Emirates, demonstrating human presence in eastern Arabia during the last interglacial. The tool kit found at Jebel Faya has affinities to the late Middle Stone Age in northeast Africa, indicating that technological innovation was not necessary to facilitate migration into Arabia. Instead, we propose that low eustatic sea level and increased rainfall during the transition between marine isotope stages 6 and 5 allowed humans to populate Arabia. This evidence implies that AMH may have been present in South Asia before the Toba eruption. 相似文献
86.
Armbrust EV Berges JA Bowler C Green BR Martinez D Putnam NH Zhou S Allen AE Apt KE Bechner M Brzezinski MA Chaal BK Chiovitti A Davis AK Demarest MS Detter JC Glavina T Goodstein D Hadi MZ Hellsten U Hildebrand M Jenkins BD Jurka J Kapitonov VV Kröger N Lau WW Lane TW Larimer FW Lippmeier JC Lucas S Medina M Montsant A Obornik M Parker MS Palenik B Pazour GJ Richardson PM Rynearson TA Saito MA Schwartz DC Thamatrakoln K Valentin K Vardi A Wilkerson FP Rokhsar DS 《Science (New York, N.Y.)》2004,306(5693):79-86
Diatoms are unicellular algae with plastids acquired by secondary endosymbiosis. They are responsible for approximately 20% of global carbon fixation. We report the 34 million-base pair draft nuclear genome of the marine diatom Thalassiosira pseudonana and its 129 thousand-base pair plastid and 44 thousand-base pair mitochondrial genomes. Sequence and optical restriction mapping revealed 24 diploid nuclear chromosomes. We identified novel genes for silicic acid transport and formation of silica-based cell walls, high-affinity iron uptake, biosynthetic enzymes for several types of polyunsaturated fatty acids, use of a range of nitrogenous compounds, and a complete urea cycle, all attributes that allow diatoms to prosper in aquatic environments. 相似文献
87.
Peter J. Kappes Gerard J. McChesney Michael W. Parker Harry R. Carter Stephen W. Kress Richard T. Golightly 《Biological conservation》2011,(1):620-626
Social attraction is a useful technique for re-establishing or relocating waterbird colonies and other species groups. However, little information exists regarding how newly attending individuals behave when the social environment is influenced predominantly by artificial stimuli. To help assess early progress of colony re-establishment, we compared time-activity budgets of common murres (Uria aalge) at a social attraction site (Devil’s Slide Rock; DSR) in central California with two nearby reference colonies during the first 3 years (1996–1998) of efforts. Murres at all colonies spent over 95% of their time engaged in resting, comfort, courtship, and alert activities during the pre-breeding period and over 88% of their time in similar activities during the breeding period. Although patterns were similar overall, comparisons of pooled and year-specific time budget data revealed significant differences between all three colonies, especially during pre-breeding. Murres at DSR typically engaged in comfort behaviors less frequently and in alert and courtship behaviors more frequently than reference colonies. Differences likely were due to recent re-establishment, including lower bird densities and higher proportions of non-breeders and first-time breeders at DSR, along with other factors such as disturbance. Results indicate that newly attracted birds at DSR behaved “normally” even though the social environment was influenced predominantly by artificial stimuli. Furthermore, re-established breeding in the first year of efforts, subsequent colony growth, and high productivity reflected successful restoration efforts. Thus, time-activity budgets can provide important measures of early progress of social attraction efforts and as such can be used to inform adaptive management decisions. 相似文献
88.
Gunning YM Parker R Ring SG Rigby NM Wegg B Blake A 《Journal of agricultural and food chemistry》2000,48(2):395-399
The compositions at which amorphous ethanol-maltose-water mixtures exhibit liquid-liquid separation have been determined in the temperature range from 20 to 80 degrees C. At water contents below approximately 20% w/w two phases were observed, with the maltose-rich phase slightly richer in water. Partition coefficients of organic nonelectrolytes ranging in hydrophobicity from 1, 2-ethanediol and 1,2-propanediol to benzyl alcohol and propyl acetate have been measured for octanol/sorbitol, benzyl alcohol/sorbitol, and 1-butanol/sorbitol mixtures. Linear correlations were found between the log partition coefficients in the various solvent systems. Replacing water with sorbitol results in more organic partitioning into the octanol. Replacing octanol with benzyl alcohol or 1-butanol also results in more organic partitioning into the hydrophobic phase. The results establish a relationship with partition coefficients for octanol/water mixtures, which are well studied experimentally and for which predictive approaches exist. The implications of these results for flavor retention and encapsulation are discussed. 相似文献
89.
Gunning YM Gunning PA Kemsley EK Parker R Ring SG Wilson RH Blake A 《Journal of agricultural and food chemistry》1999,47(12):5198-5205
The effects of water content and temperature variation on the release of flavor components into the headspace over flavors, encapsulated by an extrusion process, in low water content carbohydrate matrixes is studied. The largest amounts of release occurred when the matrix was above its glass transition temperature, whether this was due to increased water content or elevated temperature. Under these conditions up to 70% of the sucrose in the matrix crystallized over a period of 10 days, as quantified using Fourier transform Raman spectroscopy. Smaller amounts of headspace release occurred when the water content of the encapsulated flavor system was decreased from 3. 5 to 3.1% w/w. Small amounts of release occurred from the "as prepared" materials, which were associated with the presence of small amounts of unencapsulated flavor oil with direct access to the headspace. It was concluded that release due to matrix permeability was relatively slow as compared with the above mechanisms. 相似文献
90.
Short- and long-term effects of subsoiling four soils were closely related to variations in soil structure and consistence. Penetration resistance decreased when shear failure was accomplished by fracturing. Plastic failure in a near saturated, clayey subsoil did not affect penetration resistance. Penetration resistance of a firm, clayey subsoil with weak to moderately strong structure was lower than presubsoiling levels 2 years after subsoiling. In soils with strong structure, fracturing may occur between peds, which in time settle back to their approximate original configuration. More intra-pedal fracturing probably occurs in soils with weaker structure, thus reducing average ped size and increasing interpedal macropore volume, even after settling. Subsoiling of a loose, sandy soil decreased penetration resistance for at least 4 months, but after 4 years no residual effect was found.Only in the sandy soil did subsoiling have any noticeable effect on soil moisture content. Vertical filling of the blade and mole channels with A1 horizon material resulted in increased moisture content in the mole channel 4 months after subsoiling. After 4 years, moisture content in the A1 horizon remained higher in the vicinity of the channel. This may be due to increased upward movement of water through the more highly organic A1 horizon material that had infiltrated along the blade channel. 相似文献