全文获取类型
收费全文 | 1051篇 |
免费 | 50篇 |
专业分类
林业 | 124篇 |
农学 | 24篇 |
基础科学 | 2篇 |
188篇 | |
综合类 | 124篇 |
农作物 | 42篇 |
水产渔业 | 50篇 |
畜牧兽医 | 484篇 |
园艺 | 11篇 |
植物保护 | 52篇 |
出版年
2021年 | 14篇 |
2020年 | 19篇 |
2019年 | 12篇 |
2018年 | 17篇 |
2017年 | 19篇 |
2016年 | 20篇 |
2015年 | 11篇 |
2014年 | 11篇 |
2013年 | 40篇 |
2012年 | 35篇 |
2011年 | 35篇 |
2010年 | 32篇 |
2009年 | 28篇 |
2008年 | 44篇 |
2007年 | 38篇 |
2006年 | 34篇 |
2005年 | 34篇 |
2004年 | 40篇 |
2003年 | 39篇 |
2002年 | 16篇 |
2001年 | 18篇 |
2000年 | 19篇 |
1999年 | 22篇 |
1998年 | 12篇 |
1994年 | 11篇 |
1992年 | 20篇 |
1991年 | 30篇 |
1990年 | 15篇 |
1989年 | 29篇 |
1988年 | 10篇 |
1987年 | 21篇 |
1986年 | 22篇 |
1985年 | 20篇 |
1984年 | 12篇 |
1982年 | 9篇 |
1980年 | 9篇 |
1979年 | 15篇 |
1978年 | 13篇 |
1977年 | 8篇 |
1976年 | 9篇 |
1975年 | 8篇 |
1974年 | 15篇 |
1973年 | 11篇 |
1972年 | 14篇 |
1970年 | 20篇 |
1969年 | 11篇 |
1968年 | 14篇 |
1967年 | 21篇 |
1966年 | 9篇 |
1936年 | 7篇 |
排序方式: 共有1101条查询结果,搜索用时 0 毫秒
81.
An instrument providing a new, rapid, and accurate method of determining the number and critical radii of condensation nuclei with radii under 200 angstroms is described. Based on the principle of the cloud chamber, the instrument measures transient changes in the attenuation and scattering of a monochromatic light beam by the growing fog droplets. From data obtained the absolute number concentration and radii of condensation nuclei can be calculated. Preliminary studies of aerosol formation in beta-irradiated mixtures of air and sulfur dioxide showed that carbon monoxide and methane inhibit the formation of nuclei; relative rate constants can be deduced. Some applications of this instrument for environmental and basic research are pointed out. 相似文献
82.
83.
84.
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
85.
Liu HH Schmidt S Poulsen HF Godfrey A Liu ZQ Sharon JA Huang X 《Science (New York, N.Y.)》2011,332(6031):833-834
Over the past decade, efforts have been made to develop nondestructive techniques for three-dimensional (3D) grain-orientation mapping in crystalline materials. 3D x-ray diffraction microscopy and differential-aperture x-ray microscopy can now be used to generate 3D orientation maps with a spatial resolution of 200 nanometers (nm). We describe here a nondestructive technique that enables 3D orientation mapping in the transmission electron microscope of mono- and multiphase nanocrystalline materials with a spatial resolution reaching 1 nm. We demonstrate the technique by an experimental study of a nanocrystalline aluminum sample and use simulations to validate the principles involved. 相似文献
86.
Trans-endocytosis of CD80 and CD86: a molecular basis for the cell-extrinsic function of CTLA-4 总被引:1,自引:0,他引:1
Qureshi OS Zheng Y Nakamura K Attridge K Manzotti C Schmidt EM Baker J Jeffery LE Kaur S Briggs Z Hou TZ Futter CE Anderson G Walker LS Sansom DM 《Science (New York, N.Y.)》2011,332(6029):600-603
Cytotoxic T lymphocyte antigen 4 (CTLA-4) is an essential negative regulator of T cell immune responses whose mechanism of action is the subject of debate. CTLA-4 shares two ligands (CD80 and CD86) with a stimulatory receptor, CD28. Here, we show that CTLA-4 can capture its ligands from opposing cells by a process of trans-endocytosis. After removal, these costimulatory ligands are degraded inside CTLA-4-expressing cells, resulting in impaired costimulation via CD28. Acquisition of CD86 from antigen-presenting cells is stimulated by T cell receptor engagement and observed in vitro and in vivo. These data reveal a mechanism of immune regulation in which CTLA-4 acts as an effector molecule to inhibit CD28 costimulation by the cell-extrinsic depletion of ligands, accounting for many of the known features of the CD28-CTLA-4 system. 相似文献
87.
Induction of AIDS-like disease in macaque monkeys with T-cell tropic retrovirus STLV-III 总被引:74,自引:0,他引:74
N L Letvin M D Daniel P K Sehgal R C Desrosiers R D Hunt L M Waldron J J MacKey D K Schmidt L V Chalifoux N W King 《Science (New York, N.Y.)》1985,230(4721):71-73
The T-cell tropic retrovirus of macaque monkeys STLV-III has morphologic, growth, and antigenic properties indicating that it is related to HTLV-III/LAV, the etiologic agent of the acquired immune deficiency syndrome (AIDS) in humans. Four of six rhesus monkeys died within 160 days of STLV-III inoculation with a wasting syndrome, opportunistic infections, a primary retroviral encephalitis, and immunologic abnormalities including a decrease in T4+ peripheral blood lymphocytes. These data show that an immunodeficiency syndrome can be produced experimentally in a nonhuman primate by an agent from the HTLV-III/LAV group of retroviruses. The STLV-III-macaque system will thus provide a useful model for the study of antiviral agents and vaccine development for human AIDS. 相似文献
88.
A global view of gene activity and alternative splicing by deep sequencing of the human transcriptome 总被引:6,自引:0,他引:6
89.
Beef authentication and retrospective dietary verification using stable isotope ratio analysis of bovine muscle and tail hair 总被引:2,自引:0,他引:2
Osorio MT Moloney AP Schmidt O Monahan FJ 《Journal of agricultural and food chemistry》2011,59(7):3295-3305
Stable isotope ratio analysis (SIRA) was used as an analytical tool to verify the preslaughter diet of beef cattle. Muscle and tail hair samples were collected from animals fed either pasture (P), a barley-based concentrate (C), silage followed by pasture (SiP), or silage followed by pasture with concentrate (SiPC) for 1 year (n = 25 animals per treatment). The (13)C/(12)C, (15)N/(14)N, (2)H/(1)H, and (34)S/(32)S isotope ratios in muscle clearly reflected those of the diets consumed by the animals. By applying a stepwise canonical discriminant analysis, a good discrimination of bovine meat according to dietary regimen was obtained. On the basis of the classification success rate, the (13)C/(12)C and (34)S/(32)S ratios in muscle were the best indicators for authentication of beef from animals consuming the different diets. Analysis of (13)C/(12)C and (15)N/(14)N in tail hair sections provided an archival record of changes to the diet of the cattle for periods of over 1 year preslaughter. 相似文献
90.
Matthias Schmidt Andres Kiviste Klaus von Gadow 《European Journal of Forest Research》2011,130(2):303-315
This contribution presents an approach to model individual tree height?Cdiameter relationships for Scots pine (Pinus sylvestris) in multi-size and mixed-species stands in Estonia using the Estonian Permanent Forest Research Plot Network. The dataset includes 22,347 trees. The main focus of the study was to use an approach that is spatially explicit allowing for high accuracy prediction from a minimum set of predictor variables that can be easily derived. Consequently, the height?Cdiameter relationship is modeled as a function of only the stand quadratic mean diameter (dg) and the plot geographical coordinates. A specific generalized additive model gam is employed that allows for the integration of a varying coefficient term and 2-dimensional surface estimators representing a spatial trend and a spatially varying coefficient term. The high flexibility of the model is needed due to the very few predictor variables that subsume a variety of potential influential factors. Subsequently, a linear mixed model is used that quantifies the random variation between plots and between measurement occasions within plots, respectively. Hence, our model is based on the theory of structured additive regression models (Fahrmeir et al. 2007) and separates a structured (correlated) spatial effect from an unstructured (uncorrelated) spatial effect. Additionally, the linear mixed model allows for calibration of the model using height measurements as pre-information. Model bias is small, despite the somewhat irregular distribution of experimental areas within the country. The overall model shows some similarity with earlier applications in Finland. However, there are important differences involving the model form, the predictors and the method of parameter estimation. 相似文献