全文获取类型
收费全文 | 263584篇 |
免费 | 15768篇 |
国内免费 | 917篇 |
专业分类
林业 | 23912篇 |
农学 | 14116篇 |
基础科学 | 3024篇 |
43987篇 | |
综合类 | 25526篇 |
农作物 | 17422篇 |
水产渔业 | 17012篇 |
畜牧兽医 | 104792篇 |
园艺 | 6315篇 |
植物保护 | 24163篇 |
出版年
2021年 | 2577篇 |
2020年 | 2969篇 |
2019年 | 3759篇 |
2018年 | 4223篇 |
2017年 | 4622篇 |
2016年 | 4951篇 |
2015年 | 4414篇 |
2014年 | 5777篇 |
2013年 | 16383篇 |
2012年 | 6564篇 |
2011年 | 8525篇 |
2010年 | 7567篇 |
2009年 | 8142篇 |
2008年 | 7977篇 |
2007年 | 6879篇 |
2006年 | 7427篇 |
2005年 | 6694篇 |
2004年 | 6491篇 |
2003年 | 6353篇 |
2002年 | 5586篇 |
2001年 | 6339篇 |
2000年 | 5959篇 |
1999年 | 5514篇 |
1998年 | 4171篇 |
1997年 | 4250篇 |
1996年 | 3943篇 |
1995年 | 4536篇 |
1994年 | 3924篇 |
1993年 | 3618篇 |
1992年 | 4444篇 |
1991年 | 4717篇 |
1990年 | 4412篇 |
1989年 | 4392篇 |
1988年 | 3929篇 |
1987年 | 3925篇 |
1986年 | 3907篇 |
1985年 | 4134篇 |
1984年 | 3839篇 |
1983年 | 3560篇 |
1982年 | 2938篇 |
1981年 | 2842篇 |
1980年 | 2807篇 |
1979年 | 3482篇 |
1978年 | 3069篇 |
1977年 | 2873篇 |
1976年 | 2692篇 |
1975年 | 2592篇 |
1974年 | 2812篇 |
1973年 | 2794篇 |
1972年 | 2531篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
291.
E. DUVEILLER 《EPPO Bulletin》1994,24(1):135-157
The literature related to bacterial leaf streak of cereals, due to various pathovars of Xanthomonas campestris , is confusing and dispersed. This review presents a synthesis of the knowledge that has been accumulated on the disease in cereals since the early reports on barley and wheat. The aim is to present the current status on geographical distribution of the disease, to explain possible confusions in symptoms, and to show the limited quantitative information available on losses and on host/pathogen interactions. Disease etiology and means of control are also reviewed. The objective is to define the present situation of black chaff in wheat, barley and triticale, and to delineate the major research needs in these crops. Difficulties in controlling bacterial leaf streak arise as a result of limited epidemiological knowledge and of the absence of satisfactory seed treatment permitting the eradication of the bacterium. There is an urgent need for a seed certification scheme and for more information on genetic resistance, with a view to limiting outbreaks of this sporadic but devastating disease. 相似文献
292.
Muscle biopsies were taken from the middle gluteal muscle of 28 Standardbred trotters, 3-4 years of age. The 13 horses in Group T were trained consistently from 18 months of age, whereas the 15 horses in Group UT were not exposed to any systematic training before 3 years of age. Group T horses had a lower percentage of Type IIB fibres (31%) than did Group UT horses (39%). Citrate synthase (CS) activity, representing oxidative capacity, was higher in Group T (72 mmol kg-1 min-1) than in Group UT (47 mmol kg-1 min-1). Biopsies were taken from 4 horses in each group when they were foals and then annually until 3-4 years of age. Results from this study indicate that regular training of Standardbreds from 18 months of age resulted in increased CS activity and a decrease in the percentage of Type IIB fibres. This study shows that training, not growth, is the main factor that induces a high oxidative capacity and a high Type IIA/IIB fibre ratio in muscle of Standardbred trotters. 相似文献
293.
利用自制的应力-应变试验台,对草莓果实的应力-应变特性和蠕变特性进行了测定与研究,建立了草莓果实应力-应变的数学模型,指出了草莓果实的极限屈服点变动范围。同时对草莓在不同贮藏条件下流变特性的变化规律进行了初步的研究,发现草莓的成熟度、贮藏温度及贮藏时间对果实的硬度、屈服应力均有较显著的影响,建立了屈服应力随时间变化的不同阶段的回归方程,可用于预测在相应贮藏条件下的草莓果实的屈服应力。 相似文献
294.
This paper reviews the radiological features of 50 canine incisive or maxillary tumours and discusses the value of radiological assessment in the diagnosis and staging of these tumours. The 50 tumours examined included 21 sarcomas, 15 carcinomas, three melanomas and an assortment of benign tumours of the oral cavity. There was not any site specificity for the different histological tumour types within the upper dental arcade, although fibrosarcomas had a tendency to be maxillary whereas the squamous cell carcinomas were equally distributed between the incisive and maxillary regions. Seventy-eight per cent of fibrosarcomas, 82 per cent of squamous cell carcinoma and all three melanomas examined showed radiological evidence of bone involvement. Radiographic changes were also seen in the benign tumours. The pattern of growth of tumours correlated with the radiological changes observed. Malignant tumours showed a tendency to irregular or aggressive bone loss whereas bone production predominated in the benign tumours. 相似文献
295.
An account is given of the methodology for fractionation of cock spermatozoa into head and tail fractions by ultrasonication, followed by sucrose density gradient centrifugation. Quantitative estimates of DNA attested to 89.4% purity of the head fraction and low contamination of tails with heads. Recovery of protein and malic dehydrogenase (MDH) activity, following sperm fractionation, averaged 94.3% and 95.7%, respectively. Contamination of the head fraction with tails, as assessed by MDH assay, was only 4.65%, and the purity of the tail fraction was 91%. Intensive succinic dehydrogenase (SDH) activity was histochemically localised in the separated tail fraction and in the tail portion of intact spermatozoa. However, SDH activity was discernible neither in the head fraction nor in the head of intact spermatozoa. 相似文献
296.
The relationship between the levels of CO and airborne Pb was evaluated against conditions of traffic flow, ambient air temperature and the ventilation rates in 3 underground car parks. During the study period significant variations were noted in the levels of the pollutants in each of the car parks which frequently exceeded the proposed Malaysian Air Quality Standard of 0.7 μg m?3Pb and 9 μg L?1 (8 hr average) CO for indoor environs. High correlation coefficient values were obtained between the levels of CO and traffic flow and Pb and traffic flow. High traffic flow also increased the mean air temperature inside the car parks. Reductions in the ventilation rates in the car parks caused an almost two-fold increase in the level of CO and vice versa. A simple mathematical model depicting the CO concentrations in relation to other parameters was formulated. 相似文献
297.
Quantitative determination of plant opal content in soils, using a combined method of heavy liquid separation and alkali dissolution 总被引:3,自引:1,他引:2
The aim of this study was to improve the quantitative determination of the plant opal content (i.e. phytoliths) in soils. The proposed method is based on: (i) the separation of plant opal from the silt and sand fractions of the soil, using heavy liquid flotation (aqueous solution of ZnBr2, density = 1.92 g cm?3); (ii) the subsequent determination of alkali-soluble silicon by atomic absorption spectrometry. Extraction and analytical procedures were tested on a broad sample of temperate and tropical soils with very different phytolith contents. Our investigations lead to the following conclusions: (i) a selective dissolution of opal in alkaline solutions (e.g. hot 0.5 m NaOH as proposed by Jones, 1969) is inaccurate so that a sink-float method must be used before any dissolution procedure; (ii) to dissolve opal completely, a 0.5 M NaOH dissolution treatment at 150°C can be easily and successfully carried out in steel PTFE-lined pressure vessels; (iii) the reproducibility of the determination is satisfactory for a step-by-step procedure (mean coefficient of variation = 13.4%). The comparison of this new method of quantitative assessment of soil opal with two other methods (gravimetric and phytolith-counting methods), shows very highly significant correlations (P<0.001). Therefore, this procedure is a useful tool in studies connected with pedological and environmental history. 相似文献
298.
299.
Antibody responses to avian encephalomyelitis virus vaccines when administered by different routes 总被引:1,自引:0,他引:1
Antibody responses to a commercial avian encephalomyelitis virus (AEV) vaccine administered by different routes were measured by an enzyme-linked immunosorbent assay (ELISA). Responses to single doses of vaccine administered by the ocular route to 10% of a flock were comparable with those obtained when all birds received a single dose in the drinking water. However, ocular vaccination of 5% of the flock resulted in significantly lower responses than those obtained when 10% were vaccinated. Maternal antibody was shown by the ELISA to persist in chickens from vaccinated flocks for up to 21 days after hatching. Day-old chickens with serum absorbances of < 0.3 at 492 nm, as determined by the ELISA, were shown to be susceptible to intracerebral challenge with the neurotropic Van Roekel strain of AEV. 相似文献
300.