首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   340篇
  免费   12篇
林业   13篇
农学   11篇
基础科学   2篇
  51篇
综合类   87篇
农作物   13篇
水产渔业   25篇
畜牧兽医   95篇
园艺   4篇
植物保护   51篇
  2022年   3篇
  2020年   6篇
  2019年   9篇
  2018年   6篇
  2017年   8篇
  2016年   5篇
  2015年   4篇
  2014年   9篇
  2013年   18篇
  2012年   14篇
  2011年   19篇
  2010年   4篇
  2009年   9篇
  2008年   11篇
  2007年   15篇
  2006年   9篇
  2005年   20篇
  2004年   14篇
  2003年   15篇
  2002年   22篇
  2001年   10篇
  2000年   6篇
  1999年   8篇
  1998年   5篇
  1997年   2篇
  1996年   3篇
  1995年   7篇
  1992年   5篇
  1991年   5篇
  1989年   6篇
  1988年   4篇
  1987年   3篇
  1986年   4篇
  1985年   7篇
  1984年   4篇
  1983年   3篇
  1982年   5篇
  1981年   5篇
  1977年   2篇
  1975年   2篇
  1974年   2篇
  1972年   2篇
  1971年   2篇
  1969年   3篇
  1950年   2篇
  1946年   2篇
  1923年   1篇
  1913年   1篇
  1903年   1篇
  1889年   1篇
排序方式: 共有352条查询结果,搜索用时 15 毫秒
71.
72.
Taste pathways in rat brainstem   总被引:3,自引:0,他引:3  
By means of a combination of electrophysiological and anatomical procedures, the projections of the anterior portion of the solitary nucleus were traced to the parabrachial nuclei in the pons, structures hitherto not considered to be included in the taste pathway. Responses to taste stimuli were recorded from this pontine area. Lesions in the pontine taste area resulted in degeneration of fibers reaching the lingual area in the thalamus.  相似文献   
73.
Property rights over natural resources became a distinct area of inquiry in environmental economics and policy in the last decades, but their role has not yet been investigated thoroughly. Transition countries represent an excellent material of analysis of various policies and institutional developments concerning the regime of use and management of natural resources. The processes of societal transformation had deep impacts on the forestry sector, entailing land reforms and subsequent changes to its institutional and organisational framework. This paper presents an analysis of the reciprocal relationship between the evolving forest property rights and the conduct of policy and economic actors, in connection with their outcomes. The theoretical framework consists of institutional economics, whose core concept is that patterns of interaction between institutions and actors produce physical outcomes, assessable by criteria such as equity and efficiency. The study concentrated on three distinct periods and the applicable property regimes. Within this framework, the impacts of the characteristics of property regimes and the general framework of socio-economic conditions on the exercise of property rights were analysed. The analysis of the conduct of forest owners in relation to the institutional design was completed by the analysis of the other actors influential for land reforms. Romanian forestry sector, in evolution from the period shortly before World War II to present, represents the case of the study. Research data were collected by interviewing/questioning, participant observation and literature review. They were analysed through an integrated method of content analysis and a matrix analysis. An important conclusion is that not only the regime of forest property is determinant for the outcomes of resource use and management, but also the general settings in which they are embedded. Another essential point is that land reforms driven by either efficiency or equity rationales are not mere diversions of the benefit stream, but complex processes with serious implications for the status of the resource at stake.  相似文献   
74.
Mitochondrial DNA sequences isolated from ancient dog remains from Latin America and Alaska showed that native American dogs originated from multiple Old World lineages of dogs that accompanied late Pleistocene humans across the Bering Strait. One clade of dog sequences was unique to the New World, which is consistent with a period of geographic isolation. This unique clade was absent from a large sample of modern dogs, which implies that European colonists systematically discouraged the breeding of native American dogs.  相似文献   
75.
Plague is a zoonotic disease (transmitted mainly by fleas and maintained in nature by rodents) that causes severe acute illness in humans. We present a human plague case who became infected by the bite of a wild Gunnison's prairie dog, and a good practical example of the One Health approach that resulted in a rapid public health response. The exposure occurred while the animal was being transported for relocation to a wildlife refuge after being trapped in a plague enzootic area. This is the first report of a human plague case resulting from the bite of a Gunnison's prairie dog. Additionally, we present an observation of a longer incubation period for plague in captive prairie dogs, leading to a recommendation for a longer quarantine period for prairie dogs during translocation efforts.  相似文献   
76.
Isovolumetric geochemical analyses of 26 weathering profiles (saprolites) on crystalline rocks in Brazil and South Carolina indicate that, on average, Si and Al are leached from rocks in a long-terms molar ratio of 8.3. Hewever, with the exception of the Rio Negro River, the molar ratio of Si to Al in rivers draining these regions is about 100. If saprolites are still forming, the large discrepancy in Al mobilization between the saprolite record and modern river chemistry may be due to degassing of dissolved CO2 from soil water entering streams with attendant increase in pH and Al precipitation. If saprolites are relic, the Rio Negro basin may be a model for the environmental conditions under which they form. These conditions are 1) low relief, 2) abundant rainfall and intense leaching and 3) organic rich soils which produce large concentrations of CO2 and dissolved organic acids in the soil. Abundant rainfall maintains dilute soil water; large CO2 pressure lowers pH; dissolved organic matter complexes Al (and Fe) and thus enhances the solubility of secondary clays. All of these effects hinder the precipitation of secondary weathering products and thus promote rock dissolution that is more nearly congruent. The problem with the Rio Negro model is to reconcile the low relief of the basin (which inhibits rapid groundwater movement) with the short contact time indicated by the diluteness of its chemistry.  相似文献   
77.
The area burned by wildfire in the states of Arizona and New Mexico in the southwestern US has been increasing in recent years. In many cases, high severity burns have caused dramatic increases in runoff and sediment yield from burned watersheds. This paper describes the potential and limitations of the HEC6T sediment transport model to describe changes in channel scour and deposition following the Cerro Grande fire near Los Alamos, New Mexico. Following the fire, Pueblo Canyon, near Los Alamos, was subject to a peak flow two orders of magnitudes higher than any discharge in the 7-year period of record, and twice the initial post-fire estimate of the 100-year event. HEC6T requires that the limits of scour and deposition on a cross-section be specified prior to application. This was achieved by using geomorphologic principles, predicted post-burn hydrology and long-term estimates of channel change derived from air photos, to estimate post-fire channel widths. Because significant quantities of silt and clay were present in the runoff, erosion shear stress and erosion rate parameters for cohesive sediments had to be obtained experimentally. After a sensitivity analysis, an optimization routine was used to estimate the optimal model parameter values for sensitive parameters. HEC6T was able to accurately model the change in cumulative sediment volume change derived from Airborne Laser Swath Mapping (ALSM, often called Lidar) taken before and after the large post-fire event. One discrepancy between the HEC6T model prediction and the ALSM-estimated change was that the ALSM-estimated change showed the greatest amount of deposition in a portion of the canyon with increasing slope, which the HEC6T model did not predict. Any sediment transport model will predict increased sediment transport capacity with increasing energy slope, so that it was considered to be beyond the capability of any sediment transport model to predict this deposition. Therefore, HEC6T simulated the overall changes in scour and deposition within reasonable expectation of the capabilities of physically-based sediment transport modeling indicating that it is capable of modeling sediment transport in ephemeral channels following wildfire.  相似文献   
78.
A theoretical analysis of the contribution to rainwater sulfate concentration by precipitation scavenging of gaseous S02 and sulfate containing aerosols is presented. Aspects, such as the proper choice of mean raindrop diameter, are discussed in detail, and guidelines for their use are explored. Sample calculations are provided in which emissions from a hypothetical stack are considered as the source of the gaseous S02 and sulfate aerosol. The basic assumption of irreversible sorption behavior is discussed and utilized to obtain an upper limit to the resulting sulfate concentration. The effect of raindrop diameter on the sulfate concentration is not consistent. However, for a given drop diameter the sulfate concentration decreases as the precipitation rate increases. The sulfate concentration resulting from aerosol scavenging depends on the particle diameter and the precipitation rate. The sulfate concentration shows a maximum with respect to particle size; at moderate distances from the source, it is maximum for particles having aerodynamic equivalent diameter of about 5 to 10 gm.  相似文献   
79.
Yellow passion fruit (Passiflora edulis f. flavicarpa Degener) rind pectic substances were fractionated with water, ammonium oxalate, and dilute acid solutions. The extracted pectins were rich in anhydrogalacturonic acid and had a low degree of methyl esterification. Moreover, their acetyl groups and neutral sugar contents were relatively low. Furthermore, a low amount of proteinaceous material was also found within them. Their gelling ability and viscoelastic properties as evaluated by the SAG and small amplitude oscillatory shear tests, respectively, were comparable to those of a commercial citrus low-methoxyl pectin. Hence, yellow passion fruit rind occurs as a potentially good source of naturally low-methoxyl pectin.  相似文献   
80.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号