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   499篇
  免费   54篇
  国内免费   2篇
林业   28篇
农学   3篇
基础科学   1篇
  34篇
综合类   71篇
农作物   5篇
水产渔业   45篇
畜牧兽医   336篇
园艺   5篇
植物保护   27篇
  2023年   6篇
  2021年   7篇
  2020年   17篇
  2019年   15篇
  2018年   16篇
  2017年   15篇
  2016年   11篇
  2015年   11篇
  2014年   15篇
  2013年   25篇
  2012年   19篇
  2011年   26篇
  2010年   16篇
  2009年   14篇
  2008年   22篇
  2007年   14篇
  2006年   19篇
  2005年   19篇
  2004年   16篇
  2003年   20篇
  2002年   14篇
  2001年   22篇
  2000年   9篇
  1999年   10篇
  1998年   6篇
  1997年   4篇
  1994年   5篇
  1993年   6篇
  1992年   12篇
  1991年   12篇
  1990年   9篇
  1989年   12篇
  1988年   7篇
  1987年   7篇
  1985年   5篇
  1982年   3篇
  1981年   5篇
  1980年   5篇
  1979年   11篇
  1978年   5篇
  1977年   3篇
  1976年   4篇
  1974年   3篇
  1973年   5篇
  1972年   4篇
  1971年   3篇
  1970年   5篇
  1969年   3篇
  1967年   4篇
  1958年   3篇
排序方式: 共有555条查询结果,搜索用时 15 毫秒
541.
Matrix metalloproteinases (MMPs) and their inhibitors (TIMPs) are involved in several reproductive events like oocyte–spermatozoa interaction and semen liquefaction. In order to study their role in the llama oviductal reproductive process, MMP activity in oviductal fluid (OF) was assayed. Considering that llama genome sequences are partially known, a strategy to procure cDNA sequences of MMP‐2, MMP‐9, TIMP‐1 and TIMP‐2 was designed. Afterwards, their expression patterns in the different llama oviductal segments were assayed. Gelatine zymograms detected 62 and 94 kDa protease activities that matched MMP‐2 and pro‐MMP‐9, respectively. Expression pattern analysis showed that MMP and TIMP mRNAs were present in ampulla, isthmus, utero‐tubal junction (UTJ) and papilla. Altogether, these findings support the argument that MMPs/TIMPs are produced in the oviduct and secreted into the oviductal lumen. Our results encourage further studies to elucidate the role of these proteins in reproductive oviductal events.  相似文献   
542.
Abstract

During August 1986 when there was a build up of Parasa lepida Cramer on coconut in the study area in Kerala, India, many dead larvae were found on the underside of the leaves. A few of these cadavers which were suspected to be infected by a virus were collected. Pathogenicity tests proved positive. PIBs were negative to stain by Giemsa, and section of the polyhedra shows that the virions are rod shaped. The pathogen is a multiple embedded baculovirus.  相似文献   
543.
544.
Abstract

Edwardsiella ictaluri was consistently isolated from the spleens, livers, and head kidneys of diseased Nile tilapia Oreochromis niloticus from a farm experiencing mortality events in several culture ponds. We describe the first published outbreak of E. ictaluri–induced edwardsiellosis in Nile tilapia. Pure cultures of the isolated bacteria were characterized both biochemically and molecularly. Biochemical analysis was performed using the API-20E and RapID One systems, and antimicrobial susceptibility was determined by the broth microdilution method. Molecular analysis involved sequencing of the 16S rRNA gene, species-specific real-time polymerase chain reaction (PCR), and PCR-mediated genomic fingerprinting (rep-PCR). Pairwise sequence analysis of the 16S rRNA gene identified the case isolates to be a 100% match to E. ictaluri cultured from channel catfish in the southeastern United States. However, rep-PCR analysis identified the case isolates to be genetically different from representative strains isolated from disease outbreaks in cultured channel catfish in Mississippi. Infectivity challenges (intraperitoneal injection and immersion) demonstrated that a representative E. ictaluri strain isolated from tilapia was pathogenic to naïve tilapia, reproducing clinical signs and mortality, thereby establishing Koch's postulates.

Received August 30, 2011; accepted January 30, 2012  相似文献   
545.
Abstract

A quantitative polymerase chain reaction (qPCR) assay was developed for the detection and quantification of Edwardsiella ictaluri in channel catfish Ictalurus punctatus pond water using modifications to a published E. ictaluri–specific qPCR assay and previously established protocols for the molecular detection of myxozoan parasites in catfish ponds. Genomic DNA equivalents indicative of the number of bacteria in a sample were determined and standard curves correlating to bacterial numbers were established. The assay was found to be highly repeatable and reproducible, with a linear dynamic range of five orders of magnitude. There was no interference of the assay from the presence of large quantities of nontarget DNA. Known quantities of bacteria were added to sample volumes of 40 or 500 mL of pond water collected from several different ponds. The minimum level of detection was approximately 100 cell equivalents (CE) in 40 (2.5 CE/mL) or 500 mL of pond water (0.2 CE/mL). Sample volumes of 40 mL yielded the most consistent results, which were not significantly different from those obtained from broth culture alone. Cell equivalents determined by qPCR in 40-mL pond water samples spiked with known quantities of bacteria were within one order of magnitude of the actual number of cells added. Repetitive element-based polymerase chain reaction analysis of archived isolates demonstrated the genetic homogeneity of E. ictaluri, and consistent amplification of these isolates by qPCR analysis demonstrated the stability of the PCR target. The assay described here provides a reliable method for the detection and quantification of E. ictaluri in pond water and will be an invaluable tool in epidemiological studies. Additionally, the assay provides a way to evaluate the effects that vaccination, antibiotic treatments, and restricted feeding practices have on E. ictaluri populations during an outbreak. Information obtained with these tools will aid in optimizing disease management practices designed to maximize productivity while minimizing losses.

Received October 20, 2010; accepted June 13, 2011  相似文献   
546.
Photographic observations from a network of 60 cameras in western Canada are used to derive the influx rate of meteorites on the earth's surface, the first time instrumental data have been used for this purpose. Forty-three observed events are believed to have dropped between 0.1 and 12 kilograms of meteorites each. The flux values are corrected for a minor latitude effect and agree with earlier estimates near 10 kilograms but vary more slowly with mass. Eight events per year drop at least 1 kilogram of meteorites in an area of 10(6) square kilometers.  相似文献   
547.
As compared to that in normal children a decreased activity of beta gliucuronidase, a lysosomal enzyme associated with mucopolysaccharide metabolism and with salt transport, has been detected in the epidermis and sweat gland tissues of children with cystic fibrosis.  相似文献   
548.
549.
Concentrations of elements such as manganese, scandium, lanthanum, rubidium, samarium, barium, and zirconium in obsidian samples from different flows show ranges of 1000 percent or more, whereas the variation in element content in obsidian samples from a single flow appears to be less than 40 percent. Neutron-activation analysis of these elements, as well as of sodium and iron, provides a means of identifying the geologic source of an archeological artifact of obsidian.  相似文献   
550.
The initial occupation of Eastern North America was by small bands of people who gained their livelihood by hunting and gathering. As time passed, the occupants of different regions be came increasingly familiar with the available natural resources. The de velopment or introduction of new tools and devices enabled the people to ex ploit their environments more effective ly, until, by Late Archaic times, popu lation size had increased, in terms both of density within a given area and num ber of people in individual social units. The initial agricultural productivity aided the culmination of the long cul tural traditions in the remarkable pro ductions and practices of the Hope wellian complexes of Middle Woodland times. The major Hopewellian centers reflect the marked change in societal organization and patterns. The costumes of the dead and the manner of their burial clearly reflect individual status differentiations, while artistic creativity is probably to be equated with incipient specialization of labor. Many of the Mississippian societies reached an even more advanced plateau of cultural de velopment, with fortified towns, an or ganized priesthood, dominant hereditary chiefs, political and military alliances, and a well-developed class system. The details of the cultural develop ment in Eastern North America are unique, but the general trend may be regarded as a common one in human society, and the patterns of behavior, as analogous to those developed by other peoples in other areas of the Old World and the New.  相似文献   
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