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   17597篇
  免费   91篇
  国内免费   1篇
林业   3837篇
农学   1320篇
基础科学   147篇
  2912篇
综合类   793篇
农作物   2119篇
水产渔业   1832篇
畜牧兽医   1627篇
园艺   1132篇
植物保护   1970篇
  2021年   24篇
  2020年   16篇
  2019年   11篇
  2018年   2768篇
  2017年   2728篇
  2016年   1225篇
  2015年   93篇
  2014年   52篇
  2013年   74篇
  2012年   857篇
  2011年   2190篇
  2010年   2137篇
  2009年   1274篇
  2008年   1355篇
  2007年   1617篇
  2006年   78篇
  2005年   138篇
  2004年   141篇
  2003年   187篇
  2002年   99篇
  2001年   23篇
  2000年   65篇
  1999年   20篇
  1998年   9篇
  1993年   16篇
  1992年   16篇
  1991年   18篇
  1990年   14篇
  1989年   27篇
  1988年   22篇
  1987年   20篇
  1986年   10篇
  1985年   15篇
  1983年   12篇
  1979年   13篇
  1978年   11篇
  1977年   10篇
  1975年   8篇
  1974年   9篇
  1973年   17篇
  1972年   16篇
  1971年   13篇
  1970年   12篇
  1969年   13篇
  1968年   12篇
  1967年   13篇
  1931年   8篇
  1930年   9篇
  1929年   28篇
  1926年   11篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
3.
Humoral immune responses of black-footed penguins (Spheniscus demersus) to DNA-mediated immunization with a beta-galactosidase reporter gene expression plasmid were evaluated. Six male and 6 female adult penguins received either test plasmid, pCMV-beta, containing the beta-galactosidase gene or control plasmid, pCI, lacking a gene for expression. Three birds from each group were used previously in a diluent control group and given one injection of sterile saline. All samples were screened for anti-beta-galactosidase antibodies by indirect enzyme-linked immunosorbent assay with anti-chicken immunoglobulin G as secondary antibody. Antibodies to beta-galactosidase were detected in the sera of pCMV-beta-inoculated penguins, with a peak response on day 21. Antibody titers of the test plasmid group versus both control groups on days 21, 28, and 42 differed significantly. These results demonstrate that black-footed penguins can be safely transfected with the gene encoding beta-galactosidase and will mount a humoral response against the in vivo-expressed protein. Knowledge from this initial study can be applied to the development of DNA-mediated vaccines against specific infectious diseases of penguins.  相似文献   
4.
5.
Twelve Angus X Hereford steers were assigned to either a control high-energy diet or a test diet consisting of 20% rapeseed at the expense of 20% corn. Twelve pigs were allotted to a control diet and two test diets containing either 10 or 20% canola oil (CO). Both CO and oil in the rapeseed contained 60 to 64% oleic acid. Cattle fed rapeseed exhibited little effect from the diet due to apparent indigestibility of the rapeseed. Total saturated fatty acids decreased from 40% in adipose tissue of the control pigs to 15% in the 20% CO-fed pigs. The ratio of monounsaturated to saturated fatty acids (M/S) increased from 1.19 in adipose tissue from control pigs to 3.63 with the addition of 20% CO to the diet. In muscle, the M/S ratio increased from 1.21 in control pigs to 2.46 in the 20% CO treatment group. The percentage of the saturated fatty acids in muscle decreased from 42% in the control to 23% in the 20% CO treatment. Significant increases in "oiliness" and decreases in fat firmness were observed when increasing levels of canola oil were fed. Sensory traits, cooking loss and shear-force values of pork chops were similar among treatment groups. In conclusion, monounsaturated fatty acid content can be elevated substantially in pork without adversely influencing the quality of the meat, thus producing a product perceived to be more healthful by the consumer.  相似文献   
6.
The tissue reaction to Cysticercus bovis in the lung of cattle with an experimental infection was an inflammatory rim originating in the immediate vicinity of the cysts. The cysts recovered at days 83 and 102 p.i. contained living cysticerci. The rim was composed either of a layer of high histiocytes organized in palisades (at day 83 p.i.), or a lyer of flat histiocytes (at day 102 p.i.). The outer layer of the rim consisted of fibroblasts, reticular cells and a different number of eosinophil- and neutrophil luekocytes. On the periphery, the rim was formed by granulation tissue infiltrated with lymphoplasmocytes. At the border between the layers of the inflammatory rim there were conspicuous foci of a necrotic appearance typical of a tissue reaction to C. bovis.  相似文献   
7.
Engineering resistance against various diseases and pests is hampered by the lack of suitable genes. To overcome this problem we started a research program aimed at obtaining resistance by transfecting plants with genes encoding monoclonal antibodies against pathogen specific proteins. The idea is that monoclonal antibodies will inhibit the biological activity of molecules that are essential for the pathogenesis. Potato cyst nematodes are chosen as a model and it is thought that monoclonal antibodies are able to block the function of the saliva proteins of this parasite. These proteins are, among others, responsible for the induction of multinucleate transfer cells upon which the nematode feeds. It is well documented that the ability of antibodies to bind molecules is sufficient to inactivate the function of an antigen and in view of the potential of animals to synthesize antibodies to almost any molecular structure, this strategy should be feasible for a wide range of diseases and pests.Antibodies have several desirable features with regard to protein engineering. The antibody (IgG) is a Y-shaped molecule, in which the domains forming the tips of the arms bind to antigen and those forming the stem are responsible for triggering effector functions (Fc fragments) that eliminate the antigen from the animal. Domains carrying the antigen-binding loops (Fv and Fab fragments) can be used separately from the Fc fragments without loss of affinity. The antigen-binding domains can also be endowed with new properties by fusing them to toxins or enzymes. Antibody engineering is also facilitated by the Polymerase Chain Reaction (PCR). A systematic comparison of the nucleotide sequence of more than 100 antibodies revealed that not only the 3′-ends, but also the 5′-ends of the antibody genes are relatively conserved. We were able to design a small set of primers with restriction sites for forced cloning, which allowed the amplification of genes encoding antibodies specific for the saliva proteins ofGlobodera rostochiensis. Complete heavy and light chain genes as well as single chain Fv fragments (scFv), in which the variable parts of the light (VL) and heavy chain (VH) are linked by a peptide, will be transferred to potato plants. A major challenge will be to establish a correct expression of the antibody genes with regard to three dimensional folding, assembly and intracellular location.  相似文献   
8.
L11A-Fukushima (L11A-F) derived from attenuated isolate LuA of Tomato mosaic virus (ToMV) has the highest ability to cross protect against virulent ToMV among LuA and its derivatives and is stably inherited. Growth, yield, fruit quality and symptom attenuation of inoculated tomato plants did not differ significantly between L11A-F and L11A. The infectivity of progeny viruses in tomato infected with LuA-F was less than 4% of that with virulent ToMV. From these results, L11A-F appears to possess the properties necessary for practical use. To manage L11A-F strictly, a PCR-based assay to detect trace contamination of virulent ToMV in L11A-F preparations was established. Received 10 June 2002/ Accepted in revised form 30 October 2002  相似文献   
9.
DAS-ELISA proved to be reliable enough to detect a latent infection by Tomato spotted wilt virus (TSWV) in asymptomatic stock plants of chrysanthemum. A high density of Frankliniella occidentalis, the predominant vector, in the presence of latently infected stock plants resulted in a high incidence of disease in the chrysanthemum production field. The incidence of disease was low when the vector thrips were not abundant in spite of the presence of latently infected stock plants. These results suggest that an infestation of the vector thrips causes severe secondary spread of TSWV originating from latently infected stock plants in chrysanthemum production fields. Received 27 July 2001/ Accepted in revised form 27 November 2001  相似文献   
10.
Seed-grown trees and six clonal lines of 3·5–4·5-year-old Eucalyptus marginata (jarrah) growing in a rehabilitated bauxite mine site in the jarrah forest were underbark-inoculated on lateral branches (1995) or simultaneously on lateral branches and lateral roots (1996) with isolates of Phytophthora cinnamomi in late autumn. Individual seedlings from which the clonal lines were derived had previously been assessed as either resistant (RR) or susceptible (SS) to P. cinnamomi . At harvest, the acropetal lesion and colonization lengths were measured. Overall, the length of colonization in roots and branches was more consistent as a measure of resistance than lesion length, because colonization length recorded the recovery of P. cinnamomi from macroscopically symptomless tissue ahead of the lesion which, on some occasions, was up to 6 cm. In both trials, one RR clonal line was able to contain the P. cinnamomi isolates consistently, as determined by small lesion and colonization lengths in branches and roots. In contrast, the remaining two RR clonal lines used in both trials were no different from the SS line in their ability to contain lesions or colonization. These latter two RR lines may therefore not be suitable for use in rehabilitation of P. cinnamomi -infested areas. Differences in lesion and colonization lengths among P. cinnamomi isolates occurred only in the 1995 trial. Colonization and lesion lengths in branches were up to eight times greater in 1996 than in 1995, but the relative rankings of clonal lines were consistent between trials. Although colonization was always greater in branches than roots, the relative rankings of the lines were similar between branch and root inoculations. Branch inoculations are a valid option for testing the resistance and susceptibility of young jarrah trees to P. cinnamomi .  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