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   875篇
  免费   43篇
林业   48篇
农学   25篇
  128篇
综合类   77篇
农作物   23篇
水产渔业   39篇
畜牧兽医   490篇
园艺   11篇
植物保护   77篇
  2023年   6篇
  2022年   2篇
  2021年   13篇
  2020年   19篇
  2019年   21篇
  2018年   13篇
  2017年   19篇
  2016年   19篇
  2015年   17篇
  2014年   25篇
  2013年   49篇
  2012年   60篇
  2011年   60篇
  2010年   33篇
  2009年   27篇
  2008年   60篇
  2007年   61篇
  2006年   53篇
  2005年   50篇
  2004年   69篇
  2003年   60篇
  2002年   61篇
  2001年   9篇
  2000年   6篇
  1999年   9篇
  1998年   9篇
  1997年   9篇
  1996年   5篇
  1995年   6篇
  1994年   6篇
  1993年   9篇
  1992年   5篇
  1991年   3篇
  1990年   9篇
  1989年   5篇
  1988年   4篇
  1986年   6篇
  1985年   5篇
  1984年   3篇
  1983年   1篇
  1982年   3篇
  1980年   1篇
  1979年   1篇
  1978年   2篇
  1977年   2篇
  1972年   1篇
  1970年   1篇
  1955年   1篇
排序方式: 共有918条查询结果,搜索用时 114 毫秒
81.
Cells must amplify external signals to orient and migrate in chemotactic gradient fields. We find that human neutrophils release adenosine triphosphate (ATP) from the leading edge of the cell surface to amplify chemotactic signals and direct cell orientation by feedback through P2Y2 nucleotide receptors. Neutrophils rapidly hydrolyze released ATP to adenosine that then acts via A3-type adenosine receptors, which are recruited to the leading edge, to promote cell migration. Thus, ATP release and autocrine feedback through P2Y2 and A3 receptors provide signal amplification, controlling gradient sensing and migration of neutrophils.  相似文献   
82.
The evolutionarily conserved actin-related protein (Arp2/3) complex is a key component of actin filament networks that is dynamically regulated by nucleation-promoting and inhibitory factors. Although much is known about actin assembly, the physiologic functions of inhibitory proteins are unclear. We generated coronin 1-/- mice and found that coronin 1 exerted an inhibitory effect on cellular steady-state F-actin formation via an Arp2/3-dependent mechanism. Whereas coronin 1 was required for chemokine-mediated migration, it was dispensable for T cell antigen receptor functions in T cells. Moreover, actin dynamics, through a mitochondrial pathway, was linked to lymphocyte homeostasis.  相似文献   
83.
OBJECTIVE: To determine the effects of a therapeutic riding program on psychosocial measurements among children considered at risk for poor performance or failure in school or life and among children in special education programs. DESIGN: Observational study. POPULATION: 17 at-risk children (6 boys and 11 girls) and 14 special education children (7 boys and 7 girls). PROCEDURE: For the at-risk children, anger, anxiety, perceived self-competence, and physical coordination were assessed. For the special education children, anger and cheerfulness were measured, and the children's and their mothers' perceptions of the children's behavior were assessed. Measurements were made before and after an 8-session therapeutic riding program. RESULTS: For boys enrolled in the special education program, anger was significantly decreased after completion of the riding program. The boys' mothers also perceived significant improvements in their children's behavior after completion of the program. CONCLUSIONS AND CLINICAL RELEVANCE: Results suggest that an 8-session therapeutic riding program can significantly decrease anger in adolescent boys in a special education program and positively affect their mothers' perception of the boys' behavior.  相似文献   
84.
OBJECTIVE: To determine the effect of age on reference intervals of serum biochemical values in kittens. DESIGN: Prospective clinical trial. ANIMALS: 55 kittens from 12 specific-pathogen-free queens. PROCEDURE: Kittens were allocated at birth into colostrum-fed (n = 27) and colostrum-deprived (28) groups. Blood was collected at birth and on days 1, 2, 4, 7, 14, 28, and 56. Serum samples were analyzed for activities of alkaline phosphatase, alanine aminotransferase, aspartate aminotransferase, creatine kinase, lactate dehydrogenase, gamma-glutamyltransferase, amylase, and lipase and for concentrations of albumin, total protein, bilirubin, urea nitrogen, creatinine, cholesterol, glucose, calcium, phosphorus, and triglycerides by use of an automated analyzer. Total serum solids concentrations were determined by use of refractometry. Serum IgG concentrations were quantified by use of radial immunodiffusion. RESULTS: For several analytes, reference intervals changed rapidly, most notably during the first few days of life. Reference intervals for alkaline phosphatase, creatine kinase, lactate dehydrogenase, and triglycerides were higher from birth to 8 weeks than adult reference intervals. Aspartate aminotransferase, bilirubin, urea nitrogen, and creatinine were higher than in adults at birth but were similar to or lower than adult reference intervals by 8 weeks. Compared with adult reference intervals, reference intervals for calcium and phosphorus concentrations were higher and for albumin and total protein concentrations were lower throughout the study period. CONCLUSIONS AND CLINICAL RELEVANCE: Important differences exist between reference intervals for serum biochemical values of neonatal and adult cats. Age-appropriate reference intervals should be used for accurate assessment of serum biochemical test results in cats.  相似文献   
85.
OBJECTIVE: To identify surrogate markers of passive transfer of immunity in kittens. DESIGN: Prospective clinical trial. ANIMALS: 55 kittens from 12 specific-pathogen-free queens. PROCEDURE: Kittens were allocated at birth into colostrum-fed (n = 27) and colostrum-deprived (28) groups. Blood was collected at birth and on days 1, 2, 4, 7, 14, 28, and 56. Serum samples were analyzed for activities of alkaline phosphatase (ALP), alanine aminotransferase, aspartate aminotransferase, creatine kinase, lactate dehydrogenase, gamma-glutamyltransferase, amylase, and lipase and for concentrations of albumin, total protein, bilirubin, urea nitrogen, creatinine, cholesterol, glucose, calcium, phosphorus, and triglycerides by use of an automated analyzer. Total serum solids concentrations were estimated by use of refractometry. Serum IgG concentrations were quantified by use of radial immunodiffusion. RESULTS: All kittens were agammaglobulinemic at birth. Colostrum-fed kittens had significantly higher IgG concentrations than did colostrum-deprived kittens from 1 though 28 days of age. Transient significant differences in serum biochemical variables between the colostrum-deprived and colostrum-fed groups were substantially resolved by day 4. Passive transfer of immunity could be reliably determined at 1 day of age and to a lesser extent at 2 days of age only by measurement of serum activity of ALP. CONCLUSIONS AND CLINICAL RELEVANCE: Adequacy of passive transfer in kittens initially correlated with serum activity of ALP, but quantification of serum IgG concentration was necessary after 2 days of age.  相似文献   
86.
OBJECTIVE: To examine changes between 1996 and 2004 in regard to numbers of animals handled, medical care provided, expenses, numbers of employees, and agency policies for animal care and control agencies in Ohio. DESIGN: Cross-sectional survey. SAMPLE POPULATION: 223 animal care and control agencies. PROCEDURES: A questionnaire was mailed to animal care and control agencies in Ohio to collect information for 2004; results were compared with published results of a similar survey. RESULTS: 165 of the 223 (74%) agencies responded. Estimated total number of animals handled in 2004 was 315,519, which represented a decrease of 7% compared with 1996. However, although number of dogs taken in decreased 17%, number of cats taken in increased 20%. Between 1996 and 2004, the euthanasia rate decreased from 65.3% to 56.8%, and the adoption rate increased from 24.5% to 33.6%. Number of dogs euthanatized decreased 39%, but number of cats euthanatized increased 14%. The proportion of agencies with a spay-neuter policy increased from 56% to 71%, and the proportion that maintained an association with a veterinarian increased from 39% to 80%. For dogs handled by county dog warden agencies, the odds of euthanasia were higher if the agency did not have a spay-neuter policy (odds ratio, 1.36). CONCLUSIONS: Results suggest that the status of dogs handled by animal care and control agencies in Ohio improved between 1996 and 2004, but that the status of cats deteriorated.  相似文献   
87.
Colletotrichum acutatum causes anthracnose on peppers (Capsicum spp.), resulting in severe yield losses in Taiwan. Fungal isolates Coll-153, Coll-365 and Coll-524 collected from diseased peppers were found to differ in pathogenicity. Pathogenicity assays on various index plants revealed that Coll-524 was highly virulent and Coll-153 was moderately virulent to three commercially available pepper cultivars. Both isolates induced anthracnose lesions and produced abundant conidia. Coll-365 was only weakly virulent on pepper fruit, where it caused small lesions and hardly produced conidia on pepper fruit. However, Coll-365 was highly pathogenic to tomato fruit and mango leaves, where it caused anthracnose lesions and formed acervuli and conidia. All three isolates showed similar abilities in the attachment and germination of conidia, formation of highly branched hyphae and appressoria, penetration of cuticles, and infection of epidermal cells on chili peppers. Coll-365 accumulated less turgor pressure in appressoria but produced higher levels of cutinase and protease activity than Coll-153 and Coll-524 did. All three isolates invaded the neighbouring cells through plasmodesmata in chili peppers and showed similar pectinase or cellulase activities in culture. However, the most virulent strain Coll-524 expressed stronger laccase activity and was more resistant to capsaicin compared to Coll-153 and Coll-365. The three isolates are different in numbers and sizes of double-stranded RNAs. Depending on the cultivar genotypes, cellular resistance of chili pepper to C. acutatum might rely on the ability to restrict penetration, colonization, or conidiation of the pathogen. We conclude that the differences in pathogenicity among the three C. acutatum isolates of pepper are attributed to their ability to colonize the host plant.  相似文献   
88.
Use of cross incompatibility in corn (Zea mays L.) by the Ga1-s allele may reduce cross-fertilization in specialty and conventional organic corn with pollen from genetically-modified (GM) corn. For effective use, information about environment, and genotype × environment effects on cross-fertilization by ga1 as well as heritability of cross incompatibility in maize is necessary. Our objective was to obtain this information. Four population pairs differing in their genotype at ga1 were evaluated for cross incompatibility with ga1 pollen in different environments. Populations were derived by crossing the recurrent parents B116, PHG35, ARZM16035:S19, and (CHZM05015:Mo17)Mo17 to Ga1-s donor parent Mo508W/Mo506W. Two replicates of each treatment were grown in the center of 952 m2 fields planted with purple corn as an adventitious source of ga1/ga1 pollen. Open pollination was allowed and the amount of cross-fertilization estimated by averaging the percentage of purple seeds. Environment and genotype × environment effects were not significant. Contrasts to evaluate differences in cross-fertilization between Ga1-s and ga1 populations revealed that mean percentages of cross-fertilization in Ga1-s populations of B116, ARZM16035:S19, and (CHZM05015:Mo17)Mo17 were significantly lower than in ga1 populations. The estimated broad-sense heritability on an entry-mean basis for cross incompatibility was 0.81. Results suggest differences in genotype at ga1 played a major role in cross-fertilization of populations differing in their genotype at the ga1 locus. Incompatibility may be selected effectively over different environments and the Ga1-s system may be of value to reduce cross-fertilization with GM corn pollen.  相似文献   
89.
U.S. states and localities often engage in economic development policies using incentives and abatements for specific firms or industries. Yet, there is very little empirical evidence suggesting that such policies are successful. Why, then, do governments engage in these policies? In order to answer this question, we employ a model that considers not only geographic and economic factors, but also, in a novel application, local political conditions. A unique survey of U.S. county governments forms the basis for our empirical assessment of both traditional economic development policies and new‐wave policies. Using probit, Poisson, negative binomial, and spatial econometric models, we find evidence that the use of incentives is inversely related to local economic conditions. Furthermore, we find Republican counties are more apt to use incentives, though counties dominated by one political party are less likely to use them.  相似文献   
90.
Lysosomal storage diseases (LSDs) are a group of heterogeneous disorders caused by defects in lysosomal enzymes or transporters, resulting in accumulation of undegraded macromolecules or metabolites. Macrophage numbers are expanded in several LSDs, leading to histiocytosis of unknown pathophysiology. Here, we found that mice lacking the equilibrative nucleoside transporter 3 (ENT3) developed a spontaneous and progressive macrophage-dominated histiocytosis. In the absence of ENT3, defective apoptotic cell clearance led to lysosomal nucleoside buildup, elevated intralysosomal pH, and altered macrophage function. The macrophage accumulation was partly due to increased macrophage colony-stimulating factor and receptor expression and signaling secondary to the lysosomal defects. These studies suggest a cellular and molecular basis for the development of histiocytosis in several human syndromes associated with ENT3 mutations and potentially other LSDs.  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