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   115405篇
  免费   5873篇
  国内免费   77篇
林业   5055篇
农学   3658篇
基础科学   588篇
  12787篇
综合类   21755篇
农作物   4279篇
水产渔业   5625篇
畜牧兽医   59302篇
园艺   1392篇
植物保护   6914篇
  2018年   1467篇
  2017年   1567篇
  2016年   1488篇
  2015年   1260篇
  2014年   1543篇
  2013年   4053篇
  2012年   2726篇
  2011年   3435篇
  2010年   2289篇
  2009年   2175篇
  2008年   3369篇
  2007年   3323篇
  2006年   3071篇
  2005年   2934篇
  2004年   2860篇
  2003年   2872篇
  2002年   2660篇
  2001年   3645篇
  2000年   3664篇
  1999年   2966篇
  1998年   1157篇
  1997年   1120篇
  1995年   1317篇
  1994年   1081篇
  1993年   1069篇
  1992年   2390篇
  1991年   2542篇
  1990年   2594篇
  1989年   2506篇
  1988年   2348篇
  1987年   2369篇
  1986年   2373篇
  1985年   2286篇
  1984年   1873篇
  1983年   1675篇
  1982年   1086篇
  1979年   1738篇
  1978年   1373篇
  1977年   1198篇
  1976年   1129篇
  1975年   1272篇
  1974年   1623篇
  1973年   1639篇
  1972年   1682篇
  1971年   1550篇
  1970年   1447篇
  1969年   1381篇
  1968年   1159篇
  1967年   1222篇
  1966年   1097篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
51.
The effect of salinity on seed germination, plant yield parameters, and plant Na, Cl and K concentrations of chickpea and lentil varieties was studied. Results showed that in both crops percentage emergence was significantly reduced by increasing NaCl levels (0–8dSm?1). From the plant growth studies it was found that differences existed among chickpea and lentil varieties in their response to NaCl application. In chickpea, the variety Mariye showed the comparatively lowest germination percentage and the lowest seedling shoot dry weight in response to salinity and was also among the two varieties which had the lowest relative plant height, shoot and root dry weight and grain yield at maturity. Similarly, variety DZ-10-16-2, which was the second best in germination percentage and the highest in terms of seedling shoot dry weight, also had the highest relative plant height, shoot and root dry weights, and grain yield at maturity. In lentil, however, such relationships were less pronounced. Chloride concentration (mg g?1) in the plant parts at salt levels other than the control was about 2–5 times that of Na. K concentration in the plants was significantly reduced by increasing NaCl levels. Chickpea was generally more sensitive to NaCl salinity than lentil. While no seeds were produced at salinity levels beyond 2dSm?1 in chickpea (no seeds were produced at this salt level in the most sensitive variety, Mariye), most lentil varieties could produce some seeds up to the highest level of NaCl application. Overall, varieties R-186 (lentil) and Mariye (chickpea) were the most sensitive of all varieties. On the other hand, lentil variety NEL-2704 and chickpea variety DZ-10-16-2 gave comparatively higher mean relative shoot and root dry weights, and grain yield, thus showing some degree of superiority over the others. The observed variations among the varieties may be useful indications for screening varieties of both crops for salt tolerance.  相似文献   
52.
Bluetongue virus (BTV), an arthropod-borne virus, is transmitted primarily by biting midges of the genus Culicoides. Some insect species, which might serve as a potential vector, are prevalent in Central Europe. In sheep, bluetongue is acute and mortality is high, whereas in cattle, goats and most wild ruminants the infection is usually clinically inapparent. Viremia is of short duration in sheep, but cattle experience a prolonged viremia and provide a reservoir for the dissemination of BTV. At least 24 different BTV serotypes have been identified. Antigenic variations occur in the polypeptides of the outer viral capsid and the segmented nature of the viral genome provides the potential for evolution of the virus by a mechanism of reassortment. This renders the use of polyvalent vaccines inefficient and emphasizes the significance of import/export restrictions on ruminants from BTV endemic areas.  相似文献   
53.
The technique of cystocentesis in dogs and cats was studied. The indications and contraindications of this method are discussed, using illustrations.  相似文献   
54.
55.
56.
Fibrotic degeneration was examined in the kidneys of ICR-derived glomerulonephritis (ICGN) mice, a novel inbred mouse line with a hereditary nephrotic syndrome of unknown etiology considered to be a good model of human idiopathic nephrotic syndrome. In the present study, we histochemically revealed changes in accumulation of extracellular matrix (ECM) components and in localization of integrins, cellular receptors for ECM, in the kidneys of ICGN mice with the progression of renal failure. Excessive accumulation of basement membrane (laminin and collagen IV) and interstitial (type III collagen) ECM components were demonstrated in the glomeruli and tubulointerstitum of ICGN mice. Marked deposition of type I collagen and tenascin was seen only in the glomeruli of ICGN mice but not in those of ICR mice as normal controls. Increased expression of integrin alpha1-, alpha2-, alpha5- and beta1-subunits in glomeruli with fibrotic degeneration and abnormal distribution of alpha6-subunit were noted in the kidneys of ICGN mice. Excessive laminin, a ligand of alpha6beta1-integrin, was demonstrated on the tubular basement membrane, but alpha6-subunit diffusely disappeared on the basal side of the tubular epithelial cells. We presumed that abnormal integrin expression in renal tubules causes epithelial cell detachment, and consequently tubular nephropathy, and results in disorder of ECM metabolism causing excessive accumulation of ECM components in the kidneys of ICGN mice.  相似文献   
57.
Invasive tumors of the pituitary gland associated with Cushing's disease and diabetes insipidus are described in ten dogs. All patients showed typical clinical symptoms including polyuria/polydipsia and acanthosis nigricans. All tumors led to compression und partial destruction of the posterior lobe of the pituitary gland and the infundibular stalk; two of them infiltrated the hypothalamic region. All tumors produced ACTH and caused hyperadrenocorticism. In six cases, additional expression of beta-lipoprotein and MSH were found: in pars intermedia adenomas many cells stained strongly for MSH and/or beta-lipoprotein, whereas in tumors of the pars distalis only occasional cells stained positive. The purpose of the present study was to describe the neuropathological findings and the immunohistochemistry of hormone excretion in pituitary tumors in dogs resulting in Cushing's disease associated with D.i., to review the literature and to discuss the pathogenesis.  相似文献   
58.
Effective radiographic interpretation requires a veterinarian who is knowledgeable of equine limb anatomy and the various principles that affect the resulting image. The normal and its variations must be recognized and understood before the abnormal can be confidently identified as pathologic. Proper patient positioning and sound radiographic technique are mandatory if reliable diagnostic radiographs are to be produced. This review emphasizes equine foot radiographic variations of normal and pseudolesions that occur with commonly used radiographic views performed in equine practice.  相似文献   
59.
The technique of liver puncture as described by Holtenius (1961) was assessed with regard to its practicability and safety in a preliminary study of 26 cattle and a follow-up study of 108 German black pied cattle. The sample material was smeared, stained and examined by light microscope. All animals with changes of grade 5 had to be slaughtered. Liver puncture is a simple, quick and safe technique which supplies useful information about the various stages of hepatic fatty degeneration in individual cattle.  相似文献   
60.
Abstract. Changes in chemical and physical properties and erodibility of a vertisol were studied in relation to land use. The vertisol, which occurs extensively in the semi-arid south-east of Zimbabwe, is derived from basalt and has a self mulching surface layer. Irrigated crops show static yields despite introduction of improved varieties. Four uncultivated sites were selected as controls and compared with five irrigated and four dryland sites. Surface soils were analysed for a range of chemical and physical properties, and laboratory rainfall simulation was used to measure soil erodibility under high intensity rain. The irrigated soils had greater exchangeable sodium and available phosphorus than the uncultivated soils. In contrast, dryland soils showed no such changes apart from a decrease in the amount of small water-stable aggregates. The soils are very erodible under high intensity rain but no significant differences were found between sites. We conclude that, although soil chemical changes have taken place in the irrigated soils, significant soil degradation has not occurred at the sites examined. The static yields probably result from management problems. However, immediate measures should be taken to improve drainage and irrigation management in the irrigated soils to avoid further sodium increases.  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