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   2042篇
  免费   370篇
  2412篇
  2017年   22篇
  2016年   32篇
  2015年   71篇
  2014年   49篇
  2013年   67篇
  2012年   108篇
  2011年   77篇
  2010年   58篇
  2009年   72篇
  2008年   100篇
  2007年   76篇
  2006年   51篇
  2005年   81篇
  2004年   57篇
  2003年   57篇
  2002年   57篇
  2001年   82篇
  2000年   48篇
  1999年   41篇
  1998年   30篇
  1997年   26篇
  1996年   22篇
  1995年   22篇
  1994年   22篇
  1993年   21篇
  1992年   40篇
  1991年   42篇
  1990年   49篇
  1989年   47篇
  1988年   35篇
  1987年   31篇
  1986年   36篇
  1985年   38篇
  1984年   38篇
  1983年   26篇
  1982年   25篇
  1981年   33篇
  1980年   27篇
  1979年   36篇
  1978年   33篇
  1977年   27篇
  1976年   27篇
  1975年   35篇
  1974年   30篇
  1973年   45篇
  1971年   25篇
  1970年   24篇
  1969年   28篇
  1968年   27篇
  1967年   25篇
排序方式: 共有2412条查询结果,搜索用时 0 毫秒
11.
Paramyxovirus type 2 (PMV-2) isolated from wild birds is often considered non-pathogenic, but nothing is known about its effects on overall behavior and fitness of free-flying birds. Domestically bred, African cut-throat finches (Amadina fasciata), a species from which PMV-2 has been isolated in the wild, were inoculated with a Central American field strain of PMV-2. Patterns of behavior were examined before and after viral challenge to quantify inapparent, sublethal effects of the disease. Infected birds demonstrated a significant decrease in activity (P = 0.01) followed by an apparent recovery period. Antibody titers confirmed infection in inoculated birds and indicated that sentinel birds did not become infected.  相似文献   
12.
Visceral leishmaniasis is a severe chronic disease of people and animals. The disease is caused by several subspecies of a protozoal organism, Leishmania donovani. If not treated, visceral leishmaniasis is often fatal. The most commonly used chemotherapeutic agents to treat the disease are pentavalent antimonials, which can be toxic, must be administered by parenteral routes, and are sometimes ineffective. In this study, meglumine antimoniate, a pentavalent antimony, was compared with WR 6026, an 8-aminoquinoline derivative, as to antileishmanial efficacy. The results indicate that either of these 2 drugs are effective in the suppression of amastigotes in the liver and spleen of the opossum. Despite the marked parasite suppression in the liver and spleen of the infected opossums, the experimental disease was fatal in all of the infected opossums, regardless of the therapy.  相似文献   
13.
Hanson  A.  Ellingwood  C.  Kerekes  J.  Smith  A. 《Hydrobiologia》1994,279(1):521-524
The Sackville Waterfowl Park, contains a 19 hectare shallow freshwater wetland created by reflooding a saltwater marsh that was drained three centuries ago. Its primary purpose is to provide wetland habitat and wildlife viewing opportunities to tourists and residents. This newly created, eutrophic wetland supports high densities of waterfowl, 2.1 and 3.3 brood ha–1 in 1991 and 1992 respectively. It is hoped that long term monitoring of the Park's waterfowl population and wetland habitat will contribute to a better understanding of factors controlling breeding waterfowl populations.  相似文献   
14.
15.
10-Deacetylabaccatin III (10 DAB), an important precursor for paclitaxel semisynthesis, is enhanced in yew extracts using C10-deacetylase and C13-deacylase enzymes.(4) C10-deacetylase is an intracellular enzyme produced by the fermentation of a soil microorganism, Nocardioides luteus (SC 13912). During the fermentation of Nocardioides luteus, the growth of cells reaches a maximum growth at 28 h. C10-deacetylase enzyme activity starts at 26 h and peaks at 38 h of the fermentation. The cells are recovered by centrifugation. The C10-deacetylase enzyme was purified from the Nocardioides luteus cells. The enzyme was purified 190-fold to near homogeneity. The purified enzyme appeared as a single band on 12.5% SDS-PAGE analysis with a molecular weight of 40,000 daltons. (c) 1995 John Wiley & Sons, Inc.  相似文献   
16.
PVC-211 murine leukemia virus (MuLV) causes neurodegenerative disease following inoculation of neonatal, but not adult, mice and rats. It was previously shown that tropism for brain capillary endothelial cells (CEC) was a determinant of the viral neuropathogenicity. In this study, we demonstrate that host age-dependent replication of PVC-211 MuLV in vivo occurs in CEC in the brain as well as in other organs, such as the liver, kidney, and heart. In contrast, primary explant cultures of CEC derived from brains and livers of adult and neonatal rats could be infected by PVC-211 MuLV, suggesting that the age-dependent susceptibility was abrogated in vitro. Although CEC were generally less susceptible to MuLV-mediated gene transduction than fibroblasts, treatment of CEC with 2-deoxyglucose followed by inoculation of a PVC-211 MuLV-pseudotyped vector in the absence of heparin improved the transduction efficiency. These observations support the possibility that PVC-211 MuLV may be useful for establishing models of CEC gene transduction.  相似文献   
17.
Several mutants have been isolated from the facultative methylotroph, Methylobacterium organophilum, using either N-methyl-N'-nitro-N-nitrosoguanidine or ultraviolet light as mutagens. One of these isolates, a glutamate auxotroph lacking isocitrate dehydrogenase, has been transformed to prototrophy, using wild-type DNA, at a frequency of 0-5%. Competence and DNA uptake occur only in cultures which are near the end of exponential growth, and maximal transformation requires a DNA concentration of 100 mug ml-1.  相似文献   
18.
Hypothalamic sarcoidosis and hypopituitarism   总被引:1,自引:0,他引:1  
4 patients with presumed pituitary hypothalamic sarcoidosis are described. 3 had histological diagnoses compatible with sarcoidosis and in the other this diagnosis was strongly suspected from chest X-rays. 2 patients presented with diabetes insipidus. ACTH reserve was diminished in 3 out of 4 and growth hormone reserve was diminished in the 3 who were tested. All 4 patients developed secondary amenorrhea. 3 patients had hypothalamic hypothyroidism. Prolactin dynamics were intact. Tomograms of the sella turcica in all 4 and computerized tomography of the hypothalamic area in 2 patients failed to reveal any abnormality.  相似文献   
19.
20.
A cytochemical study has been made on the localization of ATPase activity in corn (Zea mays L.) roots. Light microscopy shows washing for 4 hours to increase the general ATPase activity in the peripheral layers of the root cortex; oligomycin and N,N-dicyclohexylcarbodiimide inhibit this activity, oligomycin being more effective. Ultrastructural studies of ATPase location show oligomycin treatment to inhibit both mitochondrial and plasmalemma ATPase, but only in the epidermis and outer cortex. Studies with lipid-soluble dyes indicate that oligomycin might not penetrate very deeply into root tissue in the time span of these experiments. It is suggested that the strong inhibition of ion absorption by oligomycin without a corresponding decline in ATP content is probably due to inhibition of ion absorption in the peripheral cell layers, thus limiting the supply of ion for symplastic transport to the uninhibited tissues.  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