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   19803篇
  免费   1830篇
  国内免费   19篇
  2021年   260篇
  2020年   133篇
  2019年   206篇
  2018年   263篇
  2017年   206篇
  2016年   380篇
  2015年   673篇
  2014年   744篇
  2013年   925篇
  2012年   1205篇
  2011年   1270篇
  2010年   768篇
  2009年   669篇
  2008年   965篇
  2007年   1075篇
  2006年   991篇
  2005年   980篇
  2004年   970篇
  2003年   945篇
  2002年   920篇
  2001年   213篇
  2000年   146篇
  1999年   211篇
  1998年   265篇
  1997年   190篇
  1996年   148篇
  1995年   204篇
  1994年   180篇
  1993年   168篇
  1992年   169篇
  1991年   147篇
  1990年   137篇
  1989年   130篇
  1988年   139篇
  1987年   138篇
  1986年   136篇
  1985年   156篇
  1984年   180篇
  1983年   146篇
  1982年   216篇
  1981年   221篇
  1980年   221篇
  1979年   143篇
  1978年   149篇
  1977年   134篇
  1976年   162篇
  1975年   122篇
  1974年   134篇
  1973年   154篇
  1970年   92篇
排序方式: 共有10000条查询结果,搜索用时 359 毫秒
41.
42.
Prostaglandins E1, F and A2 were covalently joined to the surface of Sepharose as carboxamide linkages. The insolubilized prostaglandins were shown to function effectively in the purification of 15(S) -hydroxyprostaglandin dehydrogenase.  相似文献   
43.
Drechsler , Charles . (Plant Industry Sta., Beltsville, Md.) Two new species of Conidiobolus found in plant detritus. Amer. Jour. Bot. 47(5) : 368—377. Illus. 1960.–By canopying Petri plates of maize-meal agar with small quantities of friable or mealy plant detritus 2 new species of Conidiobolus, both of moderate dimensions, were isolated. They are described as C. incongruus and C. multivagus. The former, obtained from leaf mold collected in Colorado, produces zygospores which with respect to their internal organization differ markedly from those of congeneric species but resemble rather closely the globuliferous zygospores of Basidiobolus haptosporus and B. meristosporus. Conidiobolus multivagus, obtained from decayed twigs of Casuarina equisetifolia gathered in western Florida, forms a mycelium that soon becomes conspicuously disconnected. The disconnected condition here results partly from the production of many detached slender filaments, which, by constantly withdrawing protoplasmic materials from the posterior end while elongating at the tip, migrate through the slated substratum apparently without any intake of nutrients. The detached conidia of C. incongruus are provided with a more prominent basal papilla than those of C. multivagus, though both species show equally sharp demarcation between the globose main contour of the conidium and the dome-shaped contour of the papillia.  相似文献   
44.
Mutants of Escherichia coli K12 which accumulated the haem precursor porphobilinogen are described. The mutants grew very slowly on carbon and energy sources which K12 uses only oxidatively, and they had low catalase activities, suggesting that they were deficient in haem. Extracts had one-tenth of the parental activity of the enzyme porphobilinogen deaminase. In transduction, the mutation mapped close to genes ilvD and metE at minute 84. The gene was tentatively identified as hemC, coding for porphobilinogen deaminase. The gene symbol hemC replaces the earlier and temporary symbol popE.  相似文献   
45.
46.
47.
Total daily intakes of diethyl phthalate (DEP), di(n-butyl) phthalate (DnBP), di(isobutyl) phthalate (DiBP), butyl benzyl phthalate (BBzP) and di(2-ethylhexyl) phthalate (DEHP) were calculated from phthalate metabolite levels measured in the urine of 431 Danish children between 3 and 6 years of age. For each child the intake attributable to exposures in the indoor environment via dust ingestion, inhalation and dermal absorption were estimated from the phthalate levels in the dust collected from the child’s home and daycare center. Based on the urine samples, DEHP had the highest total daily intake (median: 4.42 µg/d/kg-bw) and BBzP the lowest (median: 0.49 µg/d/kg-bw). For DEP, DnBP and DiBP, exposures to air and dust in the indoor environment accounted for approximately 100%, 15% and 50% of the total intake, respectively, with dermal absorption from the gas-phase being the major exposure pathway. More than 90% of the total intake of BBzP and DEHP came from sources other than indoor air and dust. Daily intake of DnBP and DiBP from all exposure pathways, based on levels of metabolites in urine samples, exceeded the Tolerable Daily Intake (TDI) for 22 and 23 children, respectively. Indoor exposures resulted in an average daily DiBP intake that exceeded the TDI for 14 children. Using the concept of relative cumulative Tolerable Daily Intake (TDIcum), which is applicable for phthalates that have established TDIs based on the same health endpoint, we examined the cumulative total exposure to DnBP, DiBP and DEHP from all pathways; it exceeded the tolerable levels for 30% of the children. From the three indoor pathways alone, several children had a cumulative intake that exceeded TDIcum. Exposures to phthalates present in the air and dust indoors meaningfully contribute to a child’s total intake of certain phthalates. Such exposures, by themselves, may lead to intakes exceeding current limit values.  相似文献   
48.
When energetic particles irradiate matter, it becomes activated by nuclear reactions. Radioactivation induced cellular effects are not clearly understood, but it could be a part of bystander effects. This investigation is aimed at understanding the biological effects from radioactivation in solution induced by hadron radiation. Water or phosphate buffered saline was activated by being exposed to hadron radiation including protons, carbon- and iron-ions. 1 mL of radioactivated solution was transferred to flasks with Chinese hamster ovary (CHO) cells cultured in 5 mL of complete media. The induction of sister chromatid exchanges (SCE) was used to observe any increase in DNA damage responses. The energy spectrum and the half-lives of the radioactivation were analyzed by NaI scintillation detector in order to identify generated radionuclides. In the radioactivated solution, 511 keV gamma-rays were observed, and their half-lives were approximately 2 min, 10 min, and 20 min. They respectively correspond to the beta+ decay of 15O, 13N, and 11C. The SCE frequencies in CHO cells increased depending on the amount of radioactivation in the solution. These were suppressed with a 2-hour delayed solution transfer or pretreatment with dimethyl sulfoxide (DMSO). Our results suggest that the SCE induction by radioactivated solution was mediated by free radicals produced by the annihilated gamma-rays. Since the SCE induction and DMSO modulation are also reported in radiation-induced bystander effects, our results imply that radioactivation of the solution may have some contribution to the bystander effects from hadron radiation. Further investigations are required to assess if radioactivation effects would attribute an additional level of cancer risk of the hadron radiation therapy itself.  相似文献   
49.
Via Oswalt’s system of classification, I compare the tool-kits of wild and captive capuchins with those of Tanzanian chimpanzees and Tasmanian aborigines. The results indicate that capuchins have tool-kits that are smaller, and have lower ratios of artifacts to naturefacts, than those of Tanzanian chimpanzees and Tasmanian aborigines. Accordingly, Oswalt’s system can be used productively to assess the relative technological skills of monkeys versus those of apes and humans.  相似文献   
50.
The goal of this study was to assess the relationship between Aβ deposition and white matter pathology (i.e., white matter hyperintensities, WMH) on microstructural integrity of the white matter. Fifty-seven participants (mean age: 78±7 years) from an ongoing multi-site research program who spanned the spectrum of normal to mild cognitive impairment (Clinical dementia rating 0–0.5) and low to high risk factors for arteriosclerosis and WMH pathology (defined as WMH volume >0.5% total intracranial volume) were assessed with positron emission tomography (PET) with Pittsburg compound B (PiB) and magnetic resonance and diffusion tensor imaging (DTI). Multivariate analysis of covariance were used to investigate the relationship between Aβ deposition and WMH pathology on fractional anisotropy (FA) from 9 tracts of interest (i.e., corona radiata, internal capsule, cingulum, parahippocampal white matter, corpus callosum, superior longitudinal, superior and inferior front-occipital fasciculi, and fornix). WMH pathology was associated with reduced FA in projection (i.e., internal capsule and corona radiate) and association (i.e., superior longitudinal, superior and inferior fronto-occipital fasciculi) fiber tracts. Aβ deposition (i.e., PiB positivity) was associated with reduced FA in the fornix and splenium of the corpus callosum. There were interactions between PiB and WMH pathology in the internal capsule and parahippocampal white matter, where Aβ deposition reduced FA more among subjects with WMH pathology than those without. However, accounting for apoE ε4 genotype rendered these interactions insignificant. Although this finding suggests that apoE4 may increase amyloid deposition, both in the parenchyma (resulting in PiB positivity) and in blood vessels (resulting in amyloid angiopathy and WMH pathology), and that these two factors together may be associated with compromised white matter microstructural integrity in multiple brain regions, additional studies with a longitudinal design will be necessary to resolve this issue.  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