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   1075篇
  免费   101篇
  国内免费   1篇
  2023年   1篇
  2022年   4篇
  2021年   15篇
  2020年   15篇
  2019年   18篇
  2018年   18篇
  2017年   19篇
  2016年   32篇
  2015年   47篇
  2014年   59篇
  2013年   61篇
  2012年   80篇
  2011年   77篇
  2010年   56篇
  2009年   52篇
  2008年   65篇
  2007年   61篇
  2006年   49篇
  2005年   66篇
  2004年   68篇
  2003年   59篇
  2002年   74篇
  2001年   19篇
  2000年   20篇
  1999年   6篇
  1998年   13篇
  1997年   19篇
  1996年   13篇
  1995年   6篇
  1994年   7篇
  1993年   10篇
  1992年   5篇
  1991年   8篇
  1990年   8篇
  1989年   5篇
  1988年   3篇
  1987年   4篇
  1986年   4篇
  1985年   2篇
  1984年   5篇
  1983年   3篇
  1982年   2篇
  1981年   4篇
  1980年   4篇
  1979年   3篇
  1978年   1篇
  1977年   3篇
  1975年   1篇
  1974年   2篇
  1973年   1篇
排序方式: 共有1177条查询结果,搜索用时 15 毫秒
1.
For the first time ever, the International Union for Conservation of Nature Red List Index for habitat types was calculated for an entire country, Finland. The RLIs were based on species threat assessments from 2000 and 2010 and included habitat definitions for all 10,131 species of 12 organism groups. The RLIs were bootstrapped to track statistically significant changes. The RLI changes of species grouped by habitats were negative for all habitat types except for forests and rural biotopes which showed a stable trend. Trends of beetles and true bugs were positive in rural and forest habitats. Other 16 observed trends of species group and habitat combinations were negative. Several trends observed were in accordance with studies focusing on particular taxa and habitats, and drivers for their change. This study demonstrates the usefulness of the RLI as a tool for observing habitat change based on species threat assessment data.  相似文献   
2.
The vegetation of semi-natural grasslands under modern, cereal/forage cropping in Finnish Karelia (n = 11) and old fashioned animal husbandry in adjacent Russian Karelia (n = 11) was compared in terms of their species composition. Each grassland site was paired with one in the other country which was as similar as possible in respect to its natural conditions. The species composition indicated differences in management between the two countries. The mean number of species was 4S.S in the Finnish sites and 52.6 in the Russian sites. A total of 12 species exhibited a statistical difference in their indicator values between the two countries. Traditional grassland species (e.g. Leontodon hispidus, Dianthus deltoides ), indicating grazing, hay making and old settlement, occurred more oñen in Russian sites, while species related to nutrient enrichment and cultivation (e.g. Urtica dioica, Elymus repens ) were more characteristic of Finnish sites.  相似文献   
3.
Vendace, Coregonus albula (L.), larvae reared in net enclosures anchored in the littoral zone of Lake Ylä-Enonvesi, eastern Finland, were observed for a period of 44 days in order to determine the effects of density on mortality, growth and feeding of the larvae. The ratio of newly-hatched fish to their food was regulated by controlling the number of fish stocked into the enclosures. Prey animals were expected to move freely in and out of the enclosures. The effect of nylon mesh (500 μm) on the penetration of zooplankters into the enclosures was negligible. The influence of possible starvation was determined by analysing the growth of the larvae, their Fulton condition factor, the instantaneous amount of food ingested, and the food composition in their guts. All these indicators were inversely correlated with larval density. Vendace larvae were found to be resistant to mortality from direct starvation. At all densities the survival rate was over 60%. At the time of complete yolk absorption, there was no increase in mortality.  相似文献   
4.
We have identified a new mouse T-cell receptor V gamma gene segment, V gamma 4.4, which frequently undergoes rearrangements in AKR thymomas, and at a lower frequency in fetal thymocytes. V gamma 4.4 is the fourth and the most 5' V gene segment in the gamma 4 cluster, being 7.3 kb from V gamma 4.3. Surprisingly, V gamma 4.4 is more homologous to eight human V gamma genes than to the other mouse V gamma genes. It has only a 38% nucleotide and 21% amino acid sequence homology to the most homologous mouse V gamma gene (V gamma 4.1), whereas these homologies to the human V gamma 8 gene are as high as 68% and 48% respectively.  相似文献   
5.
6.
Summary Salla disease is an inherited lysosomal storage disorder caused by accumulation of free sialic acid in the lysosomes. Lamp genes, lamp A and lamp B (lysosome associated membrane proteins), are the first known genes encoding for human lysosomal membrane proteins. Absence of linkage in a large group of families shows that lamp genes are not involved in Salla disease. The lamp genes were localized, using Southern hybridization in hamster — human hybrid cell panels, to chromosomes 13 (lamp A) and X (lamp B).  相似文献   
7.
Experiments were performed to study the effect of glucose on the production of the fimbrial colonization factor CFA/II of enterotoxigenicEscherichia coli (ETEC). The production of the CFA/II antigen was examined by electron microscopy, quantitative ELISA, and hemagglutination. The results showed that addition of 1% glucose to the growth medium drastically decreased CFA/II production, whereas addition of glycerol or sodium acetate did not have any effect. Bacteria grown in the presence of 1% glucose were essentially devoid of CFA/II fimbriae when examined under the electron microscope. Addition of 1 mM cAMP reversed the repressive effect of glucose, suggesting that the glucose suppression on CFA/II synthesis is via the mechanism of catabolite repression.  相似文献   
8.
Primary mouse embryo fibroblasts of C57BL/6 origin (B6/MEF) were transformed in vitro by transfection with simian virus 40 (SV40) DNA. The transformation frequency was markedly reduced if the SV40 DNA-transfected cultures were briefly exposed to K11 cells, an SV40-specific clone of cytotoxic T lymphocytes. This abrogation of SV40 transformation in vitro by cytotoxic T-lymphocyte clone K11 was specific, since transformation of B6/MEF cells by adenovirus type 5 DNA was not affected. The approach described here should serve as an ideal model of dissecting immunological events during in vivo tumorigenesis.  相似文献   
9.
Summary A polychlorophenol degrader, Rhodococcus chlorophenolicus, was shown to metabolize five different chlorinated guaiacols, namely tetrachloroguaiacol, 3,4,6-trichloroguaiacol, 3,5,6-trichloroguaiacol, 3,5-dichloroguaiacol and 3,6-dichloroguaiacol. Seven different intermediate metabolites, each with three hydroxyl or methoxyl groups, were identified. Four of these metabolites were also dehalogenation products, three carrying one chlorine atom less than the parent compound, and one metabolite from tetrachloroguaiacol where two chlorine atoms had been removed. Tetrachloroguaiacol was shown to undergo reductive dehalogenation. Demethylation of guaiacol to catechol was observed with the dichloroguaiacols, but not with polychloroguaiacols.Abbreviations DCG dichloroguaiacol - TCG trichloroguaiacol - TeCG tetrachloroguaiacol - DCC dichlorocatechol - TCC trichlorocatechol - TeCC tetrachlorocatechol - TCP trichlorophenol - TeCP tetrachlorophenol - PCP pentachlorophenol. An example of numeration - 346-TCG 3,4,6-trichloroguaiacol - GLC gas liquid chromatography  相似文献   
10.
Summary An actinomycete, Rhodococcus chlorophenolicus, isolated from a pentachlorophenol-degrading mixed bacterial culture is a polychlorophenol degrader. It was shown to oxidize pentachlorophenol into carbon dioxide and to metabolize also 2,3,4,5-,2,3,4,6-, and 2,3,5,6-tetrachlorophenol, 2,3,4-, 2,3,5-, 2,3,6-, 2,4,6-, and 2,4,5-trichlorophenol, 2,5-, and 2,6-dichlorophenol and tetrachloro-p-hydroquinone in an inducible manner. Pentachlorophenol set on the synthesis of enzymes required for the metabolism of all these chlorophenols and of tetrachloro-p-hydroquinone. 2,4,5-, and 2,4,6-trichlorophenol and 2,5-, and 2,6-dichlorophenol were degraded by R. chlorophenolicus cells only if these had previous contact to pentachlorophenol. Other chlorophenols mentioned were able to set on the synthesis of enzymes for their own degradation. 2,3,4,5-, and 2,3,4,6-tetrachlorophenol, and 2,3,5-, 2,4,5-, and 3,4,5-trichlorophenol were more toxic to R. chlorophenolicus than the other chlorophenols, but nevertheless 2,3,4,5-, and 2,3,4,6-tetrachlorophenol and 2,3,5-trichlorophenol were readily degraded by the bacteria.Abbreviations DCP dichlorophenol - TCP trichlorophenol - TeCP tetrachlorophenol - PCP pentachlorophenol - TeCH tetrachloro-p-hydroquinone An example of numeration: 2345-TeCP, 2,3,4,5-tetrachlorophenol  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