全文获取类型
收费全文 | 566532篇 |
免费 | 59161篇 |
国内免费 | 211篇 |
出版年
2016年 | 6867篇 |
2015年 | 8549篇 |
2014年 | 10378篇 |
2013年 | 14455篇 |
2012年 | 16463篇 |
2011年 | 17214篇 |
2010年 | 11858篇 |
2009年 | 10986篇 |
2008年 | 15728篇 |
2007年 | 16455篇 |
2006年 | 15450篇 |
2005年 | 14747篇 |
2004年 | 14883篇 |
2003年 | 14115篇 |
2002年 | 13928篇 |
2001年 | 23720篇 |
2000年 | 23978篇 |
1999年 | 18711篇 |
1998年 | 6638篇 |
1997年 | 6894篇 |
1996年 | 6358篇 |
1995年 | 6016篇 |
1994年 | 5771篇 |
1993年 | 5909篇 |
1992年 | 15713篇 |
1991年 | 15730篇 |
1990年 | 15276篇 |
1989年 | 14928篇 |
1988年 | 14213篇 |
1987年 | 13611篇 |
1986年 | 12598篇 |
1985年 | 12627篇 |
1984年 | 10486篇 |
1983年 | 9067篇 |
1982年 | 6764篇 |
1981年 | 6068篇 |
1980年 | 5873篇 |
1979年 | 10030篇 |
1978年 | 7962篇 |
1977年 | 7274篇 |
1976年 | 6912篇 |
1975年 | 7714篇 |
1974年 | 8665篇 |
1973年 | 8469篇 |
1972年 | 7954篇 |
1971年 | 7285篇 |
1970年 | 6432篇 |
1969年 | 6243篇 |
1968年 | 5998篇 |
1967年 | 5228篇 |
排序方式: 共有10000条查询结果,搜索用时 488 毫秒
811.
812.
B. E. Silkstone 《Oecologia》1987,74(1):149-152
Summary Low levels of artificial damage were imposed on the leaves of Betla pubescens and B. pendula in early summer 1985. Sixteen tress were used and the damage was distributed throughout the canopy in two ways. It was either randomly dispersed on the tree, or restricted to a localised region of the canopy. Up to 250 leaves/tree were damaged and the position of control leaves was marked as appropriate. At the end of the summer the experimental and control leaves were collected for analysis. Subsequent patterns of grazing by naturally-occurring herbivores were affected by the previous damage to the foliage. Fewer artificially-damaged leaves received subsequent insect grazing damage. The frequecy distribution of insect damage to previously-damaged leaves was significantly different from that of the control leaves, and less leaf tissue was removed from those experimental leaves which did receive subsequent insect attack. The evidence presented suggests that wound-induced changes in leaf palatability do occur in B. pubescens and B. pendula in the field and that a major role is to disperse the damage throughout the canopy. 相似文献
813.
Regulation of ornithine decarboxylase activity by spermidine and the spermidine analogue N1N8-bis(ethyl)spermidine. 总被引:7,自引:6,他引:1
下载免费PDF全文
![点击此处可从《The Biochemical journal》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Polyamine biosynthesis in intact cells can be exquisitely controlled with exogenous polyamines through the regulation of rate-limiting biosynthetic enzymes, particularly ornithine decarboxylase (ODC). In an attempt to exploit this phenomenon as an antiproliferative strategy, certain polyamine analogues have been identified [Porter, Cavanaugh, Stolowich, Ganis, Kelly & Bergeron (1985) Cancer Res. 45, 2050-2057] which lower ODC activity in intact cells, have no direct inhibitory effects on ODC, are incapable of substituting for spermidine (SPD) in supporting cell growth, and are growth-inhibitory at micromolar concentrations. In the present study, the most effective of these analogues, N1N8-bis(ethyl)SPD (BES), is compared with SPD in its ability to regulate ODC activity in intact L1210 cells and in the mechanism(s) by which this is accomplished. With respect to time and dose-dependence of ODC suppression, both polyamines closely paralleled one another in their response curves, although BES was slightly less effective than SPD. Conditions of minimal treatment leading to near-maximal ODC suppression (70-80%) were determined and found to be 3 microM for 2 h with either SPD or BES. After such treatment, ODC activity was fully recovered within 2-4 h when cells were re-seeded in drug-free media. By assessing BES or [3H]SPD concentrations in treated and recovered cells, it was possible to deduce that an intracellular accumulation of BES or SPD equivalent to less than 6.5% of the combined cellular polyamine pool was sufficient to invoke ODC regulatory mechanisms. Decreases in ODC activity after BES or SPD treatment were closely paralleled by concomitant decreases in ODC protein. Since cellular ODC mRNA was not similarly decreased by either BES or SPD, it was concluded that translational and/or post-translational mechanisms, such as increased degradation of ODC protein or decreased translation of ODC mRNA, were probably responsible for regulation of enzyme activity. Experimental evidence indicated that neither of these mechanisms seemed to be mediated by cyclic AMP or ODC-antizyme induction. On the basis of the consistent similarities between BES and SPD in all parameters studied, it is concluded that the analogue most probably acts by the same mechanisms as SPD in regulating polyamine biosynthesis. 相似文献
814.
815.
During a 6-month period, 7252 faeces specimens were examined for Escherichia coli serotype O157:H7. Forty-nine specimens (0.7%) from 19 patients yielded this organism. Escherichia coli O157:H7 was the third most frequently isolated bacterial pathogen, following Campylobacter jejuni and (or) Salmonella sp. Although regional variation between laboratories determined whether Campylobacter jejuni or Salmonella was the primary bacterial pathogen isolated, E. coli O157:H7 was consistently isolated more frequently than either Shigella or Yersinia enterocolitica. 相似文献
816.
Danielle E. Haulsee Dewayne A. Fox Matthew W. Breece Tonya M. Clauss Matthew J. Oliver 《PloS one》2016,11(2)
We developed a long-term tagging method that can be used to understand species assemblages and social groupings associated with large marine fishes such as the Sand Tiger shark Carcharias taurus. We deployed internally implanted archival VEMCO Mobile Transceivers (VMTs; VEMCO Ltd. Nova Scotia, Canada) in 20 adult Sand Tigers, of which two tags were successfully recovered (10%). The recovered VMTs recorded 29,646 and 44,210 detections of telemetered animals respectively. To our knowledge, this is the first study to demonstrate a method for long-term (~ 1 year) archival acoustic transceiver tag implantation, retention, and recovery in a highly migratory marine fish. Results show low presumed mortality (n = 1, 5%), high VMT retention, and that non-lethal recovery after almost a year at liberty can be achieved for archival acoustic transceivers. This method can be applied to study the social interactions and behavioral ecology of large marine fishes. 相似文献
817.
818.
819.
820.