全文获取类型
收费全文 | 3065篇 |
免费 | 283篇 |
专业分类
3348篇 |
出版年
2022年 | 17篇 |
2021年 | 37篇 |
2020年 | 13篇 |
2019年 | 26篇 |
2018年 | 35篇 |
2017年 | 29篇 |
2016年 | 53篇 |
2015年 | 90篇 |
2014年 | 140篇 |
2013年 | 166篇 |
2012年 | 203篇 |
2011年 | 194篇 |
2010年 | 126篇 |
2009年 | 123篇 |
2008年 | 156篇 |
2007年 | 168篇 |
2006年 | 182篇 |
2005年 | 174篇 |
2004年 | 167篇 |
2003年 | 143篇 |
2002年 | 166篇 |
2001年 | 32篇 |
2000年 | 32篇 |
1999年 | 53篇 |
1998年 | 61篇 |
1997年 | 47篇 |
1996年 | 45篇 |
1995年 | 41篇 |
1994年 | 33篇 |
1993年 | 36篇 |
1992年 | 47篇 |
1991年 | 41篇 |
1990年 | 40篇 |
1989年 | 32篇 |
1988年 | 37篇 |
1987年 | 17篇 |
1986年 | 16篇 |
1985年 | 30篇 |
1984年 | 34篇 |
1983年 | 21篇 |
1982年 | 33篇 |
1981年 | 26篇 |
1980年 | 24篇 |
1979年 | 16篇 |
1978年 | 17篇 |
1977年 | 16篇 |
1976年 | 11篇 |
1974年 | 10篇 |
1973年 | 12篇 |
1972年 | 10篇 |
排序方式: 共有3348条查询结果,搜索用时 15 毫秒
11.
Agar diffusion of imbibed seeds yielded significant amounts of diffusible Gibberellin-like substances. An analysis of the extractable and diffusible gibberellin-like substance, including an analysis of the remaining imbibition water of the seeds, indicated that a significant part of these gibberellin-like substances could be attributed to a net biosynthesis of these substances in the imbibing seeds. At the same time it was found that water diffusion yielded considerably more gibberellin-like activities than comparable agar diffusions i.e. 10 to 12 fold in general.Agar as well as water diffusion showed a temperature effect with regard to the yield of gibberellin-like substances particularly during the first 6 h of diffusion. The yield of these substances is lower at 10°C, and remains lower as shown with consecutive diffusions, in comparison with the yields at 20°C or 30°C.With both agar and water diffusion the sum of activities obtained with consecutive diffusions is always higher, often considerably higher, than equal periods of continuous diffusion which is probably due to inactivation and/or interference of inhibitory substances with the bioassay responses. Finally, water diffusates of both seeds and seedlings of the normal growing cv. Violet of Japanese morning glory contained considerably more gibberellin-like activities than those of the dwarf cv. Kidachi which indicated that normals synthesize more gibberellins than dwarfs. 相似文献
12.
Gerard N. Burrow Jeffrey Hopkins Yeshi Dhonden Lobsang Dolma 《The Yale journal of biology and medicine》1978,51(4):441-447
The visit of two Tibetan physicians provided a unique opportunity to gain insight into a practice of medicine very different from that of Western civilization. Initial discussions indicated that the practice of medicine and mysticism were inextricably interwoven in the Tibetan culture. Accordingly, the focus of the study was directed to goiter, which is both common in the Himalayas and easy to define.In Tibetan medical practice, illness is considered to be derived from both proximate and distant causes. Three humors, “wind,” “bile,” and “phlegm” are thought to be responsible for normal mental and physical functions when in balance, but disease when out of balance. Goiter was thought to be due to an imbalance of these humors. The Western discovery that endemic goiter in the Himalayas was due to iodine deficiency explained the proximate cause but did not explain why some individuals have goiter and others do not in the same iodine deficient village. 相似文献
13.
14.
Gerard J. Niemann Jos B. M. Pureveen Gert B. Eijkel Hendrik Poorter Jaap J. Boon 《Oecologia》1992,89(4):567-573
Summary Eleven grass species varying in potential relative growth rate (RGR) were investigated for differences in chemical composition by pyrolysis mass spectrometry. The spectral data revealed correlations between RGR and the relative composition of several biopolymers. Species with a low potential RGR contained relatively more cell wall material such as lignin, hemicellulose, cellulose, polysaccharide-bound ferulic acid and hydroxyproline-rich protein, whereas species with a high potential RGR showed relatively more cytoplasmic elements such as protein (other than those incorporated in cell walls) and sterols. 相似文献
15.
Recent reports have revealed that the asymmetric nature of the nuclear genome of somatic hybrids, produced following the irradiation of one of the parents with X- or gamma rays, is generally much less than had been anticipated. As a consequence, we have begun to investigate whether UV radiation might be used as an alternative or indeed a supplement to the presently-used ionising radiation techniques in such experiments. Cell culture studies have revealed that UV radiation induces the desired physiological effects in sugar beet ( Beta vulgaris ) protoplasts, namely, a prevention of cell division without immediate cytotoxicity. Preliminary studies using denaturing and pulsed field gel electrophoresis have shown that UV can also induce substantial physical fragmentation of DNA. When using the same techniques, less breakdown was observed following gamma radiation. All results were highly reproducible. Such results augur well for the potential use of UV in asymmetric somatic cell fusion experiments. 相似文献
16.
We describe selected aspects of the ecology of the copepod Eurytemora affinis in tide pools of an inland salt marsh near L'Isle-Verte, Québec along the southern shore of the St. Lawrence estuary. E. affinis performed daily horizontal migrations moving from the centers of pools to the banks and into dense algae. Male E. affinis were mainly found in the center of the pools during twilight (21 : 00 hrs) and in dense algae in daylight (12: 00 hrs) whereas most females and copepodites were found near the banks at all three sampling periods. Although these daily movements among sites may have minimized predation by diurnally foraging sticklebacks (Pisces: Gasterosteidae), other explanations for the movements can not be excluded. We also quantified the effects of fish predation upon the population structure of E. affinis. Densities of all stages (nauplius, copepodite, adult) were significantly lower in pools with fish than in pools without fish. Female E. affinis were significantly smaller (mean length) in pools with fish than in pools without fish, indicating that the sticklebacks selectively ate larger females. Male-biased sex ratios were found in both types of pools, which excluded the possibility that biased ratios in this species are caused by selective predation upon the females. 相似文献
17.
Gerard Housden 《BMJ (Clinical research ed.)》1982,285(6356):1744-1745
18.
In a permanent cell line derived from Drosophila embryos, cytoplasmic actin is produced as an unstable precursor, which is subsequently converted to a stable form. This conversion results in a reduction in isoelectric point, with no apparent change in molecular weight. The conversion involves an enzymatic acetylation, and results in an insensitivity to aminopeptidase digestion, suggesting N-terminal blockage. Both the acetylated and unacetylated actins can participate in the assembly of F-actin, but with different efficiencies.This work was supported by a grant from the NIH (GM 22866). 相似文献
19.
20.
Evaluation of Bottlenecks in the Late Stages of Protein Secretion in Bacillus subtilis 总被引:3,自引:0,他引:3 下载免费PDF全文
Albert Bolhuis Harold Tjalsma Hilde E. Smith Anne de Jong Rob Meima Gerard Venema Sierd Bron Jan Maarten van Dijl 《Applied microbiology》1999,65(7):2934-2941
Despite a high capacity for secretion of homologous proteins, the secretion of heterologous proteins by Bacillus subtilis is frequently inefficient. In the present studies, we have investigated and compared bottlenecks in the secretion of four heterologous proteins: Bacillus lichenifomis α-amylase (AmyL), Escherichia coli TEM β-lactamase (Bla), human pancreatic α-amylase (HPA), and a lysozyme-specific single-chain antibody. The same expression and secretion signals were used for all four of these proteins. Notably, all identified bottlenecks relate to late stages in secretion, following translocation of the preproteins across the cytoplasmic membrane. These bottlenecks include processing by signal peptidase, passage through the cell wall, and degradation in the wall and growth medium. Strikingly, all translocated HPA was misfolded, its stability depending on the formation of disulfide bonds. This suggests that the disulfide bond oxidoreductases of B. subtilis cannot form the disulfide bonds in HPA correctly. As the secretion bottlenecks differed for each heterologous protein tested, it is anticipated that the efficient secretion of particular groups of heterologous proteins with the same secretion bottlenecks will require the engineering of specifically optimized host strains. 相似文献