全文获取类型
收费全文 | 5889篇 |
免费 | 254篇 |
国内免费 | 1篇 |
专业分类
6144篇 |
出版年
2023年 | 27篇 |
2022年 | 21篇 |
2021年 | 47篇 |
2020年 | 28篇 |
2019年 | 47篇 |
2018年 | 117篇 |
2017年 | 97篇 |
2016年 | 133篇 |
2015年 | 115篇 |
2014年 | 150篇 |
2013年 | 271篇 |
2012年 | 504篇 |
2011年 | 705篇 |
2010年 | 327篇 |
2009年 | 110篇 |
2008年 | 472篇 |
2007年 | 457篇 |
2006年 | 397篇 |
2005年 | 366篇 |
2004年 | 286篇 |
2003年 | 274篇 |
2002年 | 212篇 |
2001年 | 77篇 |
2000年 | 151篇 |
1999年 | 85篇 |
1998年 | 59篇 |
1997年 | 20篇 |
1996年 | 22篇 |
1995年 | 22篇 |
1994年 | 18篇 |
1993年 | 26篇 |
1992年 | 25篇 |
1991年 | 20篇 |
1990年 | 18篇 |
1989年 | 21篇 |
1987年 | 21篇 |
1985年 | 20篇 |
1984年 | 23篇 |
1982年 | 26篇 |
1981年 | 13篇 |
1980年 | 18篇 |
1979年 | 17篇 |
1978年 | 14篇 |
1977年 | 11篇 |
1976年 | 19篇 |
1975年 | 12篇 |
1974年 | 20篇 |
1973年 | 11篇 |
1972年 | 11篇 |
1971年 | 12篇 |
排序方式: 共有6144条查询结果,搜索用时 15 毫秒
1.
Further studies on theisfA mutation responsible for anti-SOS and antimutagenic activities inEscherichia coli are described. We have previously shown that theisfA mutation inhibits mutagenesis and other SOS-dependent phenomena, possibly by interfering with RecA coprotease activity. TheisfA mutation has now been demonstrated also to suppress mutator activity inE. coli recA730 andrecA730 lexA51(Def) strains that constitutively express RecA coprotease activity. We further show that the antimutator activity of theisfA mutation is related to inhibition of RecA coprotease-dependent processing of UmuD. Expression of UmuD' from plasmid pGW2122 efficiently restores UV-induced mutagenesis in therecA730 isfA strain and partially restores its mutator activity. On the other hand, overproduction of UmuD'C proteins from pGW2123 plasmid markedly enhances UV sensitivity with no restoration of mutability. 相似文献
2.
The level of urine selenium in healthy adult population, 230 persons, was examined. Persons were selected regarding sex, ages, and smoking habits. No differences versus these observations have been found. For a total, group values are 16.96 Se nmol/creatinine mmol, SD=5.44. It is possible from a single-void specimen to express daily excretion of selenium. 相似文献
3.
In vitro culture was established from shoot tips ofDigitalis lanata cotyledonous plants. The propagated plant material was rooted, transplanted into soil and grown under field conditions. Lanatoside
C content was determined in a total of 20 clones and statistically evaluated by means of variance analysis of unequal-sized
samples.In vitro clonal propagation ofD. lanata was found not to affect lanatoside C content. Drug level was dependent on a plant genotype. 相似文献
4.
S. Živanović 《Human Evolution》1988,3(5):329-334
Studies of the morphological features of the temporal region of mesolithic skulls from Padina in the Iron Gate Gorge of the
Danube revealed a very prominent and large supramastoid ridge which is the most striking feature in skulls of both sexes.
Mastoid processes were larger in male skulls, but in 25% of the cases there was an overlap between the size of the processes
in male and female specimens. The mastoid ridge was prominent in both sexes. The digastric fossa was always well defined in
both sexes and in the two thirds of the skull specimens it was deep. The posterior root of the zygoma was prominent in all
the skulls, but it was better developed in the male specimens. The parietotemporal suture in both sexes rises above the level
of the pterion. There were no morphological or anthropometrical differences between the left and the right side of individual
skulls outside the limits of natural asymmetry. All these morphological characteristics of the temporal region may help in
racial and sexual diagnosis of the Mesolithic skulls from the Iron Gate Gorge. 相似文献
5.
In order to improve the interpretation of the results of pollen analyses concerning mediaeval anthropogenous sediments a set of recent seeds of the principal cereals was pollenanalytically elaborated. The aim was to find out whether pollen grains can be preserved on mature seeds. The pollen spectra obtained from the material of mediaeval anthropogenous sediments include, in most cases, a large amount of pollen of certain plants including cereals. It is suggested that part of the pollen spectra from anthropogenous sediments need not come from air transport, i.e. from the surrounding vegetation; it could get into waste pits in another way. Above all, some foodstuffs were considered to be the sources of pollen grains. The origin of some grains coming from honey and from plant infusions has, at least partly, been demonstrated (Jankovská 1987). As the next step, attention was paid to other products, mainly those of cereals. An experimental pollen analysis of recent seeds of selected cereals showed that pollen grains are preserved on their surface, often in high concentrations. This is due, above all, to the cleistogamy of most cereals. The highest values of pollen grains have so far been found inAvena sativa andPanicum milliaceum. Triticum aestivum andHordeum vulgare showed lower values of pollen grains. The lowest amounts were found in heterogamousSecale cereale. Simultaneously, the pollen of plants growing in a neighbourhood, above all the pollen of field weeds, was found sticking to the seeds. 相似文献
6.
Effect of applied and internal hormones on vitrification and apical necrosis of different plants cultured in vitro 总被引:2,自引:0,他引:2
Natalia V. Kataeva Irena G. Alexandrova Raisa G. Butenko Elena V. Dragavtceva 《Plant Cell, Tissue and Organ Culture》1991,27(2):149-154
Development of vitrification and apical necrosis was followed in Camellia sinensis, Gerbera jamesonii, Malus domestica and hybrid Populus tremula x P. alba shoots cultured in vitro on Murashige & Skoog (MS) medium with different concentrations of growth regulators. High humidity in the culture vessels and excess of BA in the medium were found to be the major factors influencing vitrification. Lack of exogenous cytokinin in the medium during successive subcultures induced apical necrosis in poor-rooting species (Malus domestica, Camellia sinensis). The level of internal phytohormones (ABA, IAA, IPA, 2iP, Z, ZR) was determined in the apple shoots by means of ELISA. The content of internal cytokinins in the vitrified apple shoots was several times greater than in normal ones, which supports the hypothesis that excess of cytokinins, inducing rapid divisions of cells in meristems in the atmosphere with high humidity, is responsible for vitrification. Apical necrosis of the plantlets that appeared after cultivation on cytokinin-free medium is the result of deficiency in endogenous hormones in apple shoots and this being confirmed by analysis of endogenous hormones in apple shoots.Abbreviations BA
benzyladenine
- BHT
butylated hydroxy-toluene
- ABA
abscisic acid
- IAA
indole-3-acetic acid
- ELISA
enzyme-linked immunosorbent assay
- IPA
isopentenyladenosine
- 2iP
isopentenyladenine
- NAA
naphthyl-3-acetic acid
- TBS
trishydroxymethylaminomethane buffered saline
- TLC
thin layer chromatography
- Z
zeatin
- ZR
zeatin riboside 相似文献
7.
The administration of diazepam 1.0 mg/kg decreased the level of plasma corticosterone in female but not in male Wistar rats. Picrotoxin, another drug affecting GABAergic transmission, also brought about an increase of plasma corticosterone in both sexes. However, in order to achieve a plasma corticosterone increase of similar magnitude (more than 500%) a threefold higher dose of picrotoxin had to be given to males. When the convulsive properties of picrotoxin were tested, it became evident that the dose of picrotoxin (2.5 mg/kg) which was subconvulsive in male was almost 100% convulsive in female rats. The existing sex differences in the response of rats to drugs affecting GABAergic transmission might have possible implications in the treatment of GABA system dysfunction. 相似文献
8.
9.
Dr. Radivoj Krstić 《Cell and tissue research》1985,241(3):677-681
Summary Autophagosome formation in rat and gerbil pinealocytes is described. It starts with the setting up of a tubular acid phosphatase-rich cisterna which gradually wraps around cytoplasmic areas to be catabolized. In light of obtained findings, it seems that the autophagosome formation in pinealocytes is a type of lysosome wrapping mechanism. 相似文献
10.