全文获取类型
收费全文 | 5629篇 |
免费 | 57篇 |
国内免费 | 168篇 |
出版年
2024年 | 6篇 |
2023年 | 11篇 |
2022年 | 26篇 |
2021年 | 24篇 |
2020年 | 22篇 |
2019年 | 26篇 |
2018年 | 35篇 |
2017年 | 28篇 |
2016年 | 28篇 |
2015年 | 32篇 |
2014年 | 38篇 |
2013年 | 49篇 |
2012年 | 261篇 |
2011年 | 427篇 |
2010年 | 199篇 |
2009年 | 40篇 |
2008年 | 360篇 |
2007年 | 456篇 |
2006年 | 494篇 |
2005年 | 469篇 |
2004年 | 487篇 |
2003年 | 516篇 |
2002年 | 508篇 |
2001年 | 314篇 |
2000年 | 519篇 |
1999年 | 274篇 |
1998年 | 41篇 |
1997年 | 18篇 |
1996年 | 4篇 |
1995年 | 4篇 |
1994年 | 6篇 |
1992年 | 8篇 |
1991年 | 4篇 |
1990年 | 12篇 |
1989年 | 6篇 |
1988年 | 13篇 |
1987年 | 7篇 |
1986年 | 5篇 |
1984年 | 4篇 |
1983年 | 6篇 |
1979年 | 6篇 |
1978年 | 4篇 |
1973年 | 7篇 |
1972年 | 6篇 |
1971年 | 3篇 |
1970年 | 4篇 |
1969年 | 6篇 |
1967年 | 3篇 |
1966年 | 4篇 |
1964年 | 3篇 |
排序方式: 共有5854条查询结果,搜索用时 15 毫秒
1.
2.
Summary Two winter wheat genotypes (Diószegi 200 and Mv 15) were compared for their in vitro androgenic capacity. On average, the induction frequency of embryogenic structures was 71.7% in Diószegi 200 and only 4.3% in Mv 15. The haploid induction ability of the two genotypes differed considerably, with Diószegi 200 being much higher. The difference in the in vitro inductability of the microspores may result from genetic differences which are manifested in the survival rate of the microspores during the culture period and their adaptability to in vitro conditions. Special DNA fluorochrornes were suitable for studying the different pathways of in vitro androgenesis. Our data indicate that the repeated equal divisions of the microspore nucleus might lead to pollen embryo formation, and subsequent divisions of the vegetative portion of the pollen grain after the first asymmetric microspore mitosis can result in pollen callus formation. 相似文献
3.
Summary A similar pattern of variation with time in observed maxima of daily dose equivalent rates in human thyroids (TD - µSv·d–1) and of daily fallout radioactivities (FR - kBq·m–2) has been found after the Chernobyl accident. An estimate of the time-lag between the maxima in TD lines and the preceding FR peaks was made of about seven days for adult and nine days for juveniles. Applying this time-lag it was possible to estimate transfer factors from the fall-out to thyroid dose equivalent: the highest estimated values were 221 µSv/kBq·m–2 for adult and 641 µSv/kBq·m–2 for juvenile thyroids. These values differ from those published by UNSCEAR (United Nations 1988), which have been calculated for various regions of Czechoslovakia, from ingestion and inhalation intake estimates. A broad variation of transfer factor values could be expected to result from such transfer calculations using ingestion and inhalation estimates. The findings also support the concept of a need for prolonged iodine prophylaxy after emissions of radioiodine into the environment.Abbreviations TD
dose equivalent rates in thyroids [µSv·d–1]
- FR
fall-out radioactivity (-ies) [kBq·m–2] 相似文献
4.
Kulikov AV Bogatyrenko VR Likhtenshteĭn GI Allakhverdiev SI Klimov VV Shuvalov VA Krasnovskiĭ AA 《Biofizika》1983,28(3):357-362
The influence of Mn on saturation curves of ESR spectra of Ph(-) and P(+)(680) at 1-200K in samples with different content of Mn has been studied. An analysis of these data and those on photoinduced changes of fluorescence yield of chlorophyll leads to the conclusion that the Mn-containing centre in Photosystem 2 is a cluster of 4 Mn atoms, two of which can be replaced by Mg(2+) or any other divalent metal. The distances between Mn Na Ph as well as between Mn and P(680) have been estimated. 相似文献
5.
Summary In contrast to the situation found in two classes of warm-blooded vertebrates, mammals and birds, the class Reptilia is not uniform with regard to total genetic content; rather, it contains two distinct categories. The close cytological kinship between snakes and birds was revealed. Both are almost identical in total genetic content, which is about 50 per cent that of placental mammals. Both have microchromosomes, as well as Z-chromosomes very similar in absolute size, comprising nearly 10 per cent of the homogametic haploid (AZ) set. This leads to the implication that snakes and birds originated from the same lineage, and that their Z-chromosomes have not changed substantially since the Jurassic period of the Mesozoic era, about 180 million years ago.Within the reptilian suborder Serpentes, the step-by-step differentiation from the primitive ZW pair to the grossly heteromorphic ZW pair could be observed. In the ancient family Boidae, the sex chromosomes were still homomorphic to each other. In the family Colubridae, the beginning of heteromorphism was manifested in two ways. In some species, a pericentric inversion on the W caused it to differ from the Z; in others, duplication of the W occurred. In the family Crotalidae, the W had apparently achieved its very specialized status; it was a distinctly smaller element.In Säo Paulo, this work was supported by Fundacão de Amparo a Pesquisa do Estado de São Paulo e Fundo de Pesquisas do Instituto Butantan. In Duarte, this work was supported in part by grant CA-05138-05, National Cancer Institute, U. S. Public Health Service. Contribution No. 36-64, Department of Biology, City of Hope Medical Center. 相似文献
6.
The mutagenic activity of more than 120 antimicrobial agents and protective components was investigated. Only Kathon showed
a consistent increase in revertant counts in the Ames test onSalmonella typhimurium. The hereditary bleaching test onEuglena gracilis used for detecting extranuclear mutations, showed positive results for Kathon, triethanolamine and diamine silver tetraborate. 相似文献
7.
Jan Nešvera Jitka Hochmannová Miroslav Pátek Alena Šroglová Věra Bečvářová 《Applied microbiology and biotechnology》1994,40(6):864-866
Gram-positive facultative methylotrophic coryneform bacterium Brevibacterium methylicum was efficiently transformed with various plasmids using electroporation of intact cells. In addition to the plasmid vectors pEC71 and pZ6-1 constructed on the basis of cryptic plasmids from coryneform bacteria, broad-host-range plasmids pLS5 (derivative of plasmid pMV158 from Streptococcus agalactiae) and RSF1010 belonging to the incompatibility group IncQ from Gram-negative bacteria were found to be present as autonomous structurally unchanged DNA molecules in B. methylicum transformants. With the exception of pZ6-1, all these plasmids were stably maintained in B. methylicum cells grown under non-selective conditions. When plasmid DNAs isolated from B. methylicum were used, the highest efficiency of transformation (105 transformants/g DNA) was achieved.
Correspondence to: J. Nevera 相似文献
8.
The isolation of viable egg cells of wheat has been achieved without enzymatic maceration of the ovules. 2,4-D applied to the stigmas resulted 3 to 7 days later in soft ovule tissues which disintegrated upon mechanical manipulation. The isolated egg cells were viable even 2 h after isolation. Their morphology corresponded to that of the in situ egg cells. The mean isolation frequency was 20% (two egg cells per ten ovules). 相似文献
9.
司光伟 《生物化学与生物物理进展》2023,50(6):1243-1243
Acetylcholine, the first identified neurotransmitter, plays crucial roles in various brain functions. One well-known case is its involvement as an activating neurotransmitter in the regulation of locomotion. However, its inhibitory regulatory role, particularly in locomotion, remains poorly understood. In a study conducted by Polat et al., the authors investigated the inhibitory role of acetylcholine in locomotion in C. elegans. In this organism, the acetylcholine-gated chloride channel receptor consists of four subunits. The authors thoroughly examined the loss-of-function of each subunit in movement regulation. Interestingly, the mutant worms were still capable of performing various movements such as forward, backward crawling, and turning, suggesting that the overall movement was not significantly affected. However, quantitative behavior analysis revealed subtle yet significant differences in the timing and postures of the movement in these mutants. Furthermore, the authors employed optogenetics to stimulate a specific neuron involved in backward crawling and demonstrated that the loss-of-function of the receptors in individual neurons affects the transitioning between locomotion modes.
This work provides evidence for the inhibitory regulatory role of acetylcholine in locomotion. The loss-of-function of acetylcholine-gated chloride channel receptors likely disrupts the balance of neuronal and circuit physiology, thereby affecting the regulation of locomotion. Moreover, this study highlights the powerful role of quantitative behavior analysis in discovering and understanding more sophisticated functions of neural circuits. 相似文献
10.
V M Glazer V M Savov S K Abilev S I Shesterin A M Be?m 《Nauchnye doklady vysshe? shkoly. Biologicheskie nauki》1984,(5):85-89
The study of mutagenic effect of 2-aminoantracene and benz(alpha)pyrene on Salmonella triphimurium TA 100 in the Ames test-system in the presence of postmitochondrial fractions S-9 from carp liver with 3-methylcholantrene induced by microsomal oxidation system has been carried out. The metabolic activity and cytochrome P450 contence in carp liver microsomes have been shown to concede considerably those in rats liver. But these characteristics are sufficient for the use of fraction S-9 from carp liver for the study of genotoxic effect of these xenobiotics in the Ames test-system. Several regimes of storage of S-9 preparations from carp liver have been compared. S-9 preparations frozen immediately after isolation preserve their metabolic activity with respect to 2-aminoantracene and benz(alpha)pyrene well. 相似文献