全文获取类型
收费全文 | 3450篇 |
免费 | 142篇 |
国内免费 | 173篇 |
出版年
2023年 | 20篇 |
2022年 | 33篇 |
2021年 | 53篇 |
2020年 | 59篇 |
2019年 | 73篇 |
2018年 | 72篇 |
2017年 | 62篇 |
2016年 | 72篇 |
2015年 | 77篇 |
2014年 | 114篇 |
2013年 | 148篇 |
2012年 | 77篇 |
2011年 | 120篇 |
2010年 | 96篇 |
2009年 | 132篇 |
2008年 | 155篇 |
2007年 | 152篇 |
2006年 | 147篇 |
2005年 | 171篇 |
2004年 | 141篇 |
2003年 | 122篇 |
2002年 | 106篇 |
2001年 | 111篇 |
2000年 | 91篇 |
1999年 | 97篇 |
1998年 | 101篇 |
1997年 | 78篇 |
1996年 | 66篇 |
1995年 | 77篇 |
1994年 | 67篇 |
1993年 | 75篇 |
1992年 | 72篇 |
1991年 | 65篇 |
1990年 | 57篇 |
1989年 | 64篇 |
1988年 | 52篇 |
1987年 | 61篇 |
1986年 | 54篇 |
1985年 | 49篇 |
1984年 | 68篇 |
1983年 | 51篇 |
1982年 | 45篇 |
1981年 | 36篇 |
1980年 | 35篇 |
1979年 | 28篇 |
1978年 | 18篇 |
1977年 | 12篇 |
1976年 | 11篇 |
1975年 | 7篇 |
1973年 | 5篇 |
排序方式: 共有3765条查询结果,搜索用时 15 毫秒
991.
The 31P nuclear magnetic resonance (NMR) spectra of benzene solutions of hydrated dipalmitoyl lecithin (DPL) inverted micelles, with and without incorporated paramagnetic lanthanide ions, have been recorded. Individual resonances for micelles containing none, one, and two ions can be resolved and observed in the presence of one another. The relative intensities of these peaks yield some information on the state of aggregation of lipid inverted micelles prepared by ultrasonic irradiation. The relative intensities and chemical shifts of resonances of unsonicated mixtures of preformed micelles containing different numbers of ions per micelle indicate that some kind of equilibration occurs. The data are consistent with a selective fusion of multi-ion micelles with ion-free micelles. The NMR spectra place constraints on the lifetimes of metal ions and lipid and water molecules within a micelle before transfer to another. 相似文献
992.
The microelectrode technique was used to follow oscillations in membrane potential in mesophyll cells of spinach (Spinacia oleracea L.) during exposure do different photoperiodic conditions. Both high-frequency oscillations and circadian variations were observed. The circadian rhythm was imposed on the period of high-frequency oscillation during short days as well as in continuous light: The free-running period was 25.2 h. The average period of high-frequency oscillation increased from 7.64 min in the dark to 19.95 min in the light within several minutes after dark to light transition. This period length coincides with the established period length for oscillations in the redox potential in the chloroplast suspensions of spinach.Abbreviations CL
continuous light
- SD
short day
- MP
membrane potential 相似文献
993.
Brian Colman 《Journal of phycology》1978,14(4):434-437
The claim that Chlorella sp. (CCAP 211/8p), sometimes referred to as C. fusca, Shihira and Krauss, does not excrete glycolate has been reexamined. Chlorella sp. grown on 5% CO2in air, excreted glycolate when incubated in light in 10 mM bicarbonate. Excretion ceased 30–60 min after transfer of the cells to air and no excretion could be detected with air-grown cells or with cells grown on 5% CO2in media buffered at pH 8.0. Incubation with 10 mM isonicotinyl hydrazide, a glycolate pathway inhibitor, caused excretion in air-grown cells and stimulated excretion in CO2-grown cells indicating that both the rate of glycolate synthesis and metabolism is higher in CO2grown cells than in air-grown cells. Enhanced glycolate synthesis and excretion in CO2-grown cells is correlated with law photosynthetic rate in 10 mM bicarbonate, and the photosynthetic rate of these cells doubles over a period of 2–2.5 h after initial transfer from high CO2to bicarbonate. This correlation of photosynthetic induction with cessation of glycolate excretion is similar to that reported in a bluegreen alga and thought to occur in other green algae. These results indicate that glycolate excretion and its regulation in this species of Chlorella is not different from that in other algae. 相似文献
994.
For dark-grown seedlings of Pharbitis nil capacity to flower in response to a single inductive dark period was established by 24 h white, far-red (FR) or ruby-red (BCJ) light and by a skeleton photoperiod of 10 min red (R)-24 h dark-10 min R. FR alone was ineffective without a brief terminal (R) irradiation, confirming that the form of phytochrome immediately prior to darkness is a crucial factor for flowering in Pharbitis. The magnitude of the flowering response was significantly greater after 24 h FR or white light (WL) (at 18° C and 27° C) than after two brief skeleton R irradiations, but the increased flowering response was not attributable to photosynthetic CO2 uptake because this could not be detected in seedlings exposed to 24 h WL at 18° C. Photophosphorylation could have contributed to the increased flowering response as photosystem I fluorescence was detectable in plants exposed to FR, BCJ, or WL, but there were large differences between flowering response and photosystem I capacity as indicated by fluorescence. We conclude that phytochrome plays a major role in photoresponses regulating flowering. There was no simple correlation between developmental changes, such as cotyledon expansion and chlorophyll formation during the 24-h irradiation period, and the capacity to flower in response to a following inductive dark period. Changes in plastid ultrastructure were considerable in light from fluorescent lamps and there was complete breakdown of the prolamellar body with or without lamellar stacking at 27 or 18° C, respectively, but plastid reorganization was minimal in FR-irradiated seedlings.Abbreviations BCJ
irradiation from photographic ruby-red lamps
- FR
far-red light
- Pfr
far-red-absorbing from of phytochrome
- P
total phytochrome content
- R
red light
- WL
white light from fluorescent lamps 相似文献
995.
L. M. Taylor 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1978,52(6):241-249
Summary Parthenogenetic seed induction was performed on one clone of Solanum tuberosum subspecies andigena (2n=4x=48) using S. phureja (2n=2x=24) marker inducer clones. The parthenogenetic population when grown was found to contain both diploid and tetraploid individuals presumably arising from reduced and unreduced gametes, respectively. Variation patterns in the diploid and tetraploid sub-populations, as well as a population obtained by selfing the parental clone, were compared to try and elucidate the origin of the tetraploid parthenotes. From the results of this one generation it appeared that the tetraploid parthenogenetic plants had been produced by a mechanism equivalent to second division restitution (SDR). 相似文献
996.
Changes in RNA concentration in the shoot apical meristem during induction and the transition to flowering were measured histochemically in Silene coeli-rosa (L.) Godron, a long-day plant. In the apices of plants induced by 7 long days the RNA concentration increased to about 25 per cent higher than in non-induced plants. Three long days did not induce flowering but resulted in a transient rise in RNA concentration. When plants were given long days interrupted by varying numbers of short days successful induction was accompanied by a sustained increase in RNA concentration but those treatments which were not inductive gave only transient increases in RNA. Gibberellic acid had no effect on induction or apical growth rates but increased the RNA concentration by 50 per cent or more in both induced and non-induced plants. Plants induced to flower at 13° C had the same RNA concentration and growth rate at the apex as in non-induced plants at 20° C. Since changes in RNA concentration in the apex could occur without changes in growth rate and without flowering, and induction could occur without a change in RNA concentration or growth rate, it is suggested that the increase in RNA and growth rate which normally occur at the transition to flowering might not be essential for the formation of a flower but may be more closely related to the rapid growth associated with the formation of the inflorescence.Abbreviations LD
long day
- SD
short-day 相似文献
997.
Induction of nitrate reductase EC 1.6.6.1 in etiolated barley (Hordeum vulgare L., var. Proctor) required continuous illumination and showed a lag period of about three hours. During the first 16 h of illumination the ratio NADH/NAD and NADPH/NADP, taken as a measure of internal oxidation reduction potential, declined. The inhibitor DCMU applied to whole leaves at concentrations shown to inhibit the reduction of cytochrome f by Photosystem 2 light did not inhibit the induction of nitrate reductase nor did it diminish the ratio of reduced to oxidised puridine nucleotides in the early hours of greening. It was concluded that light driven electron flow was not necessary for nitrate reductase induction. Chloramphenicol gave a slight inhibition of nitrate reductase induction. Laevulinic acid was added to greening barley leaves to inhibit tetrapyrrole pigment biosynthesis and plastid development. It strongly inhibited chlorophyll synthesis and nitrate reductase induction, with relatively little effect upon Photosystem 1 and 2 activities in isolated plastids. The activities of other inducible enzymes and control enzymes were little affected by laevulinic acid. Laevulinic acid also inhibited nitrate reductase induction by added nitrate in fully-greened illuminated plants grown in nitrate-free medium and so is unlikely to be acting through inhibition of plastid development. This inhibitor lowered the level of protohaem in whole leaves and plastids of greening barley and it is postulated that it may diminish the protohaem available for the assembly of a cytochrome b component of nitrate reductase.Abbreviations DCMU
3-(3:4-Dichlorophenyl)-1:1-dimethylurea
- LA
laevulinic acid 相似文献
998.
Experimental data on the content in metal ions of DNA preparations from various neoplastic and healthy tissues are summarized:
metal ions are preferentially bound to reiterative DNA sequences, where they may induce conformational variations and thus
modify the binding of effector molecules such as repressors and polymerases.
A model is described where essential and toxic metals are successively loaded on ligand acceptor groups of increasing affinity
and thus may reach the final active sites: enzymes and reiterative DNA sequences (equated at least partially to regulative
DNA sequences). The effects of some molecules, including peptides, antibiotics, growth factors, hormones, and antineoplastic
substances, on DNA conformation could be explained in part by their chelating ability.
The neoplastic state may be induced by a modification of metal ion transfer chains: quantitatively by a continuous derepression
of genes coding for metal ligands, genes that are only temporarily derepressed during development in normal cells, and qualitatively
by modifications of the nucleotidic sequence of structural genes leading to an increase of the chelating ability of the coded
metal ligand. 相似文献
999.
Summary Chondrocytes were isolated from rat epiphyseal cartilage, cultured in vitro, and exposed to exogenous tracers which accumulated in their lysosomes. The cells were then injected into the posterior tibial muscle of animals from the same outbred strain, where they reconstructed calcifying hyaline cartilage. The mineralization of the tissue was followed by ingrowth of blood capillaries from the host bed. Macrophage-like cells surrounding the vessels phagocytized degenerated chondrocytes and unmineralized matrix, whereas multinucleated chondroclasts removed some of the mineralized cartilage matrix. Mesenchyme-like cells accompanying the invading vessels attached to the remaining septa of calcified cartilage matrix and developed into osteoblasts depositing bone matrix on the surface of these septa. The apparent lack of inherent tracer labeling of the lysosomes in the different bone cells indicate that they were derived from the host. No signs of transformation of chondrocytes into bone cells were observed.When isolated rat epiphyseal chondrocytes were injected into the wall of the hamster cheek pouch, calcifying cartilage was reconstructed without signs of subsequent ossification. Transplantation of cartilage reconstructed in the hamster into the dorsal muscles of rats was, however, followed by formation of bone by a sequence analogous to that described above. Such an osteogenetic response was also obtained when the cartilage had been devitalized before transplantation.These experiments show that calcified cartilage, developing in or grafted into an intramuscular site, is able to induce and serve as a substrate for endochondral bone formation, similar to that occurring during normal development. They further indicate that bone induction by calcified cartilage does not require the presence of living chondrocytes.Financial support was obtained from the Swedish Medical Research Council (proj. no. 03355), the King Gustaf V 80th Birthday Fund, and from the funds of Karolinska Institutet. The authors thank Karin Blomgren for technical assistance and Inger Lohmander-Åhrén and Eva Pettersson for secretarial helpOn leave from the Department of Histology and Embryology, Medical Academy, Warsaw, Poland 相似文献
1000.
EcoRI restriction fragments derived from the DNA of bacteriophage lambda and Escherichia coli were fractionated by density gradient centrifugation of their mercury complexes in Cs2SO4 and subsequent electrophoresis on a horizontal agarose-gel slab. In this two-dimensional display, lambda fragments were resolved into six components and E coli fragments into more than 108 components. Bacterial chromosome regions contiguous to lambda prophage integrated at different sites were amplified by induction, and the EcoRI fragments were subjected to the two-dimensional analysis. As expected, the sets of amplified fragments were clearly different among the various lysogens. The approximate genome region affected by induction was estimated as one-tenth of the whole chromosome. 相似文献