全文获取类型
收费全文 | 7076篇 |
免费 | 433篇 |
国内免费 | 487篇 |
出版年
2024年 | 13篇 |
2023年 | 119篇 |
2022年 | 107篇 |
2021年 | 172篇 |
2020年 | 216篇 |
2019年 | 245篇 |
2018年 | 188篇 |
2017年 | 192篇 |
2016年 | 217篇 |
2015年 | 268篇 |
2014年 | 294篇 |
2013年 | 407篇 |
2012年 | 235篇 |
2011年 | 294篇 |
2010年 | 234篇 |
2009年 | 388篇 |
2008年 | 408篇 |
2007年 | 402篇 |
2006年 | 383篇 |
2005年 | 302篇 |
2004年 | 289篇 |
2003年 | 314篇 |
2002年 | 206篇 |
2001年 | 175篇 |
2000年 | 178篇 |
1999年 | 151篇 |
1998年 | 168篇 |
1997年 | 163篇 |
1996年 | 112篇 |
1995年 | 109篇 |
1994年 | 113篇 |
1993年 | 118篇 |
1992年 | 119篇 |
1991年 | 86篇 |
1990年 | 86篇 |
1989年 | 75篇 |
1988年 | 61篇 |
1987年 | 51篇 |
1986年 | 59篇 |
1985年 | 68篇 |
1984年 | 36篇 |
1983年 | 23篇 |
1982年 | 37篇 |
1981年 | 35篇 |
1980年 | 18篇 |
1979年 | 16篇 |
1978年 | 14篇 |
1977年 | 9篇 |
1976年 | 6篇 |
1972年 | 7篇 |
排序方式: 共有7996条查询结果,搜索用时 312 毫秒
71.
George P. Livi James A. Cardelli Randall L. Dimond 《Differentiation; research in biological diversity》1985,29(3):207-215
The lysosomal enzyme alpha-mannosidase-1 is one of the earliest developmentally controlled gene products in Dictyostelium discoideum. Although this enzyme is synthesized throughout the first 20 h of development, it is not required for complete morphogenesis, since structural gene (manA) mutants lacking activity develop normally. We isolated six strains deficient in alpha-mannosidase-1 activity which, unlike structural gene mutants, fail to aggregate. Fruiting revertants of these strains accumulate wild-type levels of alpha-mannosidase-1 activity, suggesting that both the enzymatic and morphological defects are caused by single mutations in nonstructural genes essential for early development. Direct genetic evidence for mutations outside of the structural gene was obtained by complementation analysis. We used alpha-mannosidase-1-specific monoclonal antibodies to analyze the biochemical defects in these mad (alpha-mannosidase-1-deficient) mutants. All mad mutants show a significantly reduced relative rate of enzyme precursor biosynthesis. The mad-404 mutation results in a complete lack of precursor biosynthesis, as well as a lack of functional alpha-mannosidase-1 mRNA. In some cases, however, the enzymatic defect results from improper post-translational modification which affects precursor processing. We conclude that a small number of aggregation-essential genes are involved in regulating the synthesis, modification, and processing of alpha-mannosidase-1 during development. 相似文献
72.
The effect of sulphide on the growth of several species of salt-marsh plants was investigated. Relative growth rates were significantly reduced in two upper-marsh species, Festuca rubra and Atriplex patula, and in the lower-marsh species Puccinellia maritima. However the growth of Salicornia europaea, a species frequently associated with sulphide-containing sediments, was unaffected. In a separate experiment the wide ranging halophyte Aster tripolium, also appeared to be tolerant of sulphide at a concentration frequently encountered in salt marshes. Sulphide pretreatment inhibited the activity of two metallo-enzymes, polyphenol oxidase and external phosphatase, in plants from the upper marsh, but had no effect on enzymes from P. maritima or S. europaea. The rate of respiration by root tissue was significantly reduced in all of the species investigated but whereas the uptake of 86rubidium was markedly inhibited in the other three species, uptake by S. europaea showed a significant stimulation. Similarly, whereas sulphide-grown plants of F. rubra, A. patula and P. maritima had a considerably reduced tissue iron content, the total iron concentration in S. europaea tissues was comparable to that of the controls. When the sulphide-tolerant species A. tripolium was grown in sulphide-containing media there was no significant effect on the tissue concentration of any of the elements investigated. These results are discussed in relation to possible mechanisms of sulphide toxicity and resistance. 相似文献
73.
74.
Jaap Dorgelo 《Aquatic Ecology》1987,21(1):95-110
The hydrobiid snailPotamopyrgus jenkinsi (E.A. Smith), characterized by parthenogenesis and ovovivipary, was quantitatively sampled monthly between June, 1982, and December, 1986, on sandy bottoms in the shallow zones of the meso-oligotrophic Lake Maarsseveen I and the eutrophic Lake Maarsseveen II. The snail demonstrated a very clumped distribution in both lakes. The mean numbers of juveniles and adults taken together fluctuated strongly. Organisms in Lake I showed relatively high densities (up to 25,000 per m2) in 1982, followed by a sudden drop to values approaching zero in December, 1982, with a subsequent rapid increase in densities, fluctuating between 2,000 and 200 per m2. In Lake II, densities of snails fluctuated between 13,000 and 300 per m2 with decreases in the spring of 1985 and 1986. The various types of decreases in the lakes are extensively discussed, but no explanation is presently available. The reduction in Lake I was of catastrophic proportions, but the speed of recovery of the population was remarkable.Floating was observed only in Lake I, and only during the occurrence of the highest densities on the sediment. Burrowing behaviour was very common, but strongly suppressed under an uninterrupted dark regime. A shift of temperature from 15 to 22°C had the same effect. A number of submerged macrophyte species from Lake I proved to attractP. jenkinsi in the absence of sandy substrate, though these plants were only covered by the snail during the period of the highest densities in 1982. Temperatures of 20°C or lower were well tolerated, unlike temperatures of 25 and 30°C. Growth was distinct at 10, 15 and 20°C. Keeled individuals were encountered in much higher numbers in Lake I than in Lake II. 相似文献
75.
P. Janardhan Reddy K. Vaidyanath 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1986,71(5):757-760
Summary The response of plant cells to salt stress was studied on embryo derived calli of rice (Oryza sativa L.) in order to identify cellular phenotypes associated with the stress. The feasability of selecting salt tolerant callus and its subsequent regeneration to plants was also studied. Callus was grown on agar-solidified media containing 0%, 1% and 2% (w/v) NaCl for 24 days. Parameters such as fresh weight, dry weight, soluble protein and proline content were measured. The callus growth decreased markedly with increasing NaCl concentration in the medium. The proline content was enhanced several fold in salt stressed calli. A prolonged exposure of callus to the salt environment led to discolouration and arrested growth in the majority of the calli and only a small number of callus cells maintained healthy and stable growth. These variants were subcultured every three weeks for a period of four months onto medium containing 1% NaCl to identify tolerant lines. At the end of the third cell passage, the tolerant calli were transferred to regeneration medium to regenerate plants. The regeneration frequency in the salt-selected lines was enhanced when compared to unselected lines. 相似文献
76.
A Cu-complex was isolated from the roots of copper-tolerant Mimulus guttatus. The elution volume of the complex determined by gel permeation chromatography was similar to that of rat-liver cadmium thionein. The complex was heat stable, had a relatively high ratio of absorbance at 254 nm: 280 nm and incorporated 35S. The complex, purified using a combination of gel permeation chromatography and anion-exchange chromatography, contained more glutamine/glutamic acid and glycine residues than mammalian metallothioneins. The amount of the complex in roots increased after 5 h growth in a solution containing 16 M Cu. Induction was preceded by an increase in the concentrations in root tissue of unknown compounds containing sulphur which may serve as precursors. The availability of these compounds appeared to regulate the rate of synthesis of this Cu-complex.Abbreviations CuBP
copper-binding protein
- HPLC
highperformance liquid chromatography
- MT
metallothionein
- Th
thionein
- Tris
2-amino-2-(hydroxymethyl)1-3-propanediol 相似文献
77.
The structural organization of Tetrahymena pyriformis is such that its cilia are remote from the main centers of lipid metabolism. As a result, the ciliary membrane lipid composition of cells exposed to low-temperature stress is initially unaffected by the significant metabolic changes induced in microsomal membranes. Nevertheless, changes in the ciliary membrane lipid composition can be detected during the first 4 h of cold exposure. A combination of in vivo and in vitro experiments has provided strong evidence for a substantial retailoring of ciliary phospholipid molecular species in situ in the absence of any importation of lipids from the cell interior or change in overall ciliary fatty acid composition. The mechanism responsible for the ciliary lipid changes is independent of the one(s) triggering internal acclimation responses. Our observations establish for the first time that chilling stress can simultaneously induce separate and distinctive lipid modification responses in different parts of a cell. This finding could be important in identifying the molecular ‘sensor’ capable of actuating stress-induced lipid changes. 相似文献
78.
David Levy 《Physiologia plantarum》1983,57(1):169-173
Potato plants ( Solanum tuberosum L. cvs 'Up-to-Date', 'Desiree', 'Alpha', 'Spunta', 'Elvira' and 'Troubadour') were exposed to cycles of water stress and relief during growth. Severe water deficit induced increased proline content 6- to 7-fold in nonturgid leaves which just started to wilt, and 8- to 27-fold in fully wilted leaves of potatoes. However, proline content was not affected during the early stages of stress development over a range of osmotic potentials in the leaves. The rising proline content was related to turgor loss of leaves independent of changes in the osmotic potentials, which indicates that proline involvement in osmoregulation of potato leaves is unlikely.
Repeated cycles of water stress and relief resulted in increased proline and α-amino nitrogen content in the tuber tissue of some of the cultivars. The smallest increase in proline content was obtained in 'Alpha' tubers and the content of α-amino nitrogen remained unaffected by the water stress. Concomitantly, 'Alpha' was the most drought-tolerant cultivar, as determined by its capacity to accumulate dry matter in tubers under stress conditions. On the other hand, in tubers of cultivars which were more susceptible to drought, a marked increase in proline and α-amino nitrogen was observed in response to water stress. The possible association of these findings with tolerance of potatoes to repeated short periods of drought is discussed. 相似文献
Repeated cycles of water stress and relief resulted in increased proline and α-amino nitrogen content in the tuber tissue of some of the cultivars. The smallest increase in proline content was obtained in 'Alpha' tubers and the content of α-amino nitrogen remained unaffected by the water stress. Concomitantly, 'Alpha' was the most drought-tolerant cultivar, as determined by its capacity to accumulate dry matter in tubers under stress conditions. On the other hand, in tubers of cultivars which were more susceptible to drought, a marked increase in proline and α-amino nitrogen was observed in response to water stress. The possible association of these findings with tolerance of potatoes to repeated short periods of drought is discussed. 相似文献
79.
Richard M. Emery 《Hydrobiologia》1974,44(4):511-516
Replidated experiments indicate an association between thermal tolerance of juvenile bluegill sunfish and adaptive pigmentation to background color. Bluegills exposed to black backgrounds were able to survive higher temperatures longer than those exposed to blond backgrounds, when water temperatures were raised at a rate of 0.1°C/h from 24 to 36°C. Plausible explanations are considered which involve heat radiation effectiveness (by the fish) related to the black body concept, and stress associated with adaptive pigmentation. 相似文献
80.
Hiroshi Yura 《Ecological Research》1989,4(3):351-360
The underlying mechanisms of drought resistance in seedlings ofLarix kaempferi as compared with those inAbies veitchii were investigated. First, seedlings of the two species were desiccated in air of differing water potential, and their survival
rates were compared to detect any difference in desiccation tolerance between them. Contrary to differences in their drought
resistance,A. veitchii showed higher survival. Next, the shoot heights and root depths of the two species were compared to detect any difference
in desiccation avoidance. Although the shoot height differed little between the two species, the roots ofL. kaempferi seedlings were much deeper than those ofA. veitchii seedlings, and the death rate under progressing drought was well correlated with the root extensibility of each species.
It was thus concluded thatL. kaempferi seedlings are able to avoid desiccation by extending their roots in deep soil faster thanA. veitchii seedlings, thereby resisting to drought.
This paper is dedicated to Prof. Toshiro Saeki to mark his fruitful career in plant ecology. 相似文献