全文获取类型
收费全文 | 44438篇 |
免费 | 2878篇 |
国内免费 | 5565篇 |
专业分类
52881篇 |
出版年
2024年 | 195篇 |
2023年 | 867篇 |
2022年 | 1185篇 |
2021年 | 1405篇 |
2020年 | 1412篇 |
2019年 | 1973篇 |
2018年 | 1588篇 |
2017年 | 1378篇 |
2016年 | 1386篇 |
2015年 | 1358篇 |
2014年 | 2285篇 |
2013年 | 3209篇 |
2012年 | 1731篇 |
2011年 | 2075篇 |
2010年 | 1650篇 |
2009年 | 2106篇 |
2008年 | 2130篇 |
2007年 | 2298篇 |
2006年 | 2017篇 |
2005年 | 1822篇 |
2004年 | 1536篇 |
2003年 | 1492篇 |
2002年 | 1349篇 |
2001年 | 978篇 |
2000年 | 893篇 |
1999年 | 851篇 |
1998年 | 821篇 |
1997年 | 714篇 |
1996年 | 620篇 |
1995年 | 637篇 |
1994年 | 610篇 |
1993年 | 561篇 |
1992年 | 526篇 |
1991年 | 517篇 |
1990年 | 400篇 |
1989年 | 428篇 |
1988年 | 383篇 |
1987年 | 318篇 |
1986年 | 339篇 |
1985年 | 561篇 |
1984年 | 699篇 |
1983年 | 417篇 |
1982年 | 520篇 |
1981年 | 499篇 |
1980年 | 439篇 |
1979年 | 315篇 |
1978年 | 252篇 |
1977年 | 250篇 |
1976年 | 224篇 |
1973年 | 183篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
161.
The effects of mineral salts constituting water hardness on fecundity ofDaphnia magna were assessed. Of the salts tested, increased concentrations of NaHCO3 and MgSO4 had no effect on fecundity, CaSO4 significantly increased fecundity, and KCl significantly reduced fecundity. The number of offspring produced per daphnid
was correlative to the CaSO4 concentration at CaSO4 concentrations between 91 and 2100 mg/ℓ. The effects of CaSO4 on daphnid fecundity could influence the interpretive outcome of industrial wastewater toxicity tests using this species
when the waste and dilution waters contain different concentrations of CaSO4. It is recommended that when performing these tests, dilution water be sampled at the intake site of the industry's water
source, thus assuring initial comparability of the waste and dilution waters. The CaSO4 content of the water prior to and after industrial use should be determined to identify any alterations of CaSO4 concentration during use. Identification of CaSO4 concentration differences can aid in the interpretation of effects associated with the wastewater. 相似文献
162.
Kinetics of micronucleus formation in relation to chromosomal aberrations in mouse bone marrow 总被引:1,自引:0,他引:1
A simulation analysis of the kinetics of micronucleus formation in polychromatic erythrocytes in mouse bone marrow was performed after a single administration of 3 chemicals--mitomycin C (MMC), 6-mercaptopurine (6-MP) and 1-beta-D-arabinofuranosylcytosine (Ara-C)--with different modes of action. The time-response patterns in the incidence of chromosomal aberrations and micronuclei after treatment with each chemical were compared and subjected to the simulation study with 3 parameters. Two of them, the time between the final mitotic metaphase of the erythroid series and nucleus expulsion (T1), and the duration of the polychromatic erythrocyte (PCE) stage in the bone marrow (T2), were almost identical for the 3 chemicals. However, the coefficients of formation rate of micronucleated cells resulting from cells with chromosomal aberration(s) (k) differed: Ara-C differed from the other two. These results indicate that chromosomal aberrations, especially chromatid breaks and probably gaps, induced by this chemical, effectively contribute to micronucleus formation. The DNA content of micronuclei was also compared to the length of acentric fragments induced by Ara-C and it was found that their distributions were comparable. These findings strongly suggest that chromosomal aberrations induced by chemicals are essential events for the induction of micronuclei in the PCE of bone marrow. 相似文献
163.
A technique involving culture in soft agar was used for the assay of forward mutation of V79 cells to 6-thioguanine (6TG) resistance. The main reason for the use of soft agar was to prevent reduction in recovery of mutants depending on the cell density plated for mutation selection, which is the chief problem in the liquid method, and which results mainly from metabolic co-operation due to cell-to-cell contact.V79 cells grew well in fortified soft agar medium (DMEM + 20% FBS) showing cloning efficiencies (>80%) as high as in liquid culture. Therefore, V79/HGPRT mutagenesis could be assayed quantitatively in soft agar culture.The frequency of 6TG-resistant colonies in agar selective medium increased linearly with increase in concentration of EMS. Toxicity and mutagenic responses were greater in soft agar than in liquid culture.In cultures of untreated and EMS-treated cells, more than 95% of the 6TG-resistant colonies isolated were aminopterin-sensitive.Use of soft agar for selection prevented the reduction in the number of mutants with increase in the size of incula on plating up to 1?2 × 106 cells per 9-cm dish: in liquid culture, even with a lower plating number (2 × 105 cells per 9-cm dish), a notable reduction in numbers of mutants was observed. This character was re-examined in a reconstruction experiment. The results show that, when up to 2 × 106 cells were plated per 9-cm dish, 6TG-resistant cells were almost completely recovered from the soft agar medium, whereas only 10% were recovered from liquid culture. 相似文献
164.
Cytokinin-induced switch in development in excised cotyledons of radiata pine cultured in vitro 总被引:2,自引:0,他引:2
Victor M. Villalobos Melvin J. Oliver Edward C. Yeung Trevor A. Thorpe 《Physiologia plantarum》1984,61(3):483-489
Cotyledons of Pinus radiata D. Don were cultured under shoot-forming (plus cytokinin) and elongating (minus cytokinin) conditions. Using. autoradiographic and precursor incorporation techniques, the sites and rate of macromolecular synthesis were examined during the first five days in culture. Active incorporation of 3 H-thymidine, 3 H-uridine and 3 H-leucine occurred. In shoot-forming cotyledons the incorporation became preferentially located in the epidermal and sub-epidermal cell layers in contact with the medium. In elongating cotyledons, in contrast, incorporation was randomly distributed, and the amount of incorporation declined with time. Biochemically, differences in DNA, RNA and total protein synthetic patterns were observed. In elongating cotyledons the rates of RNA and protein synthesis were higher during the first 48 h than in shoot-forming tissues, after which the synthetic rates were similar. Two peaks of newly formed DNA were observed in both tissues. These findings indicate that the cytokinin-induced changes in developmental pathways began within 24 h in culture. 相似文献
165.
Yohei Hashimoto Hiroyuki Ishizone Midori Moriyasu Kazuko Kawanishi Atsushi Kato Masaru Ogura 《Phytochemistry》1984,23(8):1807-1808
Three oleanane triterpenes were isolated from the roots of Periandra dulcis,and identified as 3β-hydroxy-25-al-olean-18-en-30-oic acid (periandric acid I), 3β-hydroxy-25-al-olean-12-en-30-oic acid (periandric acid II) and 3-oxo-25-hydroxy-olean-12-en-30-oic acid. The former two compounds (periandric acids I and II) were identical with the aglycones obtained by hydrolysis of periandrin I and II, respectively and the latter one was a new triterpene. 相似文献
166.
Abstract: Rats were treated with reserpine, desmethylimipramine, or carrier, either alone or in combination with tropolone. Either 10 min (t1) or 1 h (t2) after intraventricular injection of [3H]noradrenaline, they were decapitated. The total 3H activity and the recovery of [3H]noradrenaline were determined in tissue extracts from various brain regions. Maximum total 3H activity was measured at t1 in all tropolone-treated rats; the mean sum of these results served as an estimate of the initial tissue concentration of [3H]noradrenaline. At t1, 40–50% of the sum of [3H]noradrenaline and its metabolites was recovered unchanged in normal rats; reserpine and DMI reduced the recovery to 18–27%. In all groups, the decline of [3H]noradrenaline was retarded after t1. Inhibition of catechol-O-methyltransferase by tropolone caused consistently elevated [3H]noradrenaline levels, but did not affect the metabolic rate after t1 when compared with similarly pretreated, but tropolone-free rats. Thus, if catechol-O-methyltransferase was inhibited during the injection of [3H]noradrenaline, a higher percentage of the amine had been taken up into spaces with a slow noradrenaline turnover. The maximum increase was seen when the neuronal uptake, was inhibited by desmethylimipramine. This supported the hypothesis that an additional extraneuronal space exists, in addition to the known intraneuronal and extraneuronal compartments, which has a slow noradrenaline turnover. The tropolone effect on the noradrenaline recovery possibly shows that there might be a saturable “methylating system,” similar to that described for the periphery, in which catechol-O-methyltransferase is linked to the extraneuronal uptake2. By affecting the access of noradrenaline to non-neuronal cells it might influence the rate of noradrenaline elimination from the intercellular space. 相似文献
167.
Thomas M. Koval 《In vitro cellular & developmental biology. Plant》1987,23(11):733-737
Summary Insect cell lines are not well-suited to colony formation in liquid medium following low-density cell plating. The present
studies demonstrate that the time of addition of fetal bovine serum to the culture medium and the number of γ-irradiated feeder
cells added to each plate are important factors in developing a useful colony formation assay. TN-368 lepidopteran and WR69-DM-1
dipteran cell lines were used for these experiments. Both cell types display increased plating efficiencies if serum is added
to the medium one or more days prior to plating as compared to adding serum immediately before plating. Growth curves obtained
by seeding cells at higher densities also indicate that cell growth is slightly better if serum is added one or more days
before seeding. These findings are especially important for survival and toxicity studies because the results demonstrate
that even seemingly minor factors involved in cell survival assays may benefit treated cells to a greater degree than untreated
control cells, thus providing an erroneous assessment of cell survival.
This work was supported by USPHS grant R01-CA34158, awarded by the National Cancer Institute, DHHS, Bethesda, MD. 相似文献
168.
通过过聚乙二醇6000-磷酸钾缓冲液双相分离、Sephadex G-100凝胶过滤、DEAE-Sephadex A-50离子交换层析、羟基磷灰石层析及SephadexG-100凝胶过滤等提纯步骤,从海枣曲霉(Aspergillus phoenicis)麦麸培养物抽提液中提纯得到凝胶电泳均一的β-半乳糖苷酶。该酶的最适pH为3.5—4.0,最适温度为60℃(反应15分钟),在pH5.0—8.5之间及60℃以下稳定。在65℃和70℃保温时失活50%的时间分别为27和2分钟。用SDS凝胶电泳法和梯度凝胶电泳法分别测得该酶的分子量为115,000和118,000。薄层凝胶等电聚焦法测得其等电点为pH4.6。 相似文献
169.
The identification and efficiency of arthropod predator and parasites related to natural control of cotton leafworm eggs,Alabama argillacea (Huebner), were studied in a cotton field, Jaboticabal, SP, Brazil. Plants were marked randomly and the eggs found on them were indicated by arrow tapes for predation and parasitism observations. To evaluate and identify the arthropod fauna in the row-meter containing the marked plant, visual countings and collections using D-Vac ® and “beat sheet” were used. The average predation rate during the season, in the presence of 23.0 predators per row-meter, was 50.6% and the parasitism byTrichogrammatoidea annulata was 44.9% totaling 95.5% of egg reduction. In decreasing order of abundance, the arthropod predators found were the antPheidole sp., the spiderChrysso clementinae Petrunkevitch, the hemipteraOrius insidiosus Say andCeratocapsus mariliensis Carvalho & Fontes, and the coccinellidHyperaspis festiva (Mulsant). 相似文献
170.
As germination of axes of Pisum sativum L. seeds progressed, profound quantitative and qualitative changes occurred in the patterns of protein synthesis. This was shown by fluorography of gels following two-dimensional polyacrylamide gel electrophoresis separation of [35S]methioninelabelled proteins. The effects of desiccation during germination on these in-vivo protein-synthesis patterns were followed. Desiccation differentially affected the synthesis of proteins. Usually, however, upon rehydration following desiccation the types of proteins being synthesized were recognizable as those synthesized earlier during imbibition of control, once-imbibed axes: seeds imbibed for 8 h, and then dried, did not recommence synthesis of proteins typical of 8-h-imbibed control seeds, but rather of 4-h-imbibed control seeds. Seeds imbibed for 12 h, and then dried and rehydrated, synthesized proteins typical of 4-h-and 8-h-control seeds. Thus drying of germinating pea axes caused the proteinsynthesizing mechanism to revert to producing proteins typical of earlier stages of imbibition. Drying during germination never caused the seed to revert to the metabolic status of the initial mature dry state, however.Abbreviation DR
dried and rehydrated 相似文献