全文获取类型
收费全文 | 22871篇 |
免费 | 1579篇 |
国内免费 | 1085篇 |
出版年
2024年 | 59篇 |
2023年 | 320篇 |
2022年 | 668篇 |
2021年 | 1097篇 |
2020年 | 720篇 |
2019年 | 969篇 |
2018年 | 943篇 |
2017年 | 716篇 |
2016年 | 1010篇 |
2015年 | 1398篇 |
2014年 | 1577篇 |
2013年 | 1802篇 |
2012年 | 2018篇 |
2011年 | 1879篇 |
2010年 | 1047篇 |
2009年 | 926篇 |
2008年 | 1026篇 |
2007年 | 947篇 |
2006年 | 862篇 |
2005年 | 731篇 |
2004年 | 644篇 |
2003年 | 530篇 |
2002年 | 476篇 |
2001年 | 312篇 |
2000年 | 321篇 |
1999年 | 284篇 |
1998年 | 175篇 |
1997年 | 164篇 |
1996年 | 181篇 |
1995年 | 163篇 |
1994年 | 136篇 |
1993年 | 102篇 |
1992年 | 151篇 |
1991年 | 150篇 |
1990年 | 124篇 |
1989年 | 97篇 |
1988年 | 98篇 |
1987年 | 101篇 |
1986年 | 70篇 |
1985年 | 89篇 |
1984年 | 50篇 |
1983年 | 54篇 |
1982年 | 27篇 |
1981年 | 26篇 |
1980年 | 24篇 |
1979年 | 35篇 |
1978年 | 28篇 |
1977年 | 20篇 |
1975年 | 29篇 |
1974年 | 21篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
32.
Regulation of root hair density by phosphorus availability in Arabidopsis thaliana 总被引:14,自引:1,他引:13
We characterized the response of root hair density to phosphorus (P) availability in Arabidopsis thaliana. Arabidopsis plants were grown aseptically in growth media with varied phosphorus concentrations, ranging from 1 mmol m−3 to 2000 mmol m−3 phosphorus. Root hair density (number of root hairs per mm of root length) was analysed starting at 7 d of growth. Root hair density was highly regulated by phosphorus availability, increasing significantly in roots exposed to low-phosphorus availability. The initial root hairs produced by the radicle were not sensitive to phosphorus availability, but began to respond after 9 d of growth. Root hair density was about five times greater in low phosphorus (1 mmol m−3) than in high phosphorus (1000 mmol m−3) media. Root hair density decreased logarithmically in response to increasing phosphorus concentrations within that range. Root hair density also increased in response to deficiencies of several other nutrients, but not as strongly as to low phosphorus. Indoleacetic acid (IAA), the auxin transport inhibitor 2-(p-chlorophenoxy)-2-methylpropionic acid (CMPA), the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC), and the ethylene synthesis inhibitor amino-oxyacetic acid (AOA) all increased root hair density under high phosphorus but had very little effect under low phosphorus. Low phosphorus significantly changed root anatomy, causing a 9% increase in root diameter, a 31% decrease in the cross-sectional area of individual trichoblasts, a 40% decrease in the cross-sectional area of individual atrichoblasts, and 45% more cortical cells in cross-section. The larger number of cortical cells and smaller epidermal cell size in low phosphorus roots increased the number of trichoblast files from eight to 12. Two-thirds of increased root hair density in low phosphorus roots was caused by increased likelihood of trichoblasts to form hairs, and 33% of the increase was accounted for by changes in low phosphorus root anatomy resulting in an increased number of trichoblast files. These results show that phosphorus availability can fundamentally alter root anatomy, leading to changes in root hair density, which are presumably important for phosphorus acquisition. 相似文献
33.
Iron plaque enhances phosphorus uptake by rice (Oryza sativa) growing under varying phosphorus and iron concentrations 总被引:4,自引:0,他引:4
Both solution culture and pot experiments were performed to investigate (a) the effects of external Fe (II) concentrations and forms on the formation of iron plaque on the roots of rice (Oryza sativa) and subsequent P adsorption on iron plaque and shoot P concentrations and (b) the effects of soil moisture regimes on the formation of iron plaque and P adsorption on root surfaces and P accumulation in shoots. The results showed that iron plaque was significantly increased with increasing Fe2+ concentrations in the solution culture. The amounts of P adsorbed on the iron plaque were increased significantly with external Fe2+ concentrations. Although shoot P concentration was not significantly affected by Fe2+ treatment after incubation for 2 days, it was significantly increased in the Fe‐treated plants compared with Fe‐deprived ones after incubation for 4 days. Soil culture experiment showed that the formation of iron plaque on root surfaces was promoted by exogenous iron, with greater amount of iron plaque being formed by addition of ferric hydroxide than of ferric oxide. Phosphorus adsorption on iron plaque also increased with the addition of iron oxides, and increasing soil P increased the amounts of P associated with the iron plaque and shoot P concentration. The amounts of iron plaque were almost sixfold higher under flooding condition than under field capacity condition. Plants pretreated under flooding condition generally had higher shoot P concentrations when they were transplanted to solutions with varying P levels, and this was most pronounced in the treatment with highest solution P concentration. The results suggest that iron plaque acts as a nutrient reservoir for phosphorus in the rhizosphere and helps enhance P acquisition by rice. 相似文献
34.
A rapid extraction method for screening aflatoxin producing potential ofAspergillus flavus group isolates is described. The method is performed using a moist wheat medium with ca. five infected grains extracted with 2 mL of chloroform, and using thin layer chromatography. This method was proved with 95A. flavus isolates from animal feeds. 相似文献
35.
W E Collins J C Skinner M Pappaioanou N S Ma J R Broderson B B Sutton P S Stanfill 《The Journal of parasitology》1987,73(3):536-540
Twenty splenectomized Aotus vociferans (karyotype V) monkeys were infected with strains of Plasmodium vivax from New Guinea, North Korea, Indonesia, El Salvador, and Honduras. Peak parasite densities ranged from 4,840 to 75,500 per mm3. Gametocytes infective to different species of mosquitoes were produced with all strains of P. vivax studied. Two transmissions of the Chesson strain of P. vivax were made by the intravenous inoculation of dissected sporozoites from An. dirus mosquitoes. Prepatent periods were 16 days. 相似文献
36.
Hungtu Ma Minghong Gu G. H. Liang 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1987,73(3):389-394
Summary Immature embryos of 20 sorghum genotypes were cultured on MS 5 medium containing MS mineral salts supplemented with 2,4-D, zeatin, glycine, niacinamide, Ca-pantothenate, L-asparagine, and vitamins. For regeneration, calli were transferred onto the same medium with the exception that IAA was substituted for 2,4-D. In general, immature embryos obtained 9–12 days after pollination resulted in the best redifferentiation. Ability of calli to regenerate varied among genotypes; cultivars C401-1 and C625 had the highest redifferentiation frequencies. Ability to redifferentiate was heritable and acted as a dominant trait. At least two gene pairs were involved. Regenerated R0 plants were planted in a greenhouse and their selfed (R1 and R2) progenies were planted in the field and examined for morphological and cytological variations. The majority of the phenotypic variations noted in R0 were not transmitted to later generations. However, variants for plant height, degree of fertility, and midrib color persisted in R1 and R2 generations. A variation in tallness was attributable to one dominant mutant gene. Short stature and male sterility variants appeared to be consequences of recessive mutant genes controlling those traits. Minor variations in peroxidase banding patterns were found among R0 plants.This study was supported by a research grant from Kansas Sorghum Commission and by a Research Fellowship to the senior author from the Ministry of Agriculture, Animal Husbandry, and Fisheries, China. Contribution 86-456-J from the Kansas Agricultural Experiment Station 相似文献
37.
38.
N C Milos H C Wilson Y L Ma T M Mohanraj Y N Frunchak 《Pigment cell research / sponsored by the European Society for Pigment Cell Research and the International Pigment Cell Society》1987,1(3):188-196
We have investigated cell-cell and cell-substratum adhesion of Xenopus laevis neural crest cells at various stages of melanophore differentiation. Single-cell suspensions were obtained by trypsinization and aggregated in a cell-cell adhesion assay. Unpigmented cells did not adhere while the rate of adhesion of melanophores correlated with the degree of melanization. Melanophore cell-cell adhesion decreased significantly in the presence of beta-galactosidase, which suggests that cell-surface galactose is involved. Beta-galactoside-binding lectin has been isolated and purified from embryos at the stage of neural crest migration. When added to aggregating cells smaller, looser clusters formed compared to controls. When lectin was added to cells in stationary culture to test cell-substratum adhesion, melanophores spread more smoothly and formed more regular spacing patterns. These results suggest that this lectin can modulate receptors used in cell-cell and cell-substratum adhesion of melanophores. 相似文献
39.
The ultrasonic absorption coefficient per wavelength (alpha lambda), as a function of temperature and frequency, was determined for large unilamellar vesicles (LUV) in the vicinity of their phospholipid phase transition temperature, using a double crystal acoustic interferometer. (The vesicles were composed of a 4:1 (w/w) mixture of dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylglycerol (DPPG). It has been found that alpha lambda reaches a maximum (alpha lambda)max at the phase transition temperature (tm) of the phospholipids in the bilayer, at an ultrasonic relaxation frequency of 2.1 MHz. Divalent cations (Ca2+ and Mg2+), added to LUV suspensions, shifted (alpha lambda)max to higher temperatures, dependent upon the concentration of divalent cation. It was also found that the shape of the alpha lambda versus t curve was significantly changed, representing changes in the Van't Hoff enthalpy of the transition, and therefore, the cooperative unit of the transition. This suggests that divalent cations interact individually with the negatively charged phospholipid headgroups of DPPG and with DPPC headgroups, thus decreasing the cooperative unit of the transition. The observed upward shift in tm suggests an interaction that increases the activation energy and, therefore, the temperature of the phase transition. However, alpha lambda as a function of frequency did not change with the addition of divalent cations and, thus, the relaxation time of the event responsible for the absorption of ultrasound is not changed by the addition of divalent cations. 相似文献
40.