全文获取类型
收费全文 | 507篇 |
免费 | 39篇 |
专业分类
546篇 |
出版年
2021年 | 5篇 |
2017年 | 3篇 |
2016年 | 7篇 |
2015年 | 6篇 |
2014年 | 5篇 |
2013年 | 18篇 |
2012年 | 19篇 |
2011年 | 23篇 |
2010年 | 9篇 |
2009年 | 14篇 |
2008年 | 13篇 |
2007年 | 25篇 |
2006年 | 23篇 |
2005年 | 23篇 |
2004年 | 33篇 |
2003年 | 21篇 |
2002年 | 19篇 |
2001年 | 19篇 |
2000年 | 20篇 |
1999年 | 14篇 |
1997年 | 3篇 |
1996年 | 7篇 |
1995年 | 3篇 |
1994年 | 5篇 |
1993年 | 3篇 |
1992年 | 9篇 |
1991年 | 4篇 |
1989年 | 9篇 |
1988年 | 6篇 |
1987年 | 6篇 |
1986年 | 10篇 |
1985年 | 12篇 |
1984年 | 8篇 |
1982年 | 6篇 |
1981年 | 9篇 |
1980年 | 11篇 |
1979年 | 5篇 |
1978年 | 7篇 |
1977年 | 7篇 |
1976年 | 7篇 |
1975年 | 9篇 |
1974年 | 8篇 |
1973年 | 9篇 |
1972年 | 4篇 |
1969年 | 6篇 |
1966年 | 3篇 |
1960年 | 4篇 |
1945年 | 3篇 |
1940年 | 3篇 |
1934年 | 2篇 |
排序方式: 共有546条查询结果,搜索用时 15 毫秒
11.
The sibling marine snails Littorina obtusata (L.) and Littorina mariae Sacchi & Rastelli are sympatrically distributed and the shells of both species are subject to similar breaking forces by predatory crabs. Nevertheless, the two species exhibit rather different growth and defence strategies. To determine growth patterns, we measured changes in five morphological variables with increasing shell length: body whorl thickness at the point of crushing force application, shell height (related to globosity), shell mass, body mass, and apertural lip thickness. We also measured ontogenetic changes in the ability to withstand shell crushing. For most morphological variables, L. mariae showed uniformly allometric growth of juveniles into adults. In contrast, L. obtusata usually exhibited a distinct change in growth pattern upon reaching maturity. As adults, L. mariae showed a more sustained increase in overall shell mass and in body whorl thickness (defence against crushing attacks) and also had proportionally thicker apertural lips (defence against peeling attacks). Littorina obtusata , however, grew to a larger size and their shells could accommodate larger bodies at all sizes. Furthermore, the strength of L. obtusata shells increased faster than could be accounted for by either overall shell mass or thickness at the point of force application, suggesting strengthening by other means such as changes in shell microstructure or shape (other than globosity). These results illustrate the viability of two contrasting antipredator strategies, despite a highly similar phylogenetic history and selective regime. 相似文献
12.
Joseph M. Culp Cheryl L. Podemski Kevin J. Cash Richard B. Lowell 《Journal of Aquatic Ecosystem Stress and Recovery (Formerly Journal of Aquatic Ecosystem Health)》1996,5(2):117-124
Experimentation using field-based artificial streams provides a promising, complimentary approach to biomonitoring assessments because artificial streams provide control over relevant environmental variables and true replication of treatments. We have used large and small artificial stream systems, based in the field, to examine the effect of treated bleached kraft pulp mill effluent (BKME) on the benthos of three large rivers in western Canada. Under natural regimes of temperature, water chemistry, and insolation, these artificial streams provide current velocities and substrata to food chains or food webs that are representative of those in the study river. With these tools we have shown that BKME stimulated mayfly growth in the Thompson River above that which could be accounted for by fertilization of their algal food supply. In contrast, moulting frequency was inhibited at high BKME concentrations. Results from artificial streams also indicate that increased algal biomass and abundances of benthic communities downstream of BKME outfalls were induced by nutrient enrichment from the effluent. BKME treatments did not change diatom species richness in the Fraser River, or diatom species diversity in either the Athabasca or Fraser Rivers. Artificial streams provide a means of understanding the mechanisms of stressor effects over a continuum ranging from single stressor effects on specific taxa to the effects of multiple stressors on communities and ecosystems. Because riverside deployment provides environmental realism within a replicated experimental design, this approach can (i) address questions that cannot be examined using laboratory tests or field observations, (ii) improve our mechanistic understanding of stressor effects on riverine ecosystems, and (iii) can contribute directly to the development, parameterization, and testing of models for predicting ecosystem-level responses. 相似文献
13.
Xiaorong Wang Xiongfei Ding Bhuvana Gopalakrishnan Thomas D. Morgan Lowell Johnson Frank F. White Subbaratnam Muthukrishnan Karl J. Kramer 《Insect biochemistry and molecular biology》1996,26(10):1055-1064
A 46 kDa Manduca sexta (tobacco hornworm) chitinase was isolated from leaves of transgenic tobacco plants containing a recombinant insect chitinase cDNA, characterized, and tested for insecticidal activity. The enzyme was purified by ammonium sulfate fractionation, Q-Sepharose anion-exchange chromatography and mono-S cation-exchange chromatography. Although the gene for the chitinase encoded the 85 kDa full-length chitinase as previously reported by Kramer et al. [Insect Biochem. Molec. Biol. 23, 691–701 (1993)], the enzyme is produced in tobacco as a 46 kDa protein that is approximately four-fold less active than the 85 kDa chitinase. The N-terminal amino acid sequence of the 46 kDa chitinase is identical to that of the 85 kDa chitinase. The former enzyme is not glycosylated, whereas the latter contains approximately 25% carbohydrate. The pH and temperature optima of the 46 kDa chitinaseare similar to those of the 85 kDa chitinase. The former enzyme is more basic than the latter. The 46 kDa chitinase likely consists of the N-terminal catalytic domain of the 85 kDa chitinase and lacks the C-terminal domain that contains several potential sites for glycosylation. The 46 kDa chitinase is expressed in a number of plant organs, including leaves, flowers, stems and roots. Enzyme levels are higher in leaves and flowers than in stems and roots, and leaves from the middle portion of the plant have more chitinase than leaves from the top and bottom portions. Little or no enzyme is secreted outside of the plant cells because it remains in the intracellular space, even though its transit sequence is processed. When fed at a 2% dietary level, the 46 kDa chitinase caused 100% larval mortality of the merchant grain beetle, Oryzaephilis mercator. The results of this study support the hypothesis that insect chitinase is a biopesticidal protein for insect pests feeding on insect chitinase gene-containing transgenic plants. 相似文献
14.
15.
Cyanide-sensitive O2 uptake was observed in the isolated embryonicaxis of soybean [Glycine max L. merr. cv. Essex]within 5 min after wetting and the O2 uptake rate during thefirst h of imbibition was directly proportional to axis moisturecontent. Within 10 min after wetting, O2 uptake was lower inaged low vigor (LV) axes than in high vigor (HV) axes imbibedeither in water (LV axes sustain imbibition injury) or in 30%w/v polyethylene glycol (LV axes do not sustain imbibition injury).Mitochondria isolated from LV axes after 4- and 24-h imbibitionwere characterized by lower O2 uptake, ADP:O, and respiratorycontrol ratios relative to HV mitochondria. A plot of O2 uptakevs temperature between 10 and 25?C revealed a break in the slopebetween 13 and 16?C for HV, but not LV, axes. Since LV axessustained water uptake injury at all temperatures tested, whereasHV axes sustained chilling injury only below 16?C, the dataindicate two temperature-O2 uptake relationships: one for injuredtissues and one for non-injured tissues. We conclude that deteriorationper se involves impairment of respiratory metabolism in thesoybean embryonic axis. Chilling (10?C; HV and LV axes) andimbibitional (25?C; LV axes) injuries during the initial minof inbibition further disrupt respiratory metabolism and interferewith subsequent mitochondrial development and axis growth.
1Scientific Articles No. A-3548, Contribution No. 6623 of theMaryland Agricultural Experiment Station. (Received May 25, 1983; Accepted September 28, 1983) 相似文献
16.
Summary The role of nerve cell density in the regulation of bud production in hydra was examined. Animals with different rates of bud production were produced by altering the temperature, population density and illumination of their cultures. When the distribution of cell types was examined in animals with different rates of bud production, the density of nerve cells in those animals was found to be correlated with their rate of bud production. Transfer of animals from one environment to another resulted in immediate changes in the rate of differentiation of large interstitial cells into nerve cells. This suggests that the density of nerve cells may play a role in regulating the rate of bud production in hydra. 相似文献
17.
Monica M. Palcic Rick Rutter Tsunehisa Araiso Lowell P. Hager H.Brian Dunford 《Biochemical and biophysical research communications》1980,94(4):1123-1127
When compound I of chloroperoxidase is formed from the native enzyme the absorption peak in the Soret region diminishes in intensity, and shifts to a maximum absorbance at 367 nm. This unusual Soret spectrum decreases in intensity in a linear fashion as the wavelength increases. The first visible spectrum of chloroperoxidase compound I is reported which has a peak at 689 nm as its most prominent feature. 相似文献
18.
John F. Dixon Chang Ho Lee Georgyi V. Los Lowell E. Hokin 《Journal of neurochemistry》1992,59(6):2332-2335
We previously reported that lithium, in the presence of acetylcholine, increased accumulations of inositol 1,4,5-trisphosphate and inositol 1,3,4,5-tetrakisphosphate in brain cortex slices from the guinea pig, rabbit, rat, and mouse. In the mouse and rat, the Li(+)-induced increases required supplementation of the medium with inositol. This probably relates to the following facts: (a) Brain cortices of the mouse and rat contain in vivo concentrations of inositol half of that of the guinea pig. (b) Incubated rat brain cortex slices are depleted of inositol by 80%. (c) The slices require 10 mM inositol supplementation to restore in vivo concentrations. We now show that in monkey brain cortex slices, therapeutic concentrations of Li+ increase accumulation of inositol 1,4,5-trisphosphate. The inositol 1,3,4,5-tetrakisphosphate level is not increased. Neither inositol nor an agonist is required. The same effects are seen whether inositol 1,4,5-trisphosphate is quantified by the [3H]inositol prelabeling technique or by mass assay, although mass includes a pool of inositol 1,4,5-trisphosphate that is metabolically inactive. Thus, in a therapeutically relevant model for humans, Li+ increases inositol 1,4,5-trisphosphate levels in brain cortex slices, as was previously seen in lower mammals at non-rate-limiting concentrations of inositol. 相似文献
19.
In preparation for gene transfer experiments we investigated factors that might affect the production of shoots and somatic embryos from the wound callus of cultured sugarbeet leaf discs. A complex interaction was found between the leaf disc plating density, the disc culture medium, the source-shoot culture medium and the frequency of disc transfer to fresh medium. The most productive protocol utilized: source shoots maintained on MS medium containing 0.25 mg 1-1 BA; multiple leaf discs (ten 4-mm discs/plate) plated onto an enriched modification of MS medium (RV) containing 1.0 mg 1-1 BA and solidified with 0.3% Gelrite (not permitted to dry during hardening); and transfer of the discs to fresh medium every two weeks during the first month. This standard protocol produced more callus per plate and higher rates of morphogenesis per unit dry weight of callus than did the one-step method of Saunders and Doley. Water availability considerations were found to be critical to obtaining high morphogenic rates. Root induction frequency and quality was superior on shoots transplanted to MS medium containing 1 mg 1-1 NAA as the sole growth regulator compared to IAA at the same concentration.Abbreviations BA
N6-benzyladenine
- IAA
indole-3-acetic acid
- NAA
-naphthaleneacetic acid 相似文献
20.
Terrance O. Kurtz Drew J. Winston William J. Martin Lowell S. Young William L. Hewitt 《Current microbiology》1980,4(1):21-26
Moxalactam (LY127935), a novel beta-lactam antibiotic, was compared with semisynthetic penicillins, cephalosporins, and aminoglycosides
by the agar dilution method against 5,317 recent clinical isolates of facultative and anaerobic bactria. At 0.5 μg/ml, moxalactam
inhibited 90% of all Gram-negative bacilli tested except forPseudomonas aeruginosa (81% inhibited by 32 μg/ml) andAcinetobacter calcoaceticus (88% inhibited by 32 μg/ml). More than 90% ofBacteroides fragilis andStaphylococcus aureus were inhibited by 4 μg/ml and 8 μg/ml, respectively. Moxalactam was at least 16-fold more active by weight than cephalothin,
cefamandole, and cefoxitin forEscherichia coli, Klebsiella pneumoniae, andEnterobacter species, and 2- to 4-fold more active than cefoxitin forB. fragilis. Moxalactam was 4-fold less active than cefamandole and cephalothin forS. aureus and 2- to 4-fold less active than piperacillin forP. aeruginosa. Moxalactam was as active or more active than the aminoglycosides for all facultative Gram-negative bacilli except forP. aeruginosa. Moxalactam was inhibitory (minimal inhibitory concentration <16 μg/ml) for 20/27 gentamicin-resistant isolates and 8/13
amikacin-resistant organisms. Moxalactam’s in vitro activity against Gram-negative bacilli is markedly superior to presently
available cephalosporins and, except forP. aeruginosa, is comparable to the aminoglycosides. 相似文献