全文获取类型
收费全文 | 19982篇 |
免费 | 1835篇 |
国内免费 | 19篇 |
出版年
2022年 | 123篇 |
2021年 | 263篇 |
2020年 | 134篇 |
2019年 | 207篇 |
2018年 | 263篇 |
2017年 | 208篇 |
2016年 | 383篇 |
2015年 | 674篇 |
2014年 | 745篇 |
2013年 | 927篇 |
2012年 | 1207篇 |
2011年 | 1273篇 |
2010年 | 771篇 |
2009年 | 673篇 |
2008年 | 968篇 |
2007年 | 1079篇 |
2006年 | 995篇 |
2005年 | 983篇 |
2004年 | 973篇 |
2003年 | 945篇 |
2002年 | 921篇 |
2001年 | 214篇 |
2000年 | 146篇 |
1999年 | 213篇 |
1998年 | 265篇 |
1997年 | 190篇 |
1996年 | 172篇 |
1995年 | 204篇 |
1994年 | 182篇 |
1993年 | 168篇 |
1992年 | 170篇 |
1991年 | 147篇 |
1990年 | 138篇 |
1989年 | 132篇 |
1988年 | 140篇 |
1987年 | 142篇 |
1986年 | 138篇 |
1985年 | 159篇 |
1984年 | 180篇 |
1983年 | 151篇 |
1982年 | 216篇 |
1981年 | 224篇 |
1980年 | 223篇 |
1979年 | 143篇 |
1978年 | 149篇 |
1977年 | 134篇 |
1976年 | 164篇 |
1975年 | 123篇 |
1974年 | 137篇 |
1973年 | 155篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
441.
In the presence of Cl?, the severity of ammonia-induced inhibition of photosynthetic oxygen evolution is attenuated in spinach thylakoid membranes (Sandusky, P.O. and Yocum, C.F. (1983) FEBS Lett. 162, 339–343). A further examination of this phenomenon using steady-state kinetic analysis suggests that there are two sites of ammonia attack, only one of which is protected by the presence of Cl?. In the case of Tris-induced inhibition of oxygen evolution only the Cl? protected site is evident. In both cases the mechanism of Cl? protection involves the binding of Cl? in competition with the inhibitory amine. Anions (Br? and NO?3) known to reactive oxygen evolution in Cl?-depleted membranes also protect against Tris-induced inhibition, and reactivation of Cl?-depleted membranes by Cl? is competitively inhibited by ammonia. Inactivation of the oxygen-evolving complex by NH2OH is impeded by Cl?, whereas Cl? does not affect the inhibition induced by so-called ADRY reagents. We propose that Cl? functions in the oxygen-evolving complex as a ligand bridging manganese atoms to mediate electron transfer. This model accounts both for the well known Cl? requirement of oxygen evolution, and for the inhibitory effects of amines on this reaction. 相似文献
442.
Ronald S. Oremland Cindy Umberger Charles W. Culbertson Richard L. Smith 《Applied microbiology》1984,47(5):1106-1112
The acetylene block technique was employed to study denitrification in intertidal estuarine sediments. Addition of nitrate to sediment slurries stimulated denitrification. During the dry season, sediment-slurry denitrification rates displayed Michaelis-Menten kinetics, and ambient NO3− + NO2− concentrations (≤26 μM) were below the apparent Km (50 μM) for nitrate. During the rainy season, when ambient NO3− + NO2− concentrations were higher (37 to 89 μM), an accurate estimate of the Km could not be obtained. Endogenous denitrification activity was confined to the upper 3 cm of the sediment column. However, the addition of nitrate to deeper sediments demonstrated immediate N2O production, and potential activity existed at all depths sampled (the deepest was 15 cm). Loss of N2O in the presence of C2H2 was sometimes observed during these short-term sediment incubations. Experiments with sediment slurries and washed cell suspensions of a marine pseudomonad confirmed that this N2O loss was caused by incomplete blockage of N2O reductase by C2H2 at low nitrate concentrations. Areal estimates of denitrification (in the absence of added nitrate) ranged from 0.8 to 1.2 μmol of N2 m−2 h−1 (for undisturbed sediments) to 17 to 280 μmol of N2 m−2 h−1 (for shaken sediment slurries). 相似文献
443.
The relationship between epithelial cell production and growth rate was investigated in Hydra attenuata under different feeding regimes. The increase of epithelial cell number was compared to the duration of the epithelial cell cycle using standard methods of cell cycle analysis. The results indicate that cell cycle changes accompanying changes in feeding regime are not sufficient to explain the altered growth rate. Under heavy feeding regimes, epithelial cell production equals tissue growth rate. At low feeding level or under starvation conditions the epithelial cell cycle lengthens and growth rate of epithelial cell population is slowed. However, the cell cycle changes are insufficient to account for the reduction in tissue growth and thus there is an effective overproduction of epithelial cells amounting to 10% per day. Evidence suggests that these excess cells are phagocytized by neighboring cells in the tissue. Thus phagocytosis is directly or indirectly involved in regulating the growth of hydra tissue. 相似文献
444.
Susan J. Dugmore Keith Bailey Charles S. Evans 《International journal of primatology》1984,5(3):235-245
Olfaction plays an important role in the social communication of all prosimians. (The experiment reported in this paper forms
part of an intensive chemobehavioral study of olfaction in Lemur catta (ring-tailed lemur) being carried out in this laboratory.) Five male Lemur cattawere tested on their behavioral responses to paired scent stimuli. Responses measured were (1) total investigation time, (2)
arm-marking, (3) ABO/BO rubbing, and (4) flehmen. Males showed a strong discrimination between the scent stimuli,giving higher levels of response to female scent on measures 1, 3, and 4. This response suggests an olfactory-related preference
by males for female scent under controlled conditions. This preference may be a consequence of the females’ dominance over
males and the brevity of estrus in L. catta,both of which would favor such choice behavior. 相似文献
445.
Richard P. McCabe Ruth Oneson Charles H. Evans 《Cancer immunology, immunotherapy : CII》1984,17(2):76-82
Summary Nonimmunized 2/N guinea pigs respond to the presence of chemical carcinogen-transformed syngeneic tumorigenic cells with a sustained (delayed-hypersensitivity-type) 4-day intradermal induration consisting of predominantly polymorphonuclear leukocytes on day 1 and mononuclear cells by day 4, which is independent of the presence of tumor-specific antigens on the tumorigenic cells. Chemical carcinogen-induced morphologically transformed but nontumorigenic cells also induce a polymorphonuclear response by day 1, but neither induration nor a mononuclear response is present on day 4, demonstrating the specificity of the 4-day sustained indurative response for tumorigenic cells. Induration and cellular infiltrates are unaltered if tumor cells are treated prior to injection with the cytostatic lymphokine lymphotoxin or with x-irradiation to inhibit cell proliferation. The intradermal polymorphonuclear leukocyte host response on day 1, but not the mononuclear response on day 4, is also induced by mitomycin C-treated cells or a cytokine culture medium from the cells. No response is present on day 1 or day 4 when cell membranes or lyophilized cells are injected. Thus natural delayed-hypersensitivity-type skin reactivity is a mononuclear leukocyte response specifically directed against intact and metabolically active but not necessarily proliferating tumor cells. 相似文献
446.
White, pink, orange, and yellow strains ofUstilago violacea containing high and low levels of cytochrome c and various carotenes were exposed to ultraviolet light. The survival curves for all strains were of exponential decay form, but the carotene-accumulating strains were generally more resistant to UV than those strains with no carotenes at all. The UV exposure time leading to 90% loss in viability, LD90, was quantitatively related to the carotene content in the form of a power function, where LD90 increased as the fifth root of the total carotene content per cell. We also determined that the ratio of total carotenes to total cytochrome c per cell was quantitatively related to the rate of viability loss during UV exposure. 相似文献
447.
Abstract A 7-kb piece of Escherichia coli DNA that contains five genes ( entA, C, G, B and E ) required for the biosynthesis of the iron transport molecule enterochelin was isolated. A restriction map was constructed and proteins specified by the E. coli DNA were identified in mini- and maxicell systems. Plasmids containing portions of the entACGBE DNA generated by BAL31 digestion or restriction enzyme treatment were constructed; complementation studies done with these indicated that the five genes constitute an operon. The approximate site of the promoter was determined and the product of entE was tentatively identified as an M r 63000 polypeptide. 相似文献
448.
449.
Nuclear DNA amounts were determined by cytofluorometry for twelve species and subspecies of the genus Xenopus. Absolute values, in pg per nucleus, were obtained by direct comparison with human lymphocyte nuclei. The lowest DNA amount (3.55 pg) was found in X. tropicalis, which possess only 20 chromosomes, and the highest (16.25 pg), in the hexaploid X. ruwenzoriensis, with 108 chromosomes. The two recently discovered tetraploid species, X. sp.n. and X. vestitus have, respectively, 12.57 and 12.83 pg of DNA. Among the species and subspecies with 36 chromosomes, the DNA content ranges from 6.35 to 8.45 pg. 相似文献
450.
Charles R. Gunn 《Economic botany》1977,31(2):237-252
Merremia discoidesperma (Donn. Sm.) O’Donell (Mary’s-bean) is a rarely collected and inadequately described high climbing woody liana of Chiapas, Mexico; Guatemala; Costa Rica; Hispaniola; and Cuba. There is only one record of this species being cultivated and this from Guanajuato, Mexico in 1894, though seeds are used in folk remedies. These seeds are topographically unlike other known convolvulaceous seeds, thus permitting their positive identification. Because seeds remain buoyant for more than three years in seawater, those produced in the Caribbean and Atlantic drainage region may be transported by the Gulf Stream System as far north as the Norwegian coast, a distance of about 9,500 km. Seeds produced in the Pacific drainage region from Chiapas, Mexico south to Costa Rica may be transported by the North Pacific Equatorial Current as far west as Wotho Atoll in the Marshall Islands, a distance of about 11,000 km. Records of drifting and subsequent stranding of these and other tropical New World disseminules are supported by drift bottle studies and drift debris records in the Atlantic Ocean. In the Pacific Ocean region distribution of stranded Mary’s-bean seeds is supported by a similar distribution of New World pumice. There is an indication in the literature that Mary’s-bean seeds may drift to the Philippines. The record from Wotho Atoll to Norway constitutes the widest drift range of any seed or fruit which has been documented. Other tropical disseminules may drift as far or farther, but their origins cannot be ascertained with certainty. The plant is described, its synonyms listed, and its distribution as well as the distribution of its stranded seeds recorded. 相似文献