全文获取类型
收费全文 | 28756篇 |
免费 | 3369篇 |
国内免费 | 28篇 |
专业分类
32153篇 |
出版年
2022年 | 249篇 |
2021年 | 519篇 |
2020年 | 280篇 |
2019年 | 405篇 |
2018年 | 444篇 |
2017年 | 423篇 |
2016年 | 665篇 |
2015年 | 1053篇 |
2014年 | 1181篇 |
2013年 | 1508篇 |
2012年 | 1802篇 |
2011年 | 1722篇 |
2010年 | 1130篇 |
2009年 | 973篇 |
2008年 | 1367篇 |
2007年 | 1425篇 |
2006年 | 1211篇 |
2005年 | 1230篇 |
2004年 | 1161篇 |
2003年 | 1029篇 |
2002年 | 1000篇 |
2001年 | 669篇 |
2000年 | 653篇 |
1999年 | 571篇 |
1998年 | 342篇 |
1997年 | 289篇 |
1996年 | 293篇 |
1995年 | 261篇 |
1994年 | 275篇 |
1993年 | 249篇 |
1992年 | 446篇 |
1991年 | 407篇 |
1990年 | 395篇 |
1989年 | 384篇 |
1988年 | 400篇 |
1987年 | 357篇 |
1986年 | 328篇 |
1985年 | 361篇 |
1984年 | 312篇 |
1983年 | 262篇 |
1982年 | 225篇 |
1981年 | 198篇 |
1980年 | 200篇 |
1979年 | 257篇 |
1978年 | 244篇 |
1977年 | 210篇 |
1976年 | 208篇 |
1975年 | 217篇 |
1974年 | 261篇 |
1973年 | 247篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
71.
Attempts at altering plasma glucose and, as a consequence, food intake were performed in fed broiler chickens by single i.v. injection of des-His1(Glu9) glucagon amide (a glucagon antagonist) or a non-stimulating anti-insulin serum. Plasma glucose level was not altered by des-His1(Glu9) glucagon amide but was rapidly and largely increased (for at least 2 h) by the injection of the insulin-immune serum. Hour and cumulative food intake were unaltered up to 10 h post injection. These results strongly suggest that in fed chickens, plasma glucose is mainly, if not exclusively, controlled by plasma insulin, and that the transient and heavy hyperglycemia evoked by inhibiting insulin action does not alter food intake. 相似文献
72.
G U Braga S D Flint C D Miller A J Anderson D W Roberts 《Journal of invertebrate pathology》2001,78(2):98-108
The effects of irradiances of 920 and 1200 mW m(-2) (biologically effective weighted irradiance) were examined in 2 Metarhizium album strains, 26 M. anisopliae strains, 1 M. flavoviride strain, and 1 M. taii strain isolated from sites located at latitudes from 61 degrees N to 54 degrees S. Conidia were exposed to UV-B from 1 to 6 h and subsequently examined for relative percentage culturability. Total dosage received at the end of the exposure periods ranged from 3.3 to 19.9 kJ m(-2) for the lower irradiance and from 4.3 to 25.9 kJ m(-2) for the higher irradiance. Both the irradiance values and the doses are environmentally realistic and can be observed even in temperate regions. The relationships between latitude of origin and UV-B tolerance were compared for the two levels of irradiance for the data from 1 and 2 h exposure. Exposure to both irradiances drastically reduced the relative percentage culturability of all strains. Tolerance to UV-B varied widely among strains and high variation was observed for both irradiances after all periods of exposure. After 1 h of exposure, a difference between the two irradiance levels was detectable, and this difference was magnified at longer irradiations. A significant quadratic relationship of decreasing UV-B tolerance with increasing latitude was observed after exposure of 1 and 2 h. The shape of the relationship did not differ for the two levels of irradiance. Also, we studied the effect of 1200 mW m(-2) irradiance on conidial germination time in 1 M. album strain, 7 M. anisopliae strains, and 1 M. taii strain. Exposure to UV-B delayed the germination of surviving conidia of all strains. In general, the delay in germination was directly proportional to the dose. 相似文献
73.
74.
Some recent high-load, low-density pheromone-release devices emit an ethanolic blend of pheromone directly onto crop foliage to control insect pests by mating disruption. This study characterized the phytotoxicity associated with deposition of some pheromonal compounds in concentrated drops on the foliage of trees bearing aerosol release devices. The relative toxicity of straight-chained alkanes, alcohols, aldehydes, and acetates with chain lengths varying from C-2 to approximately C-20 was quantified in the laboratory by the severity of necrotic lesions. The order of severity for phytotoxicity caused by pheromonal compounds was alkanes < acetates = aldehydes < or = alcohols. Within compound classes tested, pheromones with chain lengths of 6-13 carbons were the most phytotoxic. Phytotoxicity was not detectable at dosages <1 mg administered in 10 microl of ethanol. Phytotoxicity of pheromones was highly correlated with presence of both a hydrophilic and lipophilic molecular domain. We postulate nonspecific membrane disruption as a likely mode of action for pheromonal phytotoxicity. Limited attempts to remediate this effect by changing carrier solvents or adding surfactants, spreaders, or nonvolatile diluents were not successful. Because the toxic action of pheromones upon plant tissues appears relatively benign, and growers have not been adverse to localized phytotoxicity to foliage and fruits on two trees per 0.4 ha, we propose that limited phytotoxicity associated with first-generation aerosol dispenser technology can be viewed as non-threatening. 相似文献
75.
Barbara L. Brody Harold J. Simon Dennis E. Smallwood 《The Western journal of medicine》1987,147(3):350-356
Private financing for long-term care now comes almost exclusively from out-of-pocket payments. Long-term-care costs quickly impoverish most elderly, resulting in Medicaid dependency. The consequences are profound for the western Sun Belt with its rapidly growing elderly population. Key private financing options are long-term-care individual retirement accounts (LTC/IRAs), home equity conversion, social-health maintenance organizations and long-term-care insurance. Study of data from the past half century suggests that the LTC/IRA approach would prove unsatisfactory for the purpose despite the intuitive appeal of this mechanism. Experience with home equity conversions is still very limited, and unresolved questions limit this approach to the role of a reserve option for now. While promising, social-health maintenance organizations are still in the experimental stages and not yet commercially available. Long-term-care insurance is currently sold on a thin market and emphasizes nursing home coverage. New approaches to private financing through long-term-care insurance seem to offer the best approach for immediate implementation. 相似文献
76.
D J Saye O Ogunseitan G S Sayler R V Miller 《Applied and environmental microbiology》1987,53(5):987-995
Transduction of Pseudomonas aeruginosa plasmid Rms149 by the generalized transducing bacteriophage phi DS1 was shown to occur during a 9-day incubation of environmental test chambers in a freshwater reservoir. Plasmid DNA was transferred from a nonlysogenic plasmid donor to a phi DS1 lysogen of P. aeruginosa that served both as the source of the transducing phage and as the recipient of the plasmid DNA. When the concentration of donors introduced into the chambers was varied while the recipient concentration in each chamber was at a level equivalent to natural concentrations of P. aeruginosa, the concentration of plasmid-containing donor cells introduced was shown to affect the frequency of transduction significantly. Transduction was observed both in the absence and in the presence of the natural microbial community. The presence of the natural community resulted in a rapid decrease in the numbers of the introduced donors and recipients and a decrease in the number of transductants recovered. These results demonstrate the potential for naturally occurring transduction in aquatic environments and indicate that donor load may be an important parameter in assessing this potential. 相似文献
77.
Primary and first passage rabbit chondrocyte cultures synthesized a "free" form of hyaluronic acid (HA-f) previously characterized in rabbit cartilage. HA-f was isolated from the [3H] glcN/35SO4-labelled cell-associated-fraction (CAF) and from the culture medium by successive equilibrium centrifugations in Cs2SO4/CsCl/Cs2SO4 under low salt conditions. The culture medium HA-f appeared in the void volume of Sepharose CL-2B eluted with low salt, (0.5M sodium acetate), and was susceptible to digestion with Streptomyces hyaluronidase. HA-f aggregated purified rabbit cartilage proteoglycan monomer. These results indicated that HA-f probably subserves hyaluronic acid already complexed with proteoglycan monomer. Newly synthesized HA-f may be required for the continual formation of proteoglycan aggregates. 相似文献
78.
A. S. Paschoa M. E. Wrenn N. P. Singh F. W. Bruenger S. C. Miller M. Cholewa K. W. Jones 《Biological trace element research》1987,13(1):275-282
Several geological formations of the Utah-Colorado mining region mined for uranium ore during and after World War II had been
mined earlier for vanadium. Therefore, most miners and millers from that region were exposed to those metals’ ores or tailings
at one time or another. Preliminary investigation to determine uranium and vanadium retained in the lungs of a former uranium
miner and miller from this region, who died of lung cancer (mesothelioma), showed a high nonuniform distribution of vanadium.
This observation led to the hypothesis that the vanadium content in the lungs could be associated with inhaled particles.
Further examination of spectra of characteristic X-rays obtained by scanning particle-induced X-ray emission (microPIXE) of
an autopsy sample of this lung indicated that vanadium was indeed present in localized sites within the 20-μm spatial resolution
of the proton beam. This work points out that the microPIXE-RBS (Rutherford backscattering) test for vanadium can be used
for site localization of inhaled particles retained in the lungs. Further studies are in progress to: (i) locate uranium-bearing
particles in lung tissues of former uranium miners and millers; and (ii) evaluate the local doses of alpha radiation received
from these particles. 相似文献
79.
L D Foil T R Klei R I Miller G E Church C S Foil D D French J N Smith 《The Journal of parasitology》1987,73(2):320-326
Seasonal changes in density and spatial distribution of Onchocerca cervicalis microfilariae were studied in ventral-midline skin of 15 infected pony mares in southern Louisiana. Triple running mean analysis of data over a 13-mo period indicated that a distinct pattern exists in total microfilariae population density and in microfilariae occurrence in different levels of the dermis. Microfilariae density reaches peak levels in the spring followed by a 58% decrease in the summer, a 19% increase in the fall, and a decrease to the lowest numbers in the winter. Microfilariae were found in all levels of the skin during the spring, summer, and fall but were not found in the superficial layers of the dermis during the winter months. The population density of Culicoides variipennis, a demonstrated vector of O. cervicalis, appeared to have seasonal fluctuations similar to the changes in microfilarial density. Harmonic wave analysis of microfilariae density data in individual ponies showed that all individuals did not follow the population trend. 相似文献
80.
A J Smith M Mondain-Monval P Simon K Andersen Berg O P Clausen P O Hofmo R Scholler 《Journal of reproduction and fertility》1987,81(2):517-524
Bromocriptine administration in the form of slow-release injections to male blue foxes during March-May abolished the normal spring rise in plasma prolactin concentrations seen in May and June. The spring moult was prevented and the treated animals retained a winter coat of varied quality and maturity until the end of the study in August. Plasma testosterone concentrations fell normally from March until August. Testicular regression was, however, delayed, although there were individual variations in response. Estimation by DNA flow cytometry in early July of the relative numbers of haploid, diploid and tetraploid cells in the testis showed that, in the treated animals, 74-80% of the cells were haploid (maturing germinal cells), 4-6% tetraploid (mainly primary spermatocytes) and the rest diploid cells (somatic cells and the remaining germinal cell types). In the control males, however, no haploid cells were detected and the majority of cells were diploid (93-99%). At castration in August, histological examination revealed various stages of testicular regression in the treated and control animals. 相似文献