全文获取类型
收费全文 | 9273篇 |
免费 | 917篇 |
国内免费 | 9篇 |
专业分类
10199篇 |
出版年
2021年 | 131篇 |
2020年 | 81篇 |
2019年 | 128篇 |
2018年 | 123篇 |
2017年 | 97篇 |
2016年 | 187篇 |
2015年 | 282篇 |
2014年 | 297篇 |
2013年 | 403篇 |
2012年 | 470篇 |
2011年 | 452篇 |
2010年 | 274篇 |
2009年 | 258篇 |
2008年 | 385篇 |
2007年 | 379篇 |
2006年 | 348篇 |
2005年 | 359篇 |
2004年 | 339篇 |
2003年 | 322篇 |
2002年 | 320篇 |
2001年 | 133篇 |
2000年 | 123篇 |
1999年 | 141篇 |
1998年 | 116篇 |
1997年 | 82篇 |
1996年 | 86篇 |
1995年 | 81篇 |
1994年 | 86篇 |
1993年 | 73篇 |
1992年 | 111篇 |
1991年 | 120篇 |
1990年 | 113篇 |
1989年 | 100篇 |
1988年 | 82篇 |
1987年 | 103篇 |
1986年 | 84篇 |
1985年 | 77篇 |
1984年 | 115篇 |
1983年 | 89篇 |
1982年 | 103篇 |
1981年 | 90篇 |
1980年 | 113篇 |
1979年 | 92篇 |
1978年 | 105篇 |
1977年 | 92篇 |
1976年 | 83篇 |
1975年 | 77篇 |
1974年 | 79篇 |
1973年 | 91篇 |
1971年 | 61篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
92.
Sizes of meals of 0·5 M sucrose taken by adults of either sex of Chortoicetes terminifera showed a strong positive correlation with the weight loss of the insects during the preceding 24 hr period of food deprivation. The relationship was less marked in males fed water, and there was no relationship in those fed 1·0 M sucrose. Meal size in females fed water was as strongly weight loss dependent as in those fed 0·5 M sucrose, but was less so in those fed 1·0 M sucrose. The results indicated that changes occurring after the completion of gut emptying influence meal size. 相似文献
93.
94.
Henry C. Orr Kyle H. Sibinovic Peter G. Probst H. Donald Hochstein Dewitt C. Littlejohn 《In vitro cellular & developmental biology. Plant》1975,11(4):230-233
Summary Bacteriological tests were made on 24 lots of unfiltered calf serum collected for subsequent use as a component of tissue
culture media. The examination included the isolation and identification of bacteria, assay of phages, and demonstration of
endotoxin material. Only Gram-positive bacteria were isolated and 96% of the sera were contaminated with bacteria. The prevalent
strains of bacteria found wereBacillus species and streptococci and 63% of the sera coagulatedLimulus amebocyte lysate. More than 90% of the lots contained phages demonstrable with the C-3000 strain ofEscherichia coli. Only one lot of the serum was found to be free from bacteria, phages, and endotoxin by the tests used. 相似文献
95.
The effects of CO2 elevation on the dynamics of fine root (FR) mass and ectomycorrhizal (EM) mass and colonization were studied in situ in a Florida scrub oak system over four years of postfire regeneration. Soil cores were taken at five dates and sorted to assess the standing crop of ectomycorrhizal and fine roots. We used ingrowth bags to estimate the effects of elevated CO2 on production of EM roots and fine roots. Elevated CO2 tended to increase EM colonization frequency but did not affect EM mass nor FR mass in soil cores (standing mass). However, elevated CO2 strongly increased EM mass and FR mass in ingrowth bags (production), but it did not affect the EM colonization frequency therein. An increase in belowground production with unchanged biomass indicates that elevated CO2 may stimulate root turnover. The CO2-stimulated increase of belowground production was initially larger than that of aboveground production. The oaks may allocate a larger portion of resources to root/mycorrhizal production in this system in elevated rather than ambient CO2. 相似文献
96.
Effects of humanization by variable domain resurfacing on the antiviral activity of a single-chain antibody against respiratory syncytial virus. 总被引:1,自引:0,他引:1
S Delagrave J Catalan C Sweet G Drabik A Henry A Rees T P Monath F Guirakhoo 《Protein engineering》1999,12(4):357-362
HNK20 is a mouse monoclonal IgA that binds to the F glycoprotein of respiratory syncytial virus (RSV) and neutralizes the virus, both in vitro and in vivo. The single-chain antibody fragment (scFv) derived from HNK20 is equally active and has allowed us to assess rapidly the effect of mutations on affinity and antiviral activity. Humanization by variable domain resurfacing requires that surface residues not normally found in a human Fv be mutated to the expected human amino acid, thereby eliminating potentially immunogenic sites. We describe the construction and characterization of two humanized scFvs, hu7 and hu10, bearing 7 and 10 mutations, respectively. Both molecules show unaltered binding affinities to the RSV antigen (purified F protein) as determined by ELISA and surface plasmon resonance measurements of binding kinetics (Ka approximately 1x10(9) M-1). A competition ELISA using captured whole virus confirmed that the binding affinities of the parental scFv and also of hu7 and hu10 scFvs were identical. However, when compared with the original scFv, hu10 scFv was shown to have significantly decreased antiviral activity both in vitro and in a mouse model. Our observations suggest that binding of the scFv to the viral antigen is not sufficient for neutralization. We speculate that neutralization may involve the inhibition or induction of conformational changes in the bound antigen, thereby interfering with the F protein-mediated fusion of virus and cell membranes in the initial steps of infection. 相似文献
97.
Snow depth manipulation and its influence on soil frost and water dynamics in a northern hardwood forest 总被引:8,自引:0,他引:8
Janet P. Hardy Peter M. Groffman Ross D. Fitzhugh Karen S. Henry Adam T. Welman Jason D. Demers Timothy J. Fahey Charles T. Driscoll Geraldine L. Tierney Scott Nolan 《Biogeochemistry》2001,56(2):151-174
Climate change will likelyresult in warmer winter temperatures leading toless snowfall in temperate forests. Thesechanges may lead to increases in soil freezingbecause of lack of an insulating snow cover andchanges in soil water dynamics during theimportant snowmelt period. In this study, wemanipulated snow depth by removing snow for twowinters, simulating the late development of thesnowpack as may occur with global warming, toexplore the relationships between snow depth,soil freezing, soil moisture, and infiltration.We established four sites, each with two pairedplots, at the Hubbard Brook Experimental Forest(HBEF) in New Hampshire, U.S.A. and instrumentedall eight plots with soil and snow thermistors,frost tubes, soil moisture probes, and soillysimeters. For two winters, we removed snowfrom the designated treatment plots untilFebruary. Snow in the reference plots wasundisturbed. The treatment winters (1997/1998 and1998/1999) were relatively mild, withtemperatures above the seasonal norm and snowdepths below average. Results show the treatedplots accumulated significantly less snow andhad more extensive soil frost than referenceplots. Snow depth was a strong regulator ofsoil temperature and frost depth at all sites.Soil moisture measured by time domainreflectometry probes and leaching volumescollected in lysimeters were lower in thetreatment plots in March and April compared tothe rest of the year. The ratio of leachatevolumes collected in the treatment plots tothat in the reference plots decreased as thesnow ablation seasons progressed. Our data showthat even mild winters with low snowfall,simulated by snow removal, will result inincreased soil freezing in the forests at theHBEF. Our results suggest that a climate shifttoward less snowfall or a shorter duration ofsnow on the ground will produce increases insoil freezing in northern hardwood forests.Increases in soil freezing will haveimplications for changes in soil biogeochemicalprocesses. 相似文献
98.
Characterization of an antigenically related family of cell-type specific proteins implicated in slug migration in Dictyostelium discoideum 总被引:1,自引:0,他引:1
Stephen Alexander Elizabeth Smith Loralie Davis rew Gooley Suzanne B. Por Lois Browne Keith L. Williams 《Differentiation; research in biological diversity》1988,38(2):82-90
The monoclonal antibody MUD50 recognizes a group of developmentally regulated proteins, which are almost exclusively expressed by prespore cells in developing aggregates of Dictyostelium discoideum. Some of these antigens are integrally associated with the cell membrane, as assessed by physical and detergent-fractionation procedures. The MUD50-reactive proteins are glycosylated and some are phosphorylated. Post-translational modification is the common antigenic feature that is recognized by the MUD50 antibody in these cell-type-specific proteins. A glycosylation-defective mutant, DL118, (modB) does not express the MUD50 epitope, but does express the MUD52 epitope, which is found on a different group of glycoproteins. Therefore, we conclude that MUD50 recognizes a particular carbohydrate epitope on a restricted group of proteins. These proteins are structurally diverse, but are apparently involved in the maintenance of structure and movement of the multicellular D. discoideum slug. 相似文献
99.
Various diffusion processes employed for modelling logistic growth are briefly summarized. A discrete-time, discrete-state space stochastic process for population growth is proposed and analyzed with either Bose-Einstein or Maxwell-Boltzmann statistics for the distribution of offspring in available sites in a restricted region. A diffusion approximation is constructed, which differs from those previously employed. The logistic law is a natural deterministic analog of the diffusion process. 相似文献
100.
G. Winfield Fairchild Richard S. Stemberger Lawrence C. Epskamp Henry A. Debaugh 《International Review of Hydrobiology》1977,62(4):511-521
The small-scale distributions of the rotifers Polyarthra vulgaris, Synchaeta stylata, Conochilus unicornis, Hexarthra mira, and Asplanchna priodonta were investigated in Lancaster Lake, Cheboygan County, Michigan, July 21, 1974. Measurements were taken for 13 depths (at 1-m intervals) and at 4 times of the day (1:00 AM, 7:00 AM, 1:00 PM, and 7:00 PM). In addition, the abundances of 6 crustacean, 2 planktonic dipteran, and 10 algal species, as well as temperature, light, oxygen, chlorophyll a, alkalinity, pH and free carbon dioxide were measured. Whereas abiotic factors appeared to control large scale occupation of the lake, and excluded most species from the deeper portions of the hypolimnion, small-scale distributional variation of the rotifers depended upon biotic interactions, particularly with the crustacean zooplankton. 相似文献