全文获取类型
收费全文 | 26884篇 |
免费 | 2220篇 |
国内免费 | 20篇 |
专业分类
29124篇 |
出版年
2023年 | 86篇 |
2022年 | 189篇 |
2021年 | 441篇 |
2020年 | 262篇 |
2019年 | 331篇 |
2018年 | 410篇 |
2017年 | 343篇 |
2016年 | 593篇 |
2015年 | 1024篇 |
2014年 | 1206篇 |
2013年 | 1443篇 |
2012年 | 1923篇 |
2011年 | 1949篇 |
2010年 | 1262篇 |
2009年 | 1086篇 |
2008年 | 1697篇 |
2007年 | 1757篇 |
2006年 | 1607篇 |
2005年 | 1565篇 |
2004年 | 1583篇 |
2003年 | 1512篇 |
2002年 | 1396篇 |
2001年 | 279篇 |
2000年 | 189篇 |
1999年 | 303篇 |
1998年 | 361篇 |
1997年 | 269篇 |
1996年 | 222篇 |
1995年 | 222篇 |
1994年 | 239篇 |
1993年 | 229篇 |
1992年 | 186篇 |
1991年 | 178篇 |
1990年 | 160篇 |
1989年 | 139篇 |
1988年 | 163篇 |
1987年 | 132篇 |
1986年 | 130篇 |
1985年 | 166篇 |
1984年 | 169篇 |
1983年 | 177篇 |
1982年 | 194篇 |
1981年 | 185篇 |
1980年 | 160篇 |
1979年 | 109篇 |
1978年 | 114篇 |
1977年 | 93篇 |
1976年 | 95篇 |
1975年 | 74篇 |
1974年 | 76篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
Distilled water suspensions of Legionella pneumophila were found to be sensitive to low doses of germicidal ultraviolet radiation. 相似文献
112.
113.
Graham S. Baldwin Alphonse Galdes H.Allen O. Hill Stephen G. Waley Edward P. Abraham 《Journal of inorganic biochemistry》1980,13(3):189-204
β-Lactamase II has two metal-binding sites. The electronic spectra of Cd(II)- and Co(II)-substituted β-lactamase II have been investigated. It is suggested that a thiol ligand is involved in metal binding at the first site. The stoichiometric dissociation constants for Co(II) binding to β-lactamase II were estimated to be 0.13 and 2.66 mM (pH 6.0, 4°C, 1 M NaCl) by equilibrium dialysis. Competition between Zn(II) and Co(II) for the first metal binding site suggests a value of 0.7 μM (pH 6.0, 30°C, 1 M NaCl) for the dissociation constant o Zn(II).The electronic spectra of the Co(II) enzyme lead to the suggestion that the coordination geometries around the metal ions in the first and second sites are related to those of a distorted tetrahedron and octahedron, respectively. 相似文献
114.
The influence of zooplankton food resources on the morphology of the estuarine clupeid Gilchristella aestuarius (Gilchrist, 1914) 总被引:1,自引:0,他引:1
Stephen J. M. Blaber Digby P. Cyrus Alan K. Whitfield 《Environmental Biology of Fishes》1981,6(3-4):351-355
Synopsis The body shape of the estuarine clupeidGilchristella aestuarius from the St. Lucia system is different from that of the same species in other estuaries. The morphology ofG. aestuarius is discussed in relation to long term food availability in the St. Lucia system. 相似文献
115.
The stylet apparatus of the nemertean Paranemertes Peregrina: Its ultrastructure and role in prey capture 总被引:2,自引:0,他引:2
Summary The nemertean Paranemertes peregrina captures prey by using an eversible proboscis that is armed with a stylet apparatus. The apparatus consists of several reserve stylet sacs and a central stylet that is attached to a granular mass, called the basis. When the proboscis is everted, the central stylet is used to stab prey such as nereid polychaetes, and paralytic neurotoxins, produced in the proboscis, are inserted in the stylet-induced wounds. The central stylet averages 85 m in length and has helically-arranged grooves along its shaft. The proximal piece of the central stylet is anchored to the basis, apparently by adhesive granules in the anterior end of the basis. A basis sheath surrounds the basis and is continuous posteriorly with a duct, called the ductus ejaculatorius. Secretions in the ductus ejaculatorius may contain some of the toxin that is used to immobilize the prey. The contents of the duct are probably injected into the prey by way of the grooves on the central stylet. In the region anterior to the central stylet, there are numerous glandular cells and anchor cells that are believed to attach the stylet apparatus to the prey during attack. Each reserve stylet sac is lined by a simple epithelium. One of the epithelial cells, called the styletocyte, is greatly enlarged and fills the lumen of the sac. Several reserve stylets are assembled in a styletocyte. Each reserve stylet is formed within a membrane-bound vacuole associated with the Golgi apparatus and is composed of an inner organic core surrounded by an inorganic cortex. A duct connects each reserve stylet sac with the area around the central stylet and provides a pathway for the transfer of reserve stylets during replacement of the central stylet. 相似文献
116.
Chronic administration of l-prolyl-l-leucyl-glycinamide (MIF-I) to mice slightly reduced morphine's antinociceptive activity in the hot-plate test and modified the biphasic motor activity response to morphine. MIF-I antagonized the initial depression of activity and potentiated the increased motor activity phase. Chronic treatment of rats with MIF-I prevented morphine's antinociceptive activity in the tail flick test. MIF-I partly antagonized the inhibition by morphine of the coaxially stimulated guinea-pig ileum preparation. The inhibition of the ileum produced by ethylketocyclazocine was weakly antagonized by MIF-I. In contrast, MIF-I had no effect on the inhibition of the stimulated mouse vas deferens produced by Leu-enkephalin. The findings show that MIF-I weakly and selectively inhibits μ-type opiate receptors which suggests that MIF-I could be an endogenous inhibitor of opiate receptors. 相似文献
117.
Summary Spermatozoa from the cauda epididymidis of gossypol-treated rats exhibit distinctive departures from the morphology of spermatozoa from control rats: wrinkled and disorganized cell membrane in the head and tail regions, cell membrane missing from segments of the tail midpiece and principal piece regions, malformed heads, decapitate spermatozoa, retention of a cytoplasmic droplet at variable loci along tail midpieces, and looped tails. The observations suggest that gossypol exerts its contraceptive effect during spermatocytogenesis and spermiogenesis, including the posttesticular development and maturation of spermatozoa in the epididymis. 相似文献
118.
Stephen N. Barnes Timothy H. Goldsmith 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1977,120(2):143-159
Summary Dark adaptation of living lobsters was measured by recording the ERG at several temperatures in the range 5–20 °C following adapting flashes that convert about 70% of the rhodopsin to metarhodopsin. Recovery of log threshold is rapid, and at 10–20° is nearly complete in 10 min. Only at 5 °C is dark adaptation significantly slowed. Comparison of dark adaptation with data on regeneration of pigment (Bruno et al., 1977) is consistent with the hypothesis that as rhodopsin concentration rises and falls, its only effect on sensitivity is to alter the probability of quantum catch. This interpretation is further bolstered by observations on winter lobsters that have a 70% deficiency of rhodopsin without the concomitant increase in metarhodopsin that accompanies light adaptation. No effect of metarhodopsin on sensitivity was detected. These experiments support the growing body of evidence indicating that the relationship between rhodopsin concentration and log threshold is fundamentally different in the rhabdomeric photoreceptors of invertebrates and the rods and cones of vertebrates.This work was supported by USPHS research grant EY 00222 to Yale University. S.N.B. was aided by NIH Postdoctoral Fellowship EY 52378, by funds made available through the Unidel Foundation, and by a grant from the University of Delaware Research Foundation. 相似文献
119.
Marsh white seedless grapefruit were treated with the 2-diethylaminoethanol esters of the following acids: benzoic, phenylacetic, hydrocinnamic, 4-phenylbutyric, 5-phenylvaleric, valeric, hexanoic, heptanoic, octanoic, nonanoic, 5-chlorovaleric, cyclohexanecarboxylic, phenoxyacetic, p-chlorophenoxyacetic, 3-phenoxypropionic, cinnamic and p-chlorocinnamic. Several of these esters, in particular the hexanoate, 4-phenylbutyrate and cinnamate, caused the accumulation of large amounts of β-carotene. The effects of the hexanoate and of 2-phenoxytriethylamine, which causes only lycopene accumulation, were studied as functions of time. The hexanoate caused the rapid accumulation of lycopene during the first day. The amount of lycopene then began to decrease and that of β-carotene increased until, after 14 days, β-carotene was the major pigment. 2-Phenoxytriethylamine caused rapid lycopene accumulation during the first day and a slow steady increase afterwards. Thus, the mode of action of the β-carotene inducers may be similar to that of the lycopene inducers except that the former are probably rapidly hydrolysed by the esterase(s) in the flavedo, so that they no longer inhibit the cyclase(s), and β-carotene is accumulated at the expanse of lycopene. 相似文献
120.
Summary Nine organophosphate and carbamate insecticides were tested for effects on ability of Azotobacter vinelandii to reduce acetylene. Only GardonaR, at higher concentrations, was significantly inhibitory. The same pesticides were tested with soybeans (Glycine max L.). Some minor phytotoxic effects were noted, but there was no inhibition of the ability of the excised nodules of the plants to reduce acetylene.Published with the approval of the Director of the North Dakota Agricultural Experiment Station as Journal Article No. 726. Portion of a thesis presented by the senior author in partial fulfillment of the requirements for the M.S. degree in bacteriology at North Dakota State University.Published with the approval of the Director of the North Dakota Agricultural Experiment Station as Journal Article No. 726. Portion of a thesis presented by the senior author in partial fulfillment of the requirements for the M.S. degree in bacteriology at North Dakota State University. 相似文献