全文获取类型
收费全文 | 7556篇 |
免费 | 575篇 |
国内免费 | 2篇 |
专业分类
8133篇 |
出版年
2023年 | 40篇 |
2022年 | 106篇 |
2021年 | 187篇 |
2020年 | 96篇 |
2019年 | 135篇 |
2018年 | 173篇 |
2017年 | 132篇 |
2016年 | 239篇 |
2015年 | 383篇 |
2014年 | 415篇 |
2013年 | 510篇 |
2012年 | 680篇 |
2011年 | 637篇 |
2010年 | 392篇 |
2009年 | 345篇 |
2008年 | 466篇 |
2007年 | 416篇 |
2006年 | 373篇 |
2005年 | 354篇 |
2004年 | 342篇 |
2003年 | 297篇 |
2002年 | 280篇 |
2001年 | 80篇 |
2000年 | 59篇 |
1999年 | 64篇 |
1998年 | 80篇 |
1997年 | 47篇 |
1996年 | 56篇 |
1995年 | 41篇 |
1994年 | 64篇 |
1993年 | 43篇 |
1992年 | 47篇 |
1991年 | 47篇 |
1990年 | 35篇 |
1989年 | 44篇 |
1988年 | 24篇 |
1987年 | 35篇 |
1986年 | 27篇 |
1985年 | 23篇 |
1984年 | 32篇 |
1983年 | 26篇 |
1982年 | 18篇 |
1981年 | 27篇 |
1980年 | 28篇 |
1979年 | 18篇 |
1978年 | 16篇 |
1977年 | 21篇 |
1975年 | 14篇 |
1974年 | 13篇 |
1973年 | 17篇 |
排序方式: 共有8133条查询结果,搜索用时 15 毫秒
71.
Jose L. Garcia-Martinez Noah Ben-Shalom Lawrence Rappaport 《Plant Growth Regulation》1984,2(3):209-216
The effect of gibberellin A1 (GA1) on production of ethylene by cowpea (Vigna sinensis cv Blackeye pea no. 5) epicotyl explants and its relationship to epicotyl elongation was investigated. The explants were placed upright in water and incubated in sealed culture tubes or in large jars. GA, and IAA in ethanol solution were injected into the subapical tissues of the decapitated epicotyls. Cowpea epicotyl explants elongated after GA but not after IAA treatment, and they were very sensitive to exogenous ethylene. As little as 0.14 1/1 ethylene reduced significantly GA1-induced epicotyl elongation.Treatment with GA1 induced the production of ethylene which began 10 h after GA application, showed a peak at about 22 h and then declined. The yield of ethylene was proportional to the amount of GA, injected. The inhibition of epicotyl elongation in closed tubes was avoided by absorbing ethylene released with Hg(Cl04)2 , or by adding AVG to the incubation solution to inhibit ethylene production. Treatment with IAA elicited a rapid production of ethylene which ceased about 10 h after application. The effects of IAA and GA1 on ethylene production were additive.Abbreviations AVG
aminoethoxyvinylglycine 2-amino-4-(2-aminoethoxy)-trans-3butenoic acid
- ACC
1-aminocyclopropane-1-carboxylic acid
- GA
gibberellin
- IAA
indole-3-acetic acid 相似文献
72.
Ernesto Bonilla Enrique Salazar Jose Joaquin Villasmil Ruddy Villalobos Magaly Gonzalez Jose Omar Davila 《Neurochemical research》1984,9(11):1543-1548
Copper concentration was determined in samples from 38 areas of 7 normal human brains. The grey matter contained higher concentrations of copper than the white matter. Identical areas of the grey and white matter of the cerebral cortex showed significant differences between individuals. In the caudate nucleus the highest concentrations of copper were found in the tail followed by the body and the head, respectively. A negative linear regression between age and brain copper levels was demonstrated. 相似文献
73.
74.
Freeze fracturing electron microscopy of Escherichia coli K12 cells showed that the outer fracture face of the outer membrane is densily occupied with particles. On the inner fracture face of the outer membrane, pits are visible, which are probably complementary to the particles at opposite fracture face. This observation suggests that the particles are micelle-like. In some mutants which lack one or more major outer membrane proteins the density of particles is reduced. The loss of protein d appeared to a prerequisite for this phenomenon. However, mutants which lack all glucose and heptose-bound phosphate in their lipopolysaccharide also have a reduction in particle density whereas, the amount of protein d is normal. Moreover, loss of lipopolysaccharide by EDTA treatment also caused a reduction in the density of particles. From these results it is hypothesized that the particles consist of lipopolysaccharide aggregates stabilized by divalent cations and probably complexed with protein and/or phospholipid. 相似文献
75.
In the pituitary of A. testudineus the rostral pars distalis (RPD), proximal pars distalis (PPD) and pars intermedia (PI) are arranged in a rostro-caudal axis. The rostral neurohypophysis (NH) extends dorsal to the pars distalis, while the caudal part vertically penetrates into the PI and ramifies in its component. The RPD mainly consists of erythrosinophils and PgH-positive cells which are comparable to the lactotropes and corticotropes mentioned in the literature. In the nonbreeding season, acidophils are the predominant cells of the PPD and are largely confined to its dorsal aspect. The two types of cyanophils present in the PPD could not be differentiated with the various techniques used in this study. However, those cyanophils which increase in number and are active during the spawning season may be the gonadotropes, while those which are cytologically inactive may be the thyrotropes. The pars intermedia consists of PAS+ and PhH+ cells. Trichrome and tetrachrome staining techniques revealed the presence of acidophils, cyanophils and amphiphils in the PI. 相似文献
76.
A rapid method for determining plasmid incompatibility group by agarose gel electrophoresis is described. This procedure requires only 4 or 5 days and is especially useful when it is not possible to distinguish two plasmids in the same cell by their antibiotic resistance patterns. 相似文献
77.
Ramon J. Rhine Guy W. Norton William J. Roertgen Henry D. Klein 《International journal of primatology》1980,1(4):401-409
The importance of mother — infant attachment in free-ranging primates is illustrated by events culminating in the deaths of
two baboon infants a few days after losing their mothers. These two cases are contrasted with those of a severely injured
infant, not separated from its mother, which lived, and an animal which lost and refound its troop. Protective behavior of
adult males is described. In captivity, separation sometimes produces infant depression; in nature, such depression may be
fatal. 相似文献
78.
Effects of the dipolar form of phloretin on potassium conductance in squid giant axons. 总被引:3,自引:0,他引:3 下载免费PDF全文
The effects of phloretin on membrane ionic conductances have been studied in the giant axon of the squid, Loligo pealei. Phloretin reversibly suppresses the potassium and sodium conductances and modifies their dependence on membrane potential (Em). Its effects on the potassium conductance (GK) are much greater than on the sodium conductance; no effects on sodium inactivation are observed. Internal perfusion of phloretin produces both greater shifts in GK(Em) and greater reductions maximum GK than does external perfusion; the effect of simultaneous internal and external perfusion is little greater than that of internal perfusion alone. Lowering the internal pH, which favors the presence of the neutral species of weakly acidic phloretin (pKa 7.4), potentiates the actions of internally perfused phloretin. Other organic cations with dipole moments similar to phloretin's have little effect on either potassium or sodium conductances in squid axons. These results can be explained by either of two mechanisms; on postulates a phloretin "receptor" near the voltage sensor component of the potassium channel which is accessible to drug molecules applied at either the outer or inner membrane surface and is much more sensitive to the neutral than the negatively charged form of the drug. The other mechanism proposes that neutral phloretin molecules are dispersed in an ordered array in the membrane interior, producing a diffuse dipole field which modifies potassium channel gating. Different experimental results support these two mechanisms, and neither hypothesis can be disproven. 相似文献
79.
Several categories of social and individual behaviors were recorded during the first 60 days of life for seven stumptail macaques.
Mother-infant interactions tended to decelerate in synchrony with increases in infant nonsocial behaviors, such as object
manipulation. Several aspects of stumptail social growth during the first two months were similar to those of rhesus and other
macaques. Approaching-leaving, which has been used previously as an index of waning of the mother-infant bond, was due more
to infants than to mothers during the second month of life 相似文献
80.
Christophe Biju-Duval Hajer Ennafaa Nicole Dennebouy Monique Monnerot Françoise Mignotte Ramon C. Soriguer Amel El Gaaïed Ali El Hili Jean-Claude Mounolou 《Journal of molecular evolution》1991,33(1):92-102
Summary A characterization was conducted on mitochondrial DNA (mtDNA) molecules extracted separately from 107 European rabbits (Oryctolagus cuniculus) both wild and domestic, 13 European hares (Lepus capensis), and 1 eastern cottontail (Sylvilagus floridanus). Experimentally this study took into account restriction site polymorphism, overall length variation of the noncoding region, and numbers of repeated sequences. Nucleotide divergences indicate that the mtDNAs from the three species derived from a common ancestor some 6–8 million years (Myr) ago. Every animal appeared heteroplasmic for a set of molecules with various lengths of the noncoding region and variable numbers of repeated sequences that contribute to them. This systematic heteroplasmy, most probably generated by a rate of localized mtDNA rearrangements high enough to counterbalance the cellular segregation of rearranged molecules, is a shared derived character of leporids.The geographic distribution of mtDNA polymorphism among wild rabbit populations over the western European basin shows that two molecular lineages are represented, one in southern Spain, the second over northern Spain, France, and Tunisia. These two lineages derived from a common ancestor some 2 Myr ago. Their present geographical distribution may be correlated to the separation of rabbits into two stocks at the time of Mindel glaciation.Finally the distribution of mtDNA diversity exhibits a mosaic pattern both at inter- and intrapopulation levels. 相似文献