全文获取类型
收费全文 | 8246篇 |
免费 | 777篇 |
国内免费 | 2篇 |
出版年
2022年 | 73篇 |
2021年 | 122篇 |
2020年 | 84篇 |
2019年 | 97篇 |
2018年 | 119篇 |
2017年 | 111篇 |
2016年 | 171篇 |
2015年 | 255篇 |
2014年 | 314篇 |
2013年 | 429篇 |
2012年 | 411篇 |
2011年 | 469篇 |
2010年 | 341篇 |
2009年 | 301篇 |
2008年 | 421篇 |
2007年 | 396篇 |
2006年 | 344篇 |
2005年 | 343篇 |
2004年 | 335篇 |
2003年 | 309篇 |
2002年 | 326篇 |
2001年 | 157篇 |
2000年 | 197篇 |
1999年 | 146篇 |
1998年 | 107篇 |
1997年 | 90篇 |
1996年 | 77篇 |
1995年 | 80篇 |
1994年 | 76篇 |
1992年 | 121篇 |
1991年 | 97篇 |
1990年 | 115篇 |
1989年 | 89篇 |
1988年 | 126篇 |
1987年 | 90篇 |
1986年 | 80篇 |
1985年 | 110篇 |
1984年 | 79篇 |
1983年 | 81篇 |
1982年 | 94篇 |
1981年 | 58篇 |
1980年 | 60篇 |
1979年 | 67篇 |
1978年 | 68篇 |
1977年 | 75篇 |
1976年 | 71篇 |
1975年 | 67篇 |
1974年 | 71篇 |
1973年 | 55篇 |
1972年 | 62篇 |
排序方式: 共有9025条查询结果,搜索用时 31 毫秒
101.
102.
Kinetics of RNA synthesis by vesicular stomatitis virus particles 总被引:25,自引:0,他引:25
103.
Chromosome Transfer and Recombinant Formation with Deoxyribonucleic Acid Temperature-Sensitive Strains of Escherichia coli 总被引:8,自引:7,他引:1
下载免费PDF全文
![点击此处可从《Journal of bacteriology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Haploid recombinant yield is reduced in matings conducted at 42.5 C between deoxyribonucleic acid (DNA) temperature-sensitive [dna−(TS)] recipients unable to synthesize DNA at 42.5 C and any of the three major donor types (Hfr, F+, F′) of Escherichia coli. No such reduction is observed in matings conducted at 42.5 C when the dna−(TS) mutation is in the donor parent. Evidence is presented which indicates that chromosome transfer from donors to recipients unable to replicate DNA at 42.5 C during vegetative growth occurs at normal frequencies when the mating is conducted at 42.5 C. It is concluded that some stage in haploid recombinant formation is adversely affected in dna−(TS) recipients mated at the temperature restrictive for DNA synthesis. 相似文献
104.
The regulation of uptake of glucose (GLU), glycerol (GLY), mannitol (MTL), and succinate (SUC) has been examined in Nocardia erythropolis 305. The apparent Km values of the uptake activities of cells subcultured in a medium with the corresponding substrate as the sole carbon source were 205, 48, 8.7, and 36 μM for GLU, GLY, MTL, and SUC, respectively. GLU and GLY uptake activities were constitutive, although there was evidence for an additional inducible component in GLY uptake. Moreover, MTL and SUC uptake activities were inducible. MTL uptake activity was markedly induced by cultivation in MTL medium and partially induced by growth in GLU medium, whereas SUC uptake was induced only by cultivation in SUC medium. SUC added to MTL medium partially repressed the formation of, or inhibited the activity of, MTL uptake. When not induced, uptake of MTL and SUC was proportional to the substrate concentration. The induced uptake of MTL and SUC and the constitutive uptake of GLU were energy dependent and carrier mediated. Uptake of GLY, constitutively or when induced, was also carrier mediated. 相似文献
105.
Two pure, acidic polysaccharides have been isolated from the hemicellulose of slash pine in yields of 1–2% and 4–5%. Their properties are compared, and the structure of one of them has been investigated by methylation analysis. The results indicate that the glycan is a β-D-(1→4)-linked xylan chain with many branch points. 4-O-Methyl-D-glucopyranosyluronic acid, L-arabinofuranose, and D-xylopyranose residues occur as non-reducing end groups. The uronic acid occurs as single-unit attachments to the main chain. Some of the D-xylose residues in the polysaccharide are doubly branched. The total hemicellulose components of the wood probably represent a complex mixture of chemical types, from which the two pure fractions described above may be separated fortuitously by careful, fractional precipitation. 相似文献
106.
M. Berger O. Z. Roy J. S. Hart 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1970,66(2):190-200
Summary Investigations in 10 species showed that respiration of birds in flight is usually co-ordinated with wing beats, but the co-ordination is not obligatory. Respiration synchronous with wing beats (11 co-ordination) was found only in pigeons and crows, the other species exhibited one of 11 other types of co-ordination. Quails, ducks and pheasants, birds with relatively high wing beat frequencies (with relatively small wings) showed a 51 co-ordination. Within species, and even during a flight the type of co-ordination changed, and simultaneously there were sudden changes in the respiration frequency. For the most part, the beginning of inspiration was linked with the (end of) upstroke and the beginning of expiration with the end of downstroke.Issued as N.R.C.C. No. 11095Dedicated to Prof. Dr. Erwin Stresemann at his 80. birthday.We are grateful to Dr. D. Jensen for enabling the work on the gulls at McMaster University in Hamilton. We also wish to thank B. A. Mackenzie and R. Charbonneau for the skillful assistance in all experiments. 相似文献
107.
108.
Nickel Substitution for Calcium in Excitation-Contraction Coupling of Skeletal Muscle 总被引:3,自引:1,他引:2
In 1962 Frank (22) reported that the addition of any one of a number of divalent cations, including Ni, to a Ca-free Ringer solution prevented the rapid loss of contractility seen in the absence of external Ca. To investigate further the Ni-Ca substitution, studies were made of 45Ca and 63Ni exchange during contraction and at rest using frog striated muscle. In contrast to 45Ca, it was found that there is no increase of 63Ni uptake associated with a K contracture of the sartorius muscle. The rates of loss of 63Ni and 45Ca from resting toe muscles previously bathed in the respective radioisotopes are not significantly different. Resting and action potentials, after 1 hr in a Ringer solution with Ni replacing Ca, closely resemble these potentials in normal Ca-Ringer's solution. Studies on the syneresis of isolated myofibrils indicate that Ni cannot replace Ca in activating this reaction. It is suggested that Ca is required for at least two steps in E-C coupling: one is the spread of excitation at the sarcolemma and transverse tubular system; the second is the activation of actomyosin ATPase. Conceivably Ni can substitute for Ca in the former but not in the latter. 相似文献
109.
110.
Littlefield , Larry J., and Roy D. Wilcoxson . (U. Minnesota, St. Paul.) Studies of necrotic lesions in corn stalks . Amer. Jour. Bot. 49(10): 1072–1078. Illus. 1962.—In 3-day-old necrotic lesions in corn stalks caused by Fusarium graminearum, ground parenchyma cells were discolored and small amounts of a dark substance were present in the cells. The walls of phloem cells were also slightly discolored and a small amount of dark substance was present in the xylem cells. In older lesions the discoloration of parenchyma and phloem cells was more intense; many of the cells contained occluding substances; many phloem protoplasts collapsed, and xylem cells were partially to completely occluded. The occluding substance filling the cells appeared to be translocated from the lesion into the vessel elements extending beyond the lesion so that the bundles appeared as long, dark streaks in the stalk. The occluding substance in xylem, but not in phloem or parenchyma, stained with ruthenium red, a result indicating presence of pectin. Pectinase, however, did not remove the occluding substance. The pectinase dissolved the parenchyma cells in healthy tissues but not in the necrotic lesions. Necrosis in naturally infected plants began as small lesions, but the parenchyma cells quickly dissolved leaving the vascular bundles free of ground parenchyma. No occlusions were found in the central vascular system; a few xylem cells in the peripheral vascular system were occluded with the same substance observed in artificially inoculated plants. Phloem was entirely destroyed by the pathogen. The necrosis prevented upward movement of dye solution in the stalk, but did not measurably affect transpiration, probably because the lesions were not large. Yield was reduced in plants when lesions involved more than 50% of the tissue in inoculated internodes. Smaller lesions had no effect on yield. 相似文献