全文获取类型
收费全文 | 5723篇 |
免费 | 686篇 |
国内免费 | 3篇 |
出版年
2021年 | 79篇 |
2020年 | 42篇 |
2019年 | 53篇 |
2018年 | 72篇 |
2017年 | 79篇 |
2016年 | 94篇 |
2015年 | 145篇 |
2014年 | 192篇 |
2013年 | 276篇 |
2012年 | 271篇 |
2011年 | 278篇 |
2010年 | 190篇 |
2009年 | 155篇 |
2008年 | 217篇 |
2007年 | 248篇 |
2006年 | 225篇 |
2005年 | 211篇 |
2004年 | 200篇 |
2003年 | 192篇 |
2002年 | 192篇 |
2001年 | 137篇 |
2000年 | 161篇 |
1999年 | 129篇 |
1998年 | 91篇 |
1997年 | 69篇 |
1996年 | 75篇 |
1995年 | 66篇 |
1994年 | 57篇 |
1993年 | 59篇 |
1992年 | 109篇 |
1991年 | 102篇 |
1990年 | 83篇 |
1989年 | 87篇 |
1988年 | 105篇 |
1987年 | 104篇 |
1986年 | 81篇 |
1985年 | 82篇 |
1984年 | 97篇 |
1983年 | 85篇 |
1982年 | 54篇 |
1981年 | 67篇 |
1980年 | 58篇 |
1979年 | 78篇 |
1978年 | 50篇 |
1977年 | 45篇 |
1976年 | 55篇 |
1975年 | 41篇 |
1974年 | 55篇 |
1973年 | 43篇 |
1969年 | 44篇 |
排序方式: 共有6412条查询结果,搜索用时 15 毫秒
111.
During field studies on the pteridophytes of Costa Rica, a peculiarly dimorphic polypodioid fern was found in the rain-forests of the Atlantic lowlands near Puerto Viejo. The variation in the fertile frond, ranging from simple and short petiolate to pinnatisect and long petiolate, coupled with peculiarly elongate and irregular sori, prompted further investigations. Additional herbarium specimens from localities in Mexico, British Honduras, Nicaragua, and Costa Rica showed intermixed variations between plants with all leaves simple (typified byPolypodium bradeorum Rosenstock) to plants with all leaves lobed or pinnatisect (typified byP. colysoides Maxon & Copeland). Other characters were judged sufficiently homogeneous to consider these individuals as conspecific underP. bradeorum. Morphological studies indicate parallel evolution of several characters in the Asiatic generaColysis, Microsorium, andLeptochilus on the one hand and the New World members ofMicro gramma, Pleopeltis, andPolypodium bradeorum on the other 相似文献
112.
David F. Murray 《Brittonia》1969,21(1):55-76
The sectionAtratae is highly differentiated in the southern Rocky Mountain region where the group has developed several endemic species. Eleven species are recognized, and three new combinations are proposed:C. parryana ssp.hallii, C. parryana ssp.idahoa, andC. norvegica ssp.stevenii. 相似文献
113.
Copolymers of γ-methyl D - and L -glutamates with various D /L ratios were prepared. Infrared absorption spectra of solid films were measured and sums of right- and left-handed helix contents were determined from intensities of amide V bands. Farultraviolet absorption spectra and optical rotatory dispersion of these copolymers in solutions are used to ascertain their helical character. Chain conformations of DL -copolypeptides are discussed. 相似文献
114.
115.
116.
1. Both the acid-soluble fraction and the nucleic acid fraction of wheat embryos were extensively labelled after incubation for 6hr. in the presence of [8-(14)C]adenine. Subsequent incubation in the absence of labelled adenine resulted in no loss of radioactivity to the medium during a 48hr. period. Radioautography indicated that during this period there was a continuous increase in the radioactivity present in the acid-insoluble fractions of the root and leaf tissues relative to that present in the coleorhiza and coleoptile. 2. During incubation at 25 degrees there was a 26-fold increase in the activity of 3'-nucleotidase between 4hr. and 24hr.; the activities of enzymes hydrolysing AMP and IMP increased to a smaller extent. The activities of adenine phosphoribosyltransferase and hypoxanthine phosphoribosyltransferase increased three- to five-fold during incubation at 25 degrees for 24hr. 3. Adenosine kinase, inosine phosphorylase and 5-phosphoribosyl pyrophosphate synthetase activities were high in extracts from dry embryos and did not increase during 48hr. at 25 degrees . 4. The increase in 3'-nucleotidase activity was prevented by cycloheximide, cryptopleurine or incubation at 4 degrees , but not by actinomycin D; these treatments did not depress the activity of the other enzymes measured. 5. The results are discussed in relation to RNA translocation within the wheat embryo during germination. 相似文献
117.
1. A fractionation of chicken erythrocyte histones was achieved simultaneously with their extraction from saline-washed nuclei by stepwise titrations to progressively lower pH values. 2. Different acids and dilute buffer solutions of comparable pH behaved similarly in stepwise extractions of histones. 3. The histone preparations so obtained were characterized by their amino acid composition and behaviour on zone electrophoresis in starch gels. 4. The fractionation by titration was quite sharp at appropriate pH ranges, and the histone fraction that is apparently unique to avian erythrocytes was obtained without contamination by other histone fractions. 5. Histones prepared by stepwise titration were fractionated further by cation-exchange and exclusion chromatography. The chromatographic behaviour and amino acid composition of the components permitted comparison with histones prepared by other methods. 6. Histone fraction IIb was resolved into its subfractions IIb(1) and IIb(2) by exclusion chromatography on Bio-Gel P-60. 7. Histone fractions III and IV, previously reported to be absent from chicken erythrocyte nuclei, were found in extracts made at pH1. 相似文献
118.
Fine Structure of Listeria monocytogenes in Relation to Protoplast Formation 总被引:19,自引:11,他引:8
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)
Among the eight strains of Listeria monocytogenes tested for lysozyme sensitivity, two were resistant to lysozyme but became sensitive after lipase pretreatment. Among the other six, one was very sensitive to lipase and another one was extremely susceptible to lysozyme. Stable protoplasts were formed from the lysozyme-resistant strain (42) by lipase and lysozyme treatment, which completely digested the cell wall. The cell wall (uranyl acetate-lead stained) was of a thick triple-layered profile, with the intermediate layer of low density. Lipase treatment for a short time (60 min) did not cause any alteration in structure, but prolonged treatment (180 min) caused extensive digestion of the plasma membrane and the cell wall, liberating cytoplasmic material. When the cells were treated with either lipase or lysozyme, a small number of protoplasts were extruded through the partly digested or weakened transverse cell wall, leaving an almost intact cell wall ghost. There were small vesicular structures in the interspace between cell wall and plasma membrane. Mesosomes of varied organization were prominent in electron micrographs, both in sections and in negatively stained preparations. These were largely everted during protoplasting in the form of tubules and as small peripheral buds; a few small vesicles also remained as intrusive structures, some of which were very unusual because they appeared to be enclosed by the inner layer of plasma membrane alone. Lysis of the protoplasts by dilution of the sucrose, while maintaining a constant ionic environment, liberated many small vesicular structures and fibrillar nuclear material. 相似文献
119.
Esters of serine and threonine in hydrolysates of histones and protamines, and attendant errors in amino acid analyses of proteins 总被引:4,自引:3,他引:1
下载免费PDF全文
![点击此处可从《The Biochemical journal》网站下载免费的PDF全文](/ch/ext_images/free.gif)
1. Partial acid hydrolysates of histones from various origins and of protamine were analysed by a two-dimensional ionophoretic procedure to reveal strongly acidic ninhydrin-positive components. 2. Histone fractions prepared by extraction with sulphuric acid gave rise to spots identified as serine O-sulphate and threonine O-sulphate. These two compounds, which were not found in hydrolysates of corresponding fractions prepared by extraction with hydrochloric acid, were artifacts. 3. Hydrolysis of proteins in the presence of traces of sulphate can lead to the formation of the O-sulphates of serine and threonine. This can cause errors, which may sometimes be serious, in amino acid analyses of proteins. 4. O-Phosphoserine was obtained in small amounts from some histone fractions and from protamine, but was undetectable in other histone fractions, notably those of lower lysine content. 相似文献
120.
ELECTRON MICROSCOPIC OBSERVATIONS OF RAT AND MOUSE CEREBELLUM IN TISSUE CULTURE 总被引:12,自引:8,他引:4
下载免费PDF全文
![点击此处可从《The Journal of cell biology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Closely ordered stages of myelin formation in cultures of newborn rat and mouse cerebellum, selected by direct light microscopy, were studied with the electron microscope. Electron micrographs of these cultures reveal the presence of neurons, axons, neuroglia, microglia, and ependymal cells. The appearance of the neuron is identical to that previously described in vivo. The neuroglial cell has long, branching processes, and its cytoplasm is characterized by packets of long, narrow fibrils. During myelin formation, a glial cell process surrounds the axon. This process may form an internal mesaxon and may spiral for several turns around the axon. Other glial cell processes may interdigitate with or overlay the innermost process to contribute to the multilamellated structure. The glial processes flatten and the cytoplasmic surfaces of the cell membrane come into contact to form the lamellae of the myelin sheath. These adhesions may be temporarily incomplete as evidenced by sequestered islands of glial cytoplasm among the myelin lamellae. Ultimately, a compact, apparently spiral, myelin sheath is formed. These findings are discussed in relation to in vivo central myelin formation. 相似文献