全文获取类型
收费全文 | 4134篇 |
免费 | 249篇 |
国内免费 | 3篇 |
出版年
2021年 | 38篇 |
2020年 | 44篇 |
2019年 | 31篇 |
2018年 | 46篇 |
2017年 | 41篇 |
2016年 | 63篇 |
2015年 | 122篇 |
2014年 | 147篇 |
2013年 | 210篇 |
2012年 | 262篇 |
2011年 | 236篇 |
2010年 | 177篇 |
2009年 | 173篇 |
2008年 | 196篇 |
2007年 | 247篇 |
2006年 | 254篇 |
2005年 | 259篇 |
2004年 | 223篇 |
2003年 | 194篇 |
2002年 | 226篇 |
2001年 | 57篇 |
2000年 | 38篇 |
1999年 | 65篇 |
1998年 | 80篇 |
1997年 | 38篇 |
1996年 | 45篇 |
1995年 | 47篇 |
1994年 | 57篇 |
1993年 | 35篇 |
1992年 | 40篇 |
1991年 | 36篇 |
1990年 | 44篇 |
1989年 | 21篇 |
1988年 | 29篇 |
1987年 | 25篇 |
1986年 | 25篇 |
1985年 | 31篇 |
1984年 | 40篇 |
1983年 | 29篇 |
1982年 | 34篇 |
1981年 | 29篇 |
1980年 | 33篇 |
1979年 | 16篇 |
1978年 | 27篇 |
1977年 | 19篇 |
1976年 | 23篇 |
1975年 | 20篇 |
1974年 | 23篇 |
1973年 | 25篇 |
1967年 | 16篇 |
排序方式: 共有4386条查询结果,搜索用时 46 毫秒
111.
A PC survey revealed the occurrence of an unusual amino acid in fruit bodies of the mushroom Lactarius quietus Fr. and a related species. This was identified as l-2-amino-4-methylpimelic acid, which has not been previously reported as a naturally occurring compound. 相似文献
112.
Dr. Rolf Dierichs 《Cell and tissue research》1975,160(3):399-410
Summary Scanning and transmission electron microscopy were used to study the inner architecture of the frog lung. In some specimens the alveolar surface mucus layer was removed to permit the examination of underlying features. The inner surface of the frog's lung is covered by a layer of microvilli belonging to only one type of epithelial cells. The boundaries of these epithelial cells are demarcated by small ridges. Different degrees of lung expansion cause variations of the surface topography. The morphology of certain surface features is examined in detail. Several methods of drying the specimens are compared.The author wishes to thank Dr. I. E. Richter, Institut für allgemeine und experimentelle Pathologie der Bundeswehr, Mainz, for the opportunity to do these investigations and for helpful discussions. 相似文献
113.
The inflorescence of Helwingia japonica (Thunb.) Dietr. is initiated adjacent to the leaf axil on the adaxial side of the base of a leaf primordium during its second plastochron. The inflorescence which develops from the resulting primordium comes to be situated on the midrib of the mature fertile leaf, through the action of a basal, intercalary meristem. In fertile leaves this meristem develops beneath, as well as above, the insertion of the inflorescence primordium on the leaf primordium. The same meristem is present in sterile leaves as well. A separate, adaxial vascular bundle departs from the leaf trace in the base of the petiole and leads to the inflorescence, in the mature fertile leaf. This adaxial vascular bundle is absent in sterile leaves. It is argued that the vascular anatomy does not conclusively confirm the hypothesis that the epiphyllous inflorescence is the congenital fusion product of a leaf and an axillary inflorescence. Instead, it is suggested that the interplay of changes in the position of primordium initiation, and intercalary growth, offers an ontogenetic explanation of the situation, which in turn may be related to the phylogeny of the species in question. It appears to be misguided and futile to look for homologies (i.e., 1:1 correspondences) between fertile and sterile leaves, since 1:1 correspondences do not exist in this case. 相似文献
114.
Localization of the diphtheria toxin receptor-binding domain to the carboxyl-terminal Mr approximately 6000 region of the toxin 总被引:4,自引:0,他引:4
The carboxyl-terminal Mr = 5982 peptide of diphtheria toxin was prepared by specific cleavage of the toxin with hydroxylamine and purified by fast performance liquid chromatography. The identity of the peptide was established by a combination of sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis, reactivity with specific monoclonal antibodies, and amino-terminal sequence analysis. The Mr = 5982 peptide was shown to protect highly toxin-sensitive Vero cells from the lethal action of diphtheria toxin. This protection was shown to be due to inhibition of the initial step in the cytotoxic process, the binding of toxin to its receptor. These results strongly suggest that the Mr = 5982 carboxyl-terminal region (amino acid residues 482-535) is, or contains, the receptor-binding domain of diphtheria toxin. 相似文献
115.
The suitability of the reductive-cleavage method for analysis of the linkage positions in d-fructofuranosyl residues of d-fructans was examined by using sucrose, chicory-root inulin, and Aerobacter levanicum levan as models. Permethylation, and reductive cleavage with triethylsilane in the presence of either boron trifluoride etherate or trimethylsilyl trifluoromethanesulfonate, gave the expected methylated derivatives of 2,5-anhydro-d-mannitol and 2,5-anhydro-d-glucitol. With either catalyst, nonreducing (terminal) d-fructofuranosyl groups and d-fructofuranosyl residues linked at O-1 gave derivatives having the manno configuration as the major product, whereas d-fructofuranosyl residues linked at O-6, and at both O-1 and O-6, gave derivatives having the gluco configuration as the major product. The independent synthesis, and n.m.r.- and mass-spectral characterization, of the methylated 2,5-anhydro-d-mannitol and 2,5-anhydro-d-glucitol derivatives formed from these residues by reductive cleavage are reported. 相似文献
116.
The cell-surface expression patterns of major histocompatibility complex (MHC) class I, class II and heat-shock protein 72
(HSP72) molecules were measured on human lung (LX-1) and mammary (MX-1) carcinoma cells. No major differences were found in
the MHC cell-surface expression pattern of both cell lines. However, they differ significantly in their capacity to express
HSP72 on their cell surface. Under physiological conditions LX-1 cells express HSP72 molecules on more than 90% of the cells,
whereas MX-1 cells exhibit no significant HSP72 cell-surface expression (less than 5%). These expression patterns remained
stable in all further cell passages tested. The sensitivity to lysis mediated by an interleukin-2 (IL-2)-stimulated, adherent
natural killer (NK) cell population could be correlated with the amount of cell-surface-expressed HSP72 molecules. By antibody-blocking
studies, using HSP72-specific monoclonal antibody (mAb), a strong inhibition of lysis was only found with LX-1 cells but not
with MX-1 cells. In contrast to the cell-surface expression, the cytoplasmic amount of HSP72 in MX-1 cells was twice as high
compared to LX-1 cells under physiological conditions. After nonlethal heat-shock the rate of induction and the total cytoplasmic
amounts of HSP72 were comparable in both cell lines. The clonogenic cell viability of LX-1 cells after incubation at temperatures
ranging from 41°C to 44°C was significantly elevated compared to that of MX-1 cells. In conclusion we state the following:
(i) HSP72 cell-surface expression on human carcinoma cells is independent of the cytoplasmic amount of HSP72; (ii) the cell-surface
expression of HSP72 is associated with an increased sensitivity of tumour cells to lysis mediated by an IL-2-stimulated, adherent
NK cell population; (iii) thermoresistance is not related to the cytoplasmic HSP72 level but might be related to the amount
of HSP72 expressed on the cell surface.
Received: 20 June 1996 / Accepted: 25 September 1996 相似文献
117.
Rolf Erhard und Michael Wink 《Journal of Ornithology》1987,128(4):477-484
Zusammenfassung Die Brutverbreitung der Vögel im Großraum Bonn wurde 1975 und 1985 in jeweils 221 Minutengitterfeldern von je 2,2 km2 Größe kartiert. Statistisch signifikante Zunahmen besetzter Gitterfelder wurden für 9 Arten, Abnahmen für 22 Arten registriert, die z. T. auch andernorts festgestellt wurden. Unter Verwendung von Kartierungsdaten 1974–1978 konnte bei 14 Arten eine signifikante Änderung (Zunahmen: Sperber, Habicht, Singdrossel; Abnahmen: Steinschmätzer, Rebhuhn, Schafstelze, Grauammer, Mehlschwalbe, Feldsperling, Fasan, Dorngrasmücke, Hänfling, Feldlerche und Zaunkönig) nachgewiesen werden. Die Untersuchung zeigt, daß die Rasterkartierung gut geeignet ist, Aussagen über großflächige und langfristige Populationsveränderungen zu machen.
Fluctuation of breeding populations in the area of Bonn: Analysis of grid mapping in 1975 and 1985
Summary The distribution of breeding birds in the area of Bonn was mapped in 1975 and 1985 in 221 grids of 2.2 km2 size each. These data were analyzed to see whether fluctuations in the breeding populations occurred during the 11 years. It was based on the assumption that strong population changes should be reflected in the number of grids occupied by a species. Statistically significant increases were recorded for 9 species, decreases for 22 species. Taking other grid-map data from 1974 til 1978 (Wink 1980) into account, a continuous and significant trend could be established for 14 species (increase:Accipiter nisus, A. gentilis, Turdus philomelos; decrease:Oenanthe oenanthe, Perdix perdix, Motacilla flava, Emberiza calandra, Delichon urbica, Passer montanus, Phasianus colchicus, Sylvia communis, Acanthis cannabina, Alauda arvensis, Troglodytes troglodytes). This study shows that grid-mapping may provide valuable data on population trends of breeding birds.相似文献
118.
The first detailed morphological study of larvae, pupae and adults of a species of the hooded beetles (Coleoptera: Corylophidae) – Sericoderus lateralis – is presented. Histological sectioning, scanning and transmission electron microscopy, laser confocal microscopy and 3D-computer reconstruction were used. For the first time we report that according to the morphometric data of S. lateralis, at least some corylophid beetles have three larval stages. A phylogenetic position of Corylophidae within a cucujoid-cleroid clade is confirmed, and also the placement of Sericoderini within a corlyophid subgroup, which does not include Periptycinae and Foadiini. The larvae of Sericoderus are mainly characterized by plesiomorphic features compared to those of other corylophid tribes, notably Peltinodini and Rypobiini. Morphological and developmental consequences of miniaturization are discussed. Corylophid beetles display much less specific and far-reaching morphological consequences of miniaturization compared to Ptiliidae. We report the presence of unique modifications in the neural system not shared with any other insects, such as a distinctly asymmetric supraoesophageal ganglion in first instar larva, and a total displacement of the brain to the thorax in the adult stage. A highly unusual feature of the digestive tract is the sclerotised, V-shaped ventral wall of the pharynx. Developmental and size dependent changes in the relative volume of different organs are addressed. All organ systems change allometrically in the development of S. lateralis. Allometric trends in the volume of organs confirm that the factors limiting miniaturization are the size of the neural system, associated with the number and size of neurons (most critical for first instar larva), the mass of the skeleton, the egg size, and consequently the volume of the reproductive system (for free-living insects). 相似文献
119.