全文获取类型
收费全文 | 4099篇 |
免费 | 332篇 |
出版年
2023年 | 19篇 |
2022年 | 63篇 |
2021年 | 103篇 |
2020年 | 56篇 |
2019年 | 66篇 |
2018年 | 88篇 |
2017年 | 78篇 |
2016年 | 132篇 |
2015年 | 229篇 |
2014年 | 231篇 |
2013年 | 268篇 |
2012年 | 388篇 |
2011年 | 406篇 |
2010年 | 234篇 |
2009年 | 208篇 |
2008年 | 228篇 |
2007年 | 218篇 |
2006年 | 180篇 |
2005年 | 172篇 |
2004年 | 186篇 |
2003年 | 148篇 |
2002年 | 124篇 |
2001年 | 23篇 |
2000年 | 18篇 |
1999年 | 26篇 |
1998年 | 36篇 |
1997年 | 34篇 |
1996年 | 22篇 |
1995年 | 21篇 |
1994年 | 17篇 |
1993年 | 31篇 |
1992年 | 15篇 |
1991年 | 21篇 |
1990年 | 17篇 |
1989年 | 18篇 |
1988年 | 24篇 |
1987年 | 12篇 |
1986年 | 19篇 |
1985年 | 15篇 |
1984年 | 22篇 |
1983年 | 12篇 |
1982年 | 26篇 |
1981年 | 18篇 |
1980年 | 11篇 |
1979年 | 13篇 |
1977年 | 18篇 |
1976年 | 11篇 |
1974年 | 11篇 |
1973年 | 15篇 |
1972年 | 11篇 |
排序方式: 共有4431条查询结果,搜索用时 15 毫秒
1.
2.
3.
4.
AbstractThe Argolis Peninsula covers the north-eastern part of Peloponnisos and is surrounded by the Gulf of Argosaronic. The area hosts three species of the genus Fritillaria: F. graeca, F. rhodocanakis and F. spetsiotica. Fritillaria graeca is a Greek endemic taxon and its distribution includes Peloponnisos, C & E Sterea Ellas, C Evia and in proximity to Sterea Ellas, Salamis and Kea islands, while the stenoendemic F. rhodocanakis and F. spetsiotica are mainly found on Idra and Spetses islands respectively. The last two taxa are included in the Red Data Book of Rare and Threatened Plants of Greece, while F. rhodocanakis is also included in the IUCN Red List. Hybridization among them is a common phenomenon in the areas where they coexist, leading to an array of morphologically and karyologically intermediate forms. The current study presents the taxa’s karyomorphometric analysis for the first time and reveals hybrids’ cytological variety, including differences in marker chromosomes, polyploidy and the number of B-chromosomes. 相似文献
5.
6.
Detergent soluble extracts of differentiated regions of the Porphyra perforata J. Ag. thallus (holdfast, rhizoidal, vegetative and reproductive cells) were fractionated on sodium dodecyl sulfate polyacrylamide gels. Glycoproteins were identified by their lectin affinity. Extracts from all areas of the thallus contained glycoproteins, but the staining patterns were different for each region with each of the lectins tested: concanavalin A, Ulex europeaus agglutinin, Ricinus communis agglutinin, soybean agglutinin and peanut agglutinin. These data indicate that the morphologically distinct regions of the thallus also differ biochemically. Analysis of the lectin blots revealed the presence of tissue-specific glycoproteins in the five thallus areas. Such unique glycoproteins could be used as markers of differentiation in this species. 相似文献
7.
8.
Down's syndrome: abnormal neuromuscular junction in tongue of transgenic mice with elevated levels of human Cu/Zn-superoxide dismutase 总被引:17,自引:0,他引:17
To investigate the possible involvement of Cu/Zn-superoxide dismutase (CuZnSOD) gene dosage in the neuropathological symptoms of Down's syndrome, we analyzed the tongue muscle of transgenic mice that express elevated levels of human CuZnSOD. The tongue neuromuscular junctions (NMJ) in the transgenic animals exhibited significant pathological changes, namely, withdrawal and destruction of some terminal axons and the development of multiple small terminals. The ratio of terminal axon area to postsynaptic membrane decreased, and secondary folds were often complex and hyperplastic. The morphological changes in the transgenic NMJ were similar to those previously seen in muscles of aging mice and rats as well as in tongue muscle of patients with Down's syndrome. The findings suggest that CuZnSOD gene dosage is involved in the pathological abnormalities of tongue NMJ observed in Down's syndrome patients. 相似文献
9.
The15N abundance of plants usually closely reflects the15N abundance of their major immediate N source(s); plant-available soil N in the case of non-N2-fixing plants and atmospheric N2 in the case of N2 fixing plants. The15N abundance values of these sources are usually sufficiently different from each other that a significant and systematic difference in the15N abundance between the two kinds of plants can be detected. This difference provides the basis for the natural15N abundance method of estimating the relative contribution of atmospheric N2 to N2-fixing plants growing in natural and agricultural settings. The natural15N abundance method has certain advantages over more conventional methods, particularly in natural ecosystems, since disturbance of the system is not required and the measurements may be made on samples dried in the field. This method has been tested mainly with legumes in agricultural settings. The tests have demonstrated the validity of this method of arriving at semi-quantitative estimates of biological N2-fixation in these settings. More limited tests and applications have been made for legumes in natural ecosystems. An understanding of the limits and utility of this method in these systems is beginning to emerge. Examples of systematic measurements of differences in15N abundance between non-legume N2-fixing systems and neighbouring non-fixing systems are more unusual. In principle, application of the method to estimate N2-fixation by nodulated non-legumes, using the natural15N abundance method, is as feasible as estimating N2-fixation by legumes. Most of the studies involving N2-fixing non-legumes are with this type of system (e.g., Ceanothus, Chamabatia, Eleagnus, Alnus, Myrica, and so forth). Resuls of these studies are described. Applicability for associative N2-fixation is an empirical question, the answer to which probably depends upon the degree to which fixed N goes predominantly to the plant rather than to the soil N pool. The natural15N abundance method is probably not well suited to assessing the contribution of N2-fixation by free-living microorganisms in their natural habitat, particularly soil microorganisms.This work was supported in part by subcontracts under grants from the US National Science Foundation (DEB79-21971 and BSR821618) 相似文献
10.
v-myc alters the response of a cloned mouse mammary epithelial cell line to lactogenic hormones 总被引:5,自引:0,他引:5
R K Ball A Ziemiecki C A Sch?nenberger E Reichmann S M Redmond B Groner 《Molecular endocrinology (Baltimore, Md.)》1988,2(2):133-142
Several oncogenes have now been implicated in mammary carcinogenesis. We investigated the phenotypic effects of expressing three representative oncogenes in mammary epithelial cells. v-myc (coding for a nuclear protein), v-Ha-ras (a G-protein homologue) and v-fgr (a tyrosine kinase) genes were introduced into the nontumorigenic clone 14 of the mouse mammary epithelial cell line COMMA-1D. Their effects upon growth and differentiation were determined. Anchorage-independent growth was induced by all three oncogenes with low efficiency. v-Ha-ras and v-fgr induced tumorigenicity in nude mice. The effect of oncogenes upon parameters unique to mammary epithelial cells in vitro was assayed. Both v-myc and v-fgr abolished the ability of clone 14 to grow as three-dimensional branching structures in hydrated collagen gel. v-fgr completely and v-myc partially inhibited the expression of the epithelium specific cytokeratins. Clone 14 can be induced to produce the beta-casein milk protein by the combination of the lactogenic hormones, dexamethasone, insulin, and PRL. Introduction of v-myc into clone 14 cells resulted in an estimated 50-fold increased induction of beta-casein protein and at least a 60-fold increase in beta-casein mRNA. The number of cells stained with anti-beta casein antibodies also showed a 10-fold increase after v-myc introduction. This still required the synergistic action of all three lactogenic hormones. Thus v-myc can alter the normal response of mammary epithelial cells to lactogenic hormones. 相似文献