全文获取类型
收费全文 | 19414篇 |
免费 | 45篇 |
国内免费 | 48篇 |
专业分类
系统科学 | 144篇 |
丛书文集 | 285篇 |
教育与普及 | 36篇 |
理论与方法论 | 76篇 |
现状及发展 | 9321篇 |
研究方法 | 781篇 |
综合类 | 8668篇 |
自然研究 | 196篇 |
出版年
2012年 | 220篇 |
2011年 | 426篇 |
2009年 | 88篇 |
2008年 | 268篇 |
2007年 | 293篇 |
2006年 | 326篇 |
2005年 | 315篇 |
2004年 | 434篇 |
2003年 | 304篇 |
2002年 | 301篇 |
2001年 | 553篇 |
2000年 | 506篇 |
1999年 | 367篇 |
1994年 | 87篇 |
1992年 | 350篇 |
1991年 | 255篇 |
1990年 | 274篇 |
1989年 | 265篇 |
1988年 | 222篇 |
1987年 | 263篇 |
1986年 | 310篇 |
1985年 | 371篇 |
1984年 | 284篇 |
1983年 | 249篇 |
1982年 | 233篇 |
1981年 | 239篇 |
1980年 | 259篇 |
1979年 | 648篇 |
1978年 | 530篇 |
1977年 | 504篇 |
1976年 | 388篇 |
1975年 | 449篇 |
1974年 | 651篇 |
1973年 | 523篇 |
1972年 | 538篇 |
1971年 | 632篇 |
1970年 | 799篇 |
1969年 | 624篇 |
1968年 | 625篇 |
1967年 | 622篇 |
1966年 | 523篇 |
1965年 | 436篇 |
1964年 | 146篇 |
1959年 | 214篇 |
1958年 | 373篇 |
1957年 | 280篇 |
1956年 | 226篇 |
1955年 | 218篇 |
1954年 | 213篇 |
1948年 | 156篇 |
排序方式: 共有10000条查询结果,搜索用时 375 毫秒
211.
212.
Wilson J. E. M. Costa Axel M. Katz José Leonardo O. Mattos Pedro F. Amorim Beatrizz O. Mesquita Paulo J. Vilardo 《Journal of Natural History》2019,53(47-48):2905-2928
ABSTRACT This study is primarily directed to the most poorly known species of the genus Trichomycterus, comprising five nominal species (T. florensis, T. immaculatus, T. nigricans, T. paquequerensis and T. santaeritae) endemic to south-eastern Brazil. One of them, T. nigricans, is the type species of the genus, involved in taxonomic problems for over 150 years. A detailed historical review, accompanied by examination of type specimens and recent collections, revealed that the correct type locality of T. nigricans is in the vicinity of Rio de Janeiro, not Santa Catarina as commonly appears in the literature; specimens previously misidentified as T. nigricans from Santa Catarina belong to a possibly undescribed species of the genus Cambeva; T. paquequerensis is a synonym of T. immaculatus, and T. florensis is a synonym of T. santaeritae; and the hypothesis that T. santaeritae is closely related to the Amazon Sarcoglanidinae is refuted. The three valid species are redescribed. These species are members of a clade also including T. caipora that is highly supported by molecular data, diagnosed by a pronounced posterior maxillary process and caudal fin emarginate at least in larger specimens. A subclade comprising T. caipora, T. nigricans and T. santaeritae is diagnosed by a long maxilla and a bifid anterior extremity of hypobranchial 3. 相似文献
213.
Water mites Limnochares aquatica (L., 1758) during maintenance in the laboratory for a long period of time in constant conditions periodically produced certain whitish flocculent material consisting of long rigid unbranched tube-like threads 1.3 ± 0.3 µm in diameter crossing freely. These threads were studied using light-optical as well as transmission electron microscopical and scanning electron microscopical methods. Microbiological staining was also applied to the threads to exclude their bacterial or fungal origin. The thread wall is built of fine fibrils arranged at different angles to the long axis of threads that is reflected in a certain stratification of the wall. Threads are mostly hollow or may contain electron-dense homogeneous material. No cell components are present in the thread composition. Numerous dermal glands with their small slit-like orifice scattered throughout the mite body surface are thought to produce these threads. Most probably the thread formation is a reaction of mites to stress under laboratory conditions, and this is expected to be a type of defensive reaction. 相似文献
214.
215.
216.
Systemic Practice and Action Research - This paper explores the usefulness of rich pictures as a method in Systemic Lean Intervention (SLI) process. It combines Lean and Systems Thinking analytical... 相似文献
217.
218.
219.
Zhitao Xu X. G. Ming Wenyan Song Lina He Miao Li 《Systemic Practice and Action Research》2014,27(2):141-164
Quite often over the last decade, heavy equipment manufacturers are trying to improve their project management performance with the use of effective project management methods. However, the challenges encountered aren’t isolated problems and a systemic approach is required. This paper exemplifies how a systemic approach can be applied to heavy equipment manufacturing project (HEMP) management, through of a multifaceted vision. The findings present in the paper are based on a two-year case study in a metallurgic equipment manufacturing enterprise in China. Based on the analysis of HEMP management problems and the contributions on systemic approach application in project management, the paper enriches existing theory by: (1) interpreting the HEMP management system with the top-down disassembly method; (2) developing the collaborative project management approach with a systems view highlighting the project characteristics management and inter-departmental collaboration; (3) making suggestions for systemic approach application in project management. For practitioners and researchers, the findings are particularly meaningful for manufacturing project management and systemic approach application. 相似文献
220.
Carol D. Curtis Reema B. Davis Kyle G. Ingram Courtney T. Griffin 《Cellular and molecular life sciences : CMLS》2012,69(23):3921-3931
Vascular development is a dynamic process that relies on the coordinated expression of numerous genes, but the factors that regulate gene expression during blood vessel development are not well defined. ATP-dependent chromatin-remodeling complexes are gaining attention for their specific temporal and spatial effects on gene expression during vascular development. Genetic mutations in chromatin-remodeling complex subunits are revealing roles for the complexes in vascular signaling pathways at discrete developmental time points. Phenotypic analysis of these models at various stages of vascular development will continue to expand our understanding of how chromatin remodeling impacts new blood vessel growth. Such research could also provide novel therapeutic targets for the treatment of vascular pathologies. 相似文献