全文获取类型
收费全文 | 75980篇 |
免费 | 7052篇 |
国内免费 | 30篇 |
专业分类
83062篇 |
出版年
2023年 | 370篇 |
2022年 | 793篇 |
2021年 | 1515篇 |
2020年 | 927篇 |
2019年 | 1195篇 |
2018年 | 1425篇 |
2017年 | 1203篇 |
2016年 | 2108篇 |
2015年 | 3471篇 |
2014年 | 3811篇 |
2013年 | 4437篇 |
2012年 | 5778篇 |
2011年 | 5458篇 |
2010年 | 3472篇 |
2009年 | 3090篇 |
2008年 | 4364篇 |
2007年 | 4397篇 |
2006年 | 4108篇 |
2005年 | 3867篇 |
2004年 | 3772篇 |
2003年 | 3359篇 |
2002年 | 3312篇 |
2001年 | 975篇 |
2000年 | 802篇 |
1999年 | 923篇 |
1998年 | 977篇 |
1997年 | 679篇 |
1996年 | 600篇 |
1995年 | 534篇 |
1994年 | 557篇 |
1993年 | 540篇 |
1992年 | 671篇 |
1991年 | 535篇 |
1990年 | 523篇 |
1989年 | 537篇 |
1988年 | 477篇 |
1987年 | 431篇 |
1986年 | 421篇 |
1985年 | 405篇 |
1984年 | 463篇 |
1983年 | 420篇 |
1982年 | 437篇 |
1981年 | 400篇 |
1980年 | 415篇 |
1979年 | 328篇 |
1978年 | 311篇 |
1977年 | 271篇 |
1976年 | 274篇 |
1974年 | 246篇 |
1973年 | 217篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
101.
102.
103.
W. Thelwall Thomas 《BMJ (Clinical research ed.)》1899,1(2005):1329-1330
104.
105.
106.
107.
It is an accepted fact that fusion between the coelomic cavities and the primary body cavity occurs during development in the Arthropoda. However, such a fusion is much disputed in the Onychophora. In order to clarify this subject, the fate of embryonic coelomic cavities has been studied in an onychophoran. Ultrastructural investigations in this paper provide evidence that embryonic coelomic cavities fuse with spaces of the primary body cavity in Epiperipatus biolleyi. During embryogenesis, the somatic and splanchnic portions of the mesoderm separate and the former coelomic linings are transformed into mesenchymatic tissue. The resulting body cavity therefore represents a mixture of primary and secondary (coelomic) body cavities, i.e. the ‘mixocoel’. The nephridial anlage is already present, when the ‘mixocoel’ is formed, although there is no trace of a sacculus yet. The lumen of the nephridial anlage, thus, communicates with the newly formed ‘mixocoel’. Accordingly, the lumen of the nephridial sacculus cannot be regarded as a kind of ‘persisting coelomic cavity’ in E. biolleyi. Our findings support the hypothesis that the ‘mixocoel’ was already present in the common stem species of the Onychophora and Euarthropoda. 相似文献
108.
109.
Aim This study aims to document the floristic changes that occurred in Iceland between 15 and 6 Ma and to establish the dispersal mechanisms for the plant taxa encountered. Using changing patterns of dispersal, two factors controlling floristic changes are tested. Possible factors are (1) climate change, and (2) the changing biogeography of Iceland over the time interval studied; that is, the presence or absence of a Miocene North Atlantic Land Bridge. Location The North Atlantic. Methods Species lists of fossil plants from Iceland in the time period 15 to 6 Ma were compiled using published data and new data. Closest living analogues were used to establish dispersal properties for the fossil taxa. Dispersal mechanisms of fossil plants were then used to reconstruct how Iceland was colonized during various periods. Results Miocene floras of Iceland (15–6 Ma) show relatively high floristic turnover from the oldest floras towards the youngest; and few taxa from the oldest floras persist in the younger floras. The frequencies of the various dispersal mechanisms seen in the 15‐Ma floras are quite different from those recorded in the 6‐Ma floras, and there is a gradual change in the prevailing mode of dispersal from short‐distance anemochory and dyschory to long‐distance anemochory. Two mechanisms can be used to explain changing floral composition: (1) climate change, and (2) the interaction between the dispersal mechanisms of plants and the increasing isolation of proto‐Iceland during the Miocene. Main conclusions Dispersal mechanisms can be used to extract palaeogeographic signals from fossil floras. The composition of floras and dispersal mechanisms indicate that Iceland was connected both to Greenland and to Europe in the early Middle Miocene, allowing transcontinental migration. The change in prevalence of dispersal modes from 15 to 6 Ma appears to reflect the break‐up of a land bridge and the increasing isolation of Iceland after 12 Ma. Concurrent gradual cooling and isolation caused changes in species composition. Specifically, the widening of the North Atlantic Ocean prevented taxa with limited dispersal capability from colonizing Iceland, while climate cooling led to the extinction of thermophilous taxa. 相似文献
110.
Gametic chromosome numbers are reported for 27 collections representing the four species of the Lobelia tupa complex (Campanulaceae, Lobelioideae) in Chile; all are n = 21. This represents the first report of chromosome numbers for L. bridgesii Hook. & Arn., L. excelsa Bonpl., and L. polyphylla Hook. & Arn., and confirms previous reports of this number in L. tupa L. As the basic chromosome number of Lobelioideae is x = 7, these species are interpreted as hexaploids. Higher polyploids are extremely rare among Lobelioideae; most of those previously reported have been either sporadic individuals or populations within an otherwise diploid or tetraploid species, or occasional species within an otherwise diploid and tetraploid lineage. This is the first report of an entire complex of lobelioid species that is uniformly hexaploid. This suggests that the Chilean endemics are relatively derived within Lobelia, and offers some support for the monophyly of the complex. 相似文献