全文获取类型
收费全文 | 60804篇 |
免费 | 1726篇 |
国内免费 | 2167篇 |
出版年
2023年 | 414篇 |
2022年 | 604篇 |
2021年 | 1998篇 |
2020年 | 794篇 |
2019年 | 1036篇 |
2018年 | 865篇 |
2017年 | 598篇 |
2016年 | 1176篇 |
2015年 | 3034篇 |
2014年 | 6266篇 |
2013年 | 5326篇 |
2012年 | 4561篇 |
2011年 | 5236篇 |
2010年 | 3689篇 |
2009年 | 3087篇 |
2008年 | 3160篇 |
2007年 | 3459篇 |
2006年 | 2116篇 |
2005年 | 1792篇 |
2004年 | 1015篇 |
2003年 | 797篇 |
2002年 | 711篇 |
2001年 | 496篇 |
2000年 | 448篇 |
1999年 | 495篇 |
1998年 | 414篇 |
1997年 | 340篇 |
1996年 | 394篇 |
1995年 | 505篇 |
1994年 | 436篇 |
1993年 | 480篇 |
1992年 | 434篇 |
1991年 | 458篇 |
1990年 | 397篇 |
1989年 | 405篇 |
1988年 | 405篇 |
1987年 | 336篇 |
1986年 | 295篇 |
1985年 | 513篇 |
1984年 | 813篇 |
1983年 | 522篇 |
1982年 | 703篇 |
1981年 | 695篇 |
1980年 | 503篇 |
1979年 | 510篇 |
1978年 | 311篇 |
1977年 | 333篇 |
1976年 | 302篇 |
1974年 | 224篇 |
1973年 | 227篇 |
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
121.
N. Fujii N. Tomaru K. Okuyama T. Koike T. Mikami K. Ueda 《Plant Systematics and Evolution》2002,232(1-2):21-33
CpDNA variation in Japanese beech, Fagus crenata (Fagaceae), was studied in 45 populations distributed throughout the species' range. Two cpDNA regions were sequenced: the
non-coding region between the trnL (UAA) 5′exon and trnF (GAA), and the trnK region (including matK). Thirteen distinct cpDNA haplotypes were recognized and each haplotype was found to be geographically structured. Two major
clades (I and II+III) were revealed in phylogenetic analyses among the haplotypes using F. sylvatica as an outgroup. The haplotypes of Clade I were distributed mainly along the Japan Sea side of the Japanese Archipelago, while
those of Clade II+III occurred chiefly along the Pacific Ocean side. Consequently, the distribution of the two major cpDNA
clades suggests that there were two migration routes in the history of F. crenata; one along the Japan Sea and the other along the Pacific Ocean side of the Japanese Islands.
Received March 19, 2001 Accepted November 22, 2001 相似文献
122.
The assembly of high voltage-activated Ca2+ channels with different β subunits influences channel properties and possibly subcellular targeting. We studied β subunit expression in the somata and axon terminals of the magnocellular neurosecretory cells, which are located in the supraoptic nucleus (SON) and neurohypophysis, respectively. Antibodies directed against the 4 CaVβ subunits (CaVβ1-CaVβ4) were used for immunoblots and for immunostaining of slices of these two tissues. We found that all 4 β subunits are expressed in both locations, but that CaVβ2 had the highest relative expression in the neurohypophysis. These data suggest that the CaVβ2 subunit is selectively targeted to axon terminals and may play a role in targeting and/or regulating the properties of Ca2+ channels. 相似文献
123.
124.
Tomer J. Czaczkes Jürgen Heinze 《Proceedings. Biological sciences / The Royal Society》2015,282(1810)
Animals must contend with an ever-changing environment. Social animals, especially eusocial insects such as ants and bees, rely heavily on communication for their success. However, in a changing environment, communicated information can become rapidly outdated. This is a particular problem for pheromone trail using ants, as once deposited pheromones cannot be removed. Here, we study the response of ant foragers to an environmental change. Ants were trained to one feeder location, and the feeder was then moved to a different location. We found that ants responded to an environmental change by strongly upregulating pheromone deposition immediately after experiencing the change. This may help maintain the colony''s foraging flexibility, and allow multiple food locations to be exploited simultaneously. Our treatment also caused uncertainty in the foragers, by making their memories less reliable. Ants which had made an error but eventually found the food source upregulated pheromone deposition when returning to the nest. Intriguingly, ants on their way towards the food source downregulated pheromone deposition if they were going to make an error. This may suggest that individual ants can measure the reliability of their own memories and respond appropriately. 相似文献
125.
126.
Aim Species capable of vigorous growth under a wide range of environmental conditions should have a higher chance of becoming invasive after introduction into new regions. High performance across environments can be achieved either by constitutively expressed traits that allow for high resource uptake under different environmental conditions or by adaptive plasticity of traits. Here we test whether invasive and non‐invasive species differ in presumably adaptive plasticity. Location Europe (for native species); the rest of the world and North America in particular (for alien species). Methods We selected 14 congeneric pairs of European herbaceous species that have all been introduced elsewhere. One species of each pair is highly invasive elsewhere in the world, particularly so in North America, whereas the other species has not become invasive or has spread only to a limited degree. We grew native plant material of the 28 species under shaded and non‐shaded conditions in a common garden experiment, and measured biomass production and morphological traits that are frequently related to shade tolerance and avoidance. Results Invasive species had higher shoot–root ratios, tended to have longer leaf‐blades, and produced more biomass than congeneric non‐invasive species both under shaded and non‐shaded conditions. Plants responded to shading by increasing shoot–root ratios and specific leaf area. Surprisingly, these shade‐induced responses, which are widely considered to be adaptive, did not differ between invasive and non‐invasive species. Main conclusions We conclude that high biomass production across different light environments pre‐adapts species to become invasive, and that this is not mediated by plasticities of the morphological traits that we measured. 相似文献
127.
A number of methods of construction of partially balanced incomplete block designs with nested rows and columns are developed and new balanced incomplete block designs with nested rows and columns are obtained as a by-product. 相似文献
128.
The course of the cellular response in the liver to nonembryonated Capillaria hepatica (Bancroft, 1893) eggs given by intravenous injection into the portal circulation of unsensitized and sensitized mice was studied qualitatively and quantitatively. A gradual infiltration of predominantly mononuclear cells occurred around the eggs in the liver, leading to the formation of distinct granulomatous lesions characterized by macrophages and lymphocytes. This was followed by an infiltration of eosinophils. Previous intraperitoneal sensitization led to an earlier and an enhanced reaction to an intravenous challenge with eggs. Specificity of the cellular response was suggested by the lack of an enhanced reaction to presensitization with eggs of a closely related species, Trichuris muris. These studies indicate that granuloma formation to C. hepatica eggs has an immunological basis and furthermore the cell composition of the granuloma would suggest that a cell-mediated component may be involved as part of the specific response. 相似文献
129.
K Folkers T Kubiak H Stepien N Sakura 《Biochemical and biophysical research communications》1980,97(2):601-606
A new approach to the study of the molecular arrangements of proteins in membranes is described. Irradiation with visible light of native erythrocytes or washed erythrocyte membranes suspended in buffers containing a) riboflavin, fluorescein or fluorescein coupled to dextran and b) 3H-labelled tryptophan resulted in incorporation of radioactivity into the membrane proteins. Polyacrylamide gel electrophoresis of solubilized membranes followed by radioactivity measurements of the separated membrane proteins revealed that in native erythrocytes the protein components known to be located at the exterior cell surface, Band 3 and the major sialoglycoproteins became specifically labelled, whereas in washed lysed cells all of the major membrane proteins were labelled. 相似文献
130.
Dr. J. D. Newstead 《Cell and tissue research》1987,250(2):401-411
Summary The medullary pyramid of renculi in kidneys of ringed seals (Phoca hispida) is enclosed by a basket composed of ribbons of stromal tissue continuous with the wall of the calyx. Branched smooth muscle cells with well-developed Golgi complexes and rough endoplasmic reticulum and only an incomplete external lamina are the principal cells in sites near the origin of the ribbons from the calycal wall. Deeper in the corticomedullary junctional region, smooth muscle is progressively replaced with stellate or spindle-shaped cells exhibiting structural characteristics intermediate between those of fibroblasts and smooth muscle fibers. These myofibroblast-like cells contain arrays of parallel microfilaments 6–8 nm thick with associated focal densities and subplasmalemmal dense plaques, caveolae, elongate, often deeply wrinkled nuclei, and well-developed Golgi complexes and rough endoplasmic reticulum. Material resembling external lamina is associated with parts of the surfaces of most myofibroblast-like cells and intermediate junctions are present. Fibroblasts lacking arrays of parallel microfilaments are a minority at any level in the stromal ribbons. Interstitial cells in the vicinity of the corticomedullary junction show similar myofibroblast-like characteristics. The smooth muscle and myofibroblast-like cells presumably assist expression of urine from the papilla and calyx, and possibly participate as pacemakers for the urinary tract. 相似文献