全文获取类型
收费全文 | 5495篇 |
免费 | 668篇 |
国内免费 | 6篇 |
专业分类
6169篇 |
出版年
2022年 | 40篇 |
2021年 | 92篇 |
2020年 | 61篇 |
2019年 | 63篇 |
2018年 | 71篇 |
2017年 | 82篇 |
2016年 | 109篇 |
2015年 | 183篇 |
2014年 | 243篇 |
2013年 | 305篇 |
2012年 | 341篇 |
2011年 | 359篇 |
2010年 | 241篇 |
2009年 | 219篇 |
2008年 | 288篇 |
2007年 | 325篇 |
2006年 | 290篇 |
2005年 | 317篇 |
2004年 | 262篇 |
2003年 | 282篇 |
2002年 | 244篇 |
2001年 | 124篇 |
2000年 | 106篇 |
1999年 | 115篇 |
1998年 | 85篇 |
1997年 | 60篇 |
1996年 | 62篇 |
1995年 | 64篇 |
1994年 | 52篇 |
1993年 | 41篇 |
1992年 | 75篇 |
1991年 | 66篇 |
1990年 | 54篇 |
1989年 | 59篇 |
1988年 | 55篇 |
1987年 | 49篇 |
1986年 | 38篇 |
1985年 | 55篇 |
1984年 | 44篇 |
1983年 | 33篇 |
1982年 | 45篇 |
1981年 | 37篇 |
1979年 | 40篇 |
1978年 | 35篇 |
1976年 | 28篇 |
1974年 | 27篇 |
1973年 | 30篇 |
1971年 | 25篇 |
1970年 | 30篇 |
1968年 | 27篇 |
排序方式: 共有6169条查询结果,搜索用时 0 毫秒
181.
Hahn WJ 《Molecular phylogenetics and evolution》2002,23(2):189-204
A phylogenetic analysis of the Arecoid Line (sensu Moore) of palms was conducted using 7 kb of coding and noncoding plastid DNA sequence data. Recovered maximum-parsimony and maximum-likelihood phylogenies support monophyly for the Arecoid Line relative to the rest of the family but paraphyly for subfamily Arecoideae and polyphyly for subfamily Ceroxyloideae (sensu Dransfield and Uhl). Tribes Cocoeae, Geonomeae, Hyophorbeae, and Iriarteae and subfamily Phytelephantoideae were identified as monophyletic as were subfamily Phytelephantoideae + Ravenea (tribe Ceroxyleae of Ceroxyloideae), Podococcus (tribe Podococceae of Arecoideae) + Pseudophoenix (tribe Cyclospatheae of Ceroxyloideae), Reinhardtia (tribe Malortieinae) + tribe Cocoeae (both of Arecoideae), and a clade containing all IndoPacific pseudomonomerous genera of tribe Areceae (Arecoideae). A few taxa show spurious resolution with noncoding plastid DNA data but noncoding data are generally congruent with protein-coding data. Biogeographic interpretation suggests a Gondwanan origin for the Arecoid Line with several lineages found on more than one fragment of the former supercontinent and primary diversification in these groups possibly due to continental breakup vicariance. Three groups involving Cocos, Orania, and the IndoPacific clade demonstrate independent dispersals into the IndoPacific region from a Gondwanan origin. 相似文献
182.
183.
Helicobacter pylori infection and gastrin and cyclooxygenase expression in gastric and colorectal malignancies 总被引:6,自引:0,他引:6
Helicobacter pylori, infecting more than 50% of the world population, results in gastritis, usually located in the antral portion of the stomach, accompanied by hypergastrinemia, the key factor in gastric and colorectal carcinogenesis. Excessive mucosal cell proliferation for many years may eventually result in gastric atrophy, cell mutation and transformation of gastric mucosal cells into gastrin-producing cells, which also express gastrin receptors serving to stimulate cell proliferation and tumor growth. These processes may be completed by the expression of cyclooxygenase-2 (COX-2) as an inflammation enzyme to release excessive amounts of PGE(2), leading to further proliferation, reduction in apoptosis, angiogenesis and tumor growth. H. pylori eradication results in complete regression of MALT lymphoma and subsequent normalisation of excessive gastrin release and COX-2 expression. Reduction of gastrin by active immunisation (gastrimmune), blocking of gastrin receptors with specific blockers and suppression of COX-2 might be helpful in inhibiting tumor growth and invasion. 相似文献
184.
Summary
In vitro plantlets of Phalaenopsis ‘Happy Valentine’, Neofinetia falcate Hu, Cymbidium kanran Makino, and Cymbidium goeringii Reichb. f. were grown under photoautotrophic [high photosynthetic photon flux (PPF), high CO2 concentration, and increased number of air exchanges] and heterotrophic (low PPF, low CO2 concentration, no air exchanges) culture conditions. After 40 d of culture, a significant difference in plantlet growth was
observed between the two cultures. Total fresh and dry mass were on average 1.5 times greater in photoautotrophic culture
than in heterotrophic culture. Higher net photosynthetic rates were also observed for Phalaenopsis in photoautotrophic culture. In photoautotrophic culture, little difference was observed in air temperature between the inside
and outside of the culture vessel, whereas in heterotrophic culture, air temperature inside the culture vessel was 1–2°C higher
than that outside the culture vessel. Relative humidity inside the culture vessel was remarkably different between the two
cultures: 83–85% in photoautotrophic culture and 97–99% in heterotrophic culture. These results indicated that growth and
net photosynthetic rate of in vitro orchid plantlets were susceptible to the culture environments such as PPF, CO2 concentration, relative humidity (RH), and the number of air exchanges, which would allow a more efficient micropropagation
system for these orchid plants. 相似文献
185.
186.
Gajewski Konrad Kriesche Björn Chaput Michelle A. Kulik Rafal Schmidt Volker 《Vegetation History and Archaeobotany》2019,28(6):635-647
Vegetation History and Archaeobotany - Between the initial colonization of North America and the European settlement period, Indigenous American land use practices shaped North American landscapes... 相似文献
187.
Low-temperature accumulation of alcohol dehydrogenase-1 mRNA and protein activity in maize and rice seedlings 总被引:6,自引:5,他引:6 下载免费PDF全文
Low-temperature stress was shown to cause a rapid increase in steady-state levels of alcohol dehydrogenase-1 message (Adh1) and protein activity (ADH1) in maize (Zea mays) (B37N, A188) and rice (Oryza sativa) (Taipei 309, Calmochi 101) seedlings. Maize roots and rice shoots and roots from 7-day seedlings shifted to low temperature (10°C) contained as much as 15-fold more Adh1 mRNA and 8-fold more ADH1 protein activity than the corresponding tissues from untreated seedlings. Time-course studies showed that these tissues accumulated Adh1 mRNA and ADH1 activity severalfold within 4- to 8-hour, levels plateaued within 20 to 24 hours, and remained elevated at 4 days of cold treatment. Within 24 hours of returning cold-stressed seedlings to ambient temperature, Adh1 mRNA and ADH1 activity decreased to pretreatment levels. Histochemical staining of maize and rice tissue imprints showed that ADH activity was enhanced along the lengths of cold-stressed maize primary roots and rice roots, and along the stems and leaves of rice shoots. Our observations suggest that short-term cold stress induces Adh1 gene expression in certain plant tissues, which, reminiscient of the anaerobic response, may reflect a fundamental shift in energy metabolism to ensure tissue survival during the stress period. 相似文献
188.
We previously reported that expression of human immunodeficiency virus type 1 strain NL4-3 (HIV-1(NL4-3))vpr causes cells to arrest in the G2 phase of the cell cycle. We examined the induction of cell cycle arrest by other HIV-1 isolates and by primary lentiviruses other than HIV-1. We demonstrate that the vpr genes from tissue culture-adapted or primary isolates of HIV-1 are capable of inducing G2 arrest. In addition, we demonstrate that induction of cell cycle arrest is a conserved function of members of two other groups of primate lentiviruses, HIV-2/simian immunodeficiency virus strain sm (SIVsm)/SIVmac and SIVagm. vpr from HIV-1, HIV-2, and SIVmac induced cell cycle arrest when transfected in human (HeLa) and monkey (CV-1) cells. vpx from HIV-2 and SIVmac did not induce detectable cell cycle arrest in either cell type, and SIVagm vpx was capable of inducing arrest in CV-1 but not HeLa cells. These results indicate that induction of cell cycle perturbation is a general property of lentiviruses that infect primates. The conservation of this viral function throughout evolution suggests that it plays a key role in virus-host relationships, and elucidation of its mechanism may reveal important clues about pathology induced by primary lentiviruses. 相似文献
189.
Developmental and Tissue-Specific Structural Alterations of the Cell-Wall Polysaccharides of Arabidopsis thaliana Roots 总被引:6,自引:1,他引:6 下载免费PDF全文
Freshour G Clay RP Fuller MS Albersheim P Darvill AG Hahn MG 《Plant physiology》1996,110(4):1413-1429
The plant cell wall is a dynamic structure that plays important roles in growth and development and in the interactions of plants with their environment and other organisms. We have used monoclonal antibodies that recognize different carbohydrate epitopes present in plant cell-wall polysaccharides to locate these epitopes in roots of developing Arabidopsis thaliana seedlings. An epitope in the pectic polysaccharide rhamnogalacturonan I is observed in the walls of epidermal and cortical cells in mature parts of the root. This epitope is inserted into the walls in a developmentally regulated manner. Initially, the epitope is observed in atrichoblasts and later appears in trichoblasts and simultaneously in cortical cells. A terminal [alpha]-fucosyl-containing epitope is present in almost all of the cell walls in the root. An arabinosylated (1->6)-[beta]-galactan epitope is also found in all of the cell walls of the root with the exception of lateral root-cap cell walls. It is striking that these three polysaccharide epitopes are not uniformly distributed (or accessible) within the walls of a given cell, nor are these epitopes distributed equally across the two walls laid down by adjacent cells. Our results further suggest that the biosynthesis and differentiation of primary cell walls in plants are precisely regulated in a temporal, spatial, and developmental manner. 相似文献
190.
An antibody Fab selected from a recombinant phage display library detects deesterified pectic polysaccharide rhamnogalacturonan II in plant cells. 总被引:3,自引:0,他引:3 下载免费PDF全文
Rhamnogalacturonan II (RG-II) is a structurally complex, low molecular weight pectic polysaccharide that is released from primary cell walls of higher plants by treatment with endopolygalacturonase and is chromatographically purified after alkaline deesterification. A recombinant monovalent antibody fragment (Fab) that specifically recognizes RG-II has been obtained by selection from a phage display library of mouse immunoglobulin genes. By itself, RG-II is not immunogenic. Therefore, mice were immunized with a neoglycoprotein prepared by covalent attachment of RG-II to modified BSA. A cDNA library of the mouse IgG1/kappa antibody repertoire was constructed in the phage display vector pComb3. Selection of antigen-binding phage particles resulted in the isolation of an antibody Fab, CCRC-R1, that binds alkali-treated RG-II with high specificity. CCRC-R1 binds an epitope found primarily at sites proximal to the plasma membrane of suspension-cultured sycamore maple cells. In cells deesterified by alkali, CCRC-R1 labels the entire wall, suggesting that the RG-II epitope recognized by CCRC-R1 is masked by esterification in most of the wall and tha such RG-II esterification is absent near the plasma membrane. 相似文献