全文获取类型
收费全文 | 76806篇 |
免费 | 6555篇 |
国内免费 | 39篇 |
出版年
2022年 | 485篇 |
2021年 | 942篇 |
2020年 | 567篇 |
2019年 | 720篇 |
2018年 | 924篇 |
2017年 | 870篇 |
2016年 | 1445篇 |
2015年 | 2395篇 |
2014年 | 2733篇 |
2013年 | 3752篇 |
2012年 | 4598篇 |
2011年 | 4753篇 |
2010年 | 3130篇 |
2009年 | 2830篇 |
2008年 | 4191篇 |
2007年 | 4311篇 |
2006年 | 4119篇 |
2005年 | 4087篇 |
2004年 | 4185篇 |
2003年 | 3772篇 |
2002年 | 3782篇 |
2001年 | 871篇 |
2000年 | 609篇 |
1999年 | 889篇 |
1998年 | 1188篇 |
1997年 | 854篇 |
1996年 | 787篇 |
1995年 | 760篇 |
1994年 | 738篇 |
1993年 | 685篇 |
1992年 | 622篇 |
1991年 | 600篇 |
1990年 | 579篇 |
1989年 | 614篇 |
1988年 | 528篇 |
1987年 | 510篇 |
1986年 | 458篇 |
1985年 | 593篇 |
1984年 | 744篇 |
1983年 | 639篇 |
1982年 | 759篇 |
1981年 | 795篇 |
1980年 | 720篇 |
1979年 | 499篇 |
1978年 | 557篇 |
1977年 | 527篇 |
1976年 | 526篇 |
1975年 | 403篇 |
1974年 | 498篇 |
1973年 | 452篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
921.
922.
Control of barley root respiration 总被引:8,自引:0,他引:8
Evidence from barley [ Hordeum distichum (L.) Lam. cv. Maris Mink], and from many other species, suggests that respiration is controlled by either supply of carbohydrate or demand for ATP. The relationship between root respiration rate (measured as O2 consumption or CO2 production) and ethanol-soluble carbohydrate content altered with time following selective pruning, and the change could not be accounted for by buffering of the cytoplasmic carbohydrate concentration by sugars in the vacuole. Exogenous sucrose supplied to the roots prevented any decline of the respiration rate in shoot-pruned plants, and if supplied for 24 h stimulated the respiration rate after any treatment. Root extension responded to sucrose in a similar manner. We suggest that respiration is under fine control by adenylates, but the capacity of the respiratory system is fixed by the supply of sucrose, possibly via coarse control of the respiratory machinery, or of the processes requiring metabolic energy. 相似文献
923.
924.
925.
Leslie matrix simulations were performed, employing idealized ungulate life histories and density-independent, either random or periodic, variation in calf survival, p0. Long-term average rate of increase was reduced whenever variation was added to p0. The iteroparous life histories strongly buffered high-frequency vital rate variations, so that large fluctuations in p0 resulted in relatively small fluctuations in population size. The buffering effect was greatest for species with the longest lifespans. The results suggest that wild ungulate populations will not fluctuate greatly in response to density-independent, environmental-driven vital rates. Such populations will reach high densities, thus experiencing strong density-dependent regulation, only a few times in many generations. 相似文献
926.
Lisa J. Samuelson Ben H. Cazell John R. Seiler 《Environmental and Experimental Botany》1990,30(4):469-473
The use of nitrogen-fixing trees such as black alder (Alnus glutinosa L. Gaertn.) as forest silvicultural tools has recently been recognized. The potential benefit of black alder in silvicultural practices may be reduced by nitrate fertilization. Fifteen-month-old, nodulated, black alder rooted cuttings were fertilized for 6 days with 0, 7.5 or 15 mM NO3 to determine the influence of nitrate on acetylene reduction, nodule respiration and net photosynthesis. Acetylene reduction, net photosynthesis and nodule respiration were measured on the second, fourth and sixth days of nitrate application. Nitrate treatment significantly reduced acetylene reduction and nodule respiration by day 4. Acetylene reduction was 75% lower and nodule respiration 36% lower for the 15 mM NO3 treatment when compared to that of the control treatment. By day 6, net photosynthesis and nodule respiration were significantly reduced by 29 and 59%, respectively, for seedlings treated with 15 mM NO3. This study suggests that nitrate fertilization has a profound influence on nitrogenase activity and that nitrogen-fixing tree species may respond to nitrate fertilization by shifting photosynthetic rates. 相似文献
927.
H L Bank J John M K Schmehl R J Dratch 《Applied and environmental microbiology》1990,56(12):3888-3889
Studies were designed to evaluate the effectiveness of pulsed modulated UV light waveforms for killing bacteria. Exposure of five strains of bacteria to the modulated information encoded in the light decreased the colony population from a confluent lawn to less than 20 colonies. However, approximately 2,000 colonies survived treatment with the same intensity and time of exposure to UV light lacking the modulated information. 相似文献
928.
929.
930.
Clare M. Baecher Karen S. Dorfman Marie-Geneviève Mattei John G. Frelinger 《Immunogenetics》1990,31(5-6):307-314
Mouse leukosialin, previously known as the 3E8 antigen, is expressed primarily on cells of the hematopoietic and lymphoid lineages and is shown to be the mouse homologue to the human leukosialin/sialophorin and rat W3/13 molecules. A partial leukosialin cDNA clone was isolated via cross-species hybridization with a portion of a human leukosialin cDNA. This mouse cDNA clone was used to demonstrate that the leukosialin isoforms are encoded by a single mRNA species of approximately 4.2 kilobases (kb) and that the leukosialin gene is located on chromosome 7. Based on these results, mouse leukosialin is given the designation Ly48.The nucleotide sequence data reported in this paper have been submitted to the GenBank nucleotide sequence database and have been assigned the accession number M30693. 相似文献