全文获取类型
收费全文 | 26395篇 |
免费 | 3033篇 |
国内免费 | 4546篇 |
出版年
2024年 | 53篇 |
2023年 | 710篇 |
2022年 | 672篇 |
2021年 | 1152篇 |
2020年 | 1204篇 |
2019年 | 1489篇 |
2018年 | 1190篇 |
2017年 | 1125篇 |
2016年 | 1213篇 |
2015年 | 1125篇 |
2014年 | 1277篇 |
2013年 | 1754篇 |
2012年 | 1085篇 |
2011年 | 1168篇 |
2010年 | 929篇 |
2009年 | 1341篇 |
2008年 | 1341篇 |
2007年 | 1454篇 |
2006年 | 1451篇 |
2005年 | 1266篇 |
2004年 | 1088篇 |
2003年 | 1061篇 |
2002年 | 987篇 |
2001年 | 868篇 |
2000年 | 833篇 |
1999年 | 693篇 |
1998年 | 574篇 |
1997年 | 454篇 |
1996年 | 480篇 |
1995年 | 432篇 |
1994年 | 431篇 |
1993年 | 386篇 |
1992年 | 349篇 |
1991年 | 301篇 |
1990年 | 264篇 |
1989年 | 223篇 |
1988年 | 179篇 |
1987年 | 179篇 |
1986年 | 131篇 |
1985年 | 213篇 |
1984年 | 170篇 |
1983年 | 95篇 |
1982年 | 136篇 |
1981年 | 106篇 |
1980年 | 89篇 |
1979年 | 65篇 |
1978年 | 39篇 |
1977年 | 35篇 |
1976年 | 35篇 |
1973年 | 23篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
101.
S. R. Saif 《Plant and Soil》1987,97(1):25-35
Summary Growth and mineral uptake of twenty-four tropical forage legumes and grasses were compared under glasshouse conditions in a sterile low P oxisol, one part inoculated and the other not inoculated with mycorrhizal fungi. Shoot and root dry weights and total uptake of P, N, K, Ca, and Mg of all the test plants were significantly increased by mycorrhizal inoculation. Mycorrhizal inoculation, with few exceptions, decreased the root/shoot ratio. Non-mycorrhizal plants contained always lower quantities of mineral elements than mycorrhizal plants. Plant species showed differences in percentage mycorrhizal root length and there was no correlation between percentage mycorrhizal infection and plant growth parameters. A great variation in dependence on mycorrhiza was observed among forage species. Total uptake of all elements by non-mycorrhizal legumes and uptake of P, N and K by non-mycorrhizal grasses correlated inversely with mycorrhizal dependency. Mycorrhizal plants of all species used significantly greater quantities of soil P than the nonmycorrhizal plants. Utilization of soil P by non-mycorrhizal plants was correlated inversely with mycorrhizal dependency. 相似文献
102.
Floral determination in the terminal bud of the short-day plant Nicotiana tabacum cv. Maryland Mammoth has been investigated. Plants grown continuously in short days flowered after producing 31.4±1.6 (SD) nodes while plants grown continuously in long days did not flower and produced 172.5±9.5 nodes after one year. At various ages, expressed as number of leaves that were at least 1.0 cm in length above the most basal 10-cm leaf, one of three treatments was performed on plants grown from seed in short days: 1) whole plants were shifted from short days to long days, 2) the terminal bud was removed and then rooted and grown in long days, and 3) the terminal bud was removed and then rooted and grown in short days. Whole plants flowered only when shifted from short days to long days at age 15 or later. Only rooted terminal buds from plants at age 15 or older produced plants that flowered when grown in long days. Only terminal buds from plants at age 15 or older that were rooted and grown in short days produced the same number of nodes as they would have produced in their original locations while buds from younger plants produced more nodes than they would have in their original locations. Thus, determination for floral development in the terminal bud, as assayed by rooting, is simultaneous with the commitment to flowering as assayed by shifting whole plants to non-inductive conditions.Abbreviations LD
long day(s)
- SD
short day(s)
- DN
dayneutral 相似文献
103.
C. P. McPhee P. C. Trappett 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1987,73(6):926-931
Summary Two lines of mice were selected for high post-weaning weight gain (3 to 6 weeks) adjusted for 3 week weight. One line (F) was grown on freely available food and the other (S) on a feeding scale set at the same level for all mice. Food intake of the S line averaged 80% of the F line. The realised heritabilities after 6 generations of selection were 0.38±0.06 and 0.33±0.07 for the F and S lines, respectively. In generation 7, mice from the F and S lines and from an unselected control line (C) were compared on both free and set levels of feeding from 3 weeks to 9 weeks of age. Measurements taken were growth rate, appetite, food conversion efficiency (weight gain/food intake) and body composition (fat, protein, ash, water). The F and S lines grew more rapidly and efficiently than the C line on both levels of feeding, each line performing best on the level of feeding on which it was selected. The average genetic correlation between growth rates of the same line on the two feeding levels was 0.54±0.10. The F line grew 19% faster and was 9% more efficient than the S line on free feeding but the S line grew 15% faster and was 15% more efficient than the F line on set feeding. Relative to the C line, food intake per day on free feeding was 4% higher in the F line and 6% lower in the S line. There was no difference between the lines in food intake/g body weight. The rate of deposition of all body components increased in both selection lines. In the F, S and C lines respectively, efficiencies of gains in body components (102x gain/food) were 1.79, 1.31 and 1.06 for fat, 1.53, 1.63 and 1.22 for protein and 5.88, 6.45 and 4.98 for protein + water. Apparently energy lost as heat was reduced in both the F and S lines. The partitioning of energy retained was altered in favour of more fat in the F line and more protein in the S line. 相似文献
104.
J. H. M. Hovenkamp-Hermelink E. Jacobsen A. S. Ponstein R. G. F. Visser G. H. Vos-Scheperkeuter E. W. Bijmolt J. N. de Vries B. Witholt W. J. Feenstra 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1987,75(1):217-221
Summary An amylose-free potato mutant was isolated after screening 12,000 minitubers. These minitubers had been induced on stem segments of adventitious shoots, which had been regenerated on leaf explants of a monoploid potato clone after Röntgen-irradiation. The mutant character is also expressed in subterranean tubers and in microspores. Starch granules from the mutant showed a strongly reduced activity of the granule bound starch synthase and loss of the major 60 kd protein from the starch granules. 相似文献
105.
E. J. Eisen 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1987,75(1):177-188
Summary Correlated responses were estimated in each of two replicate lines of mice selected within full-sib families for high (HF) or low (LF) 12-week epididymal fat pad weight as a percentage of body weight (epididymal fat pad percentage), or high (HL) or low (LL) 12-week hind carcass weight as a percentage of body weight (hind carcass percentage). Two replicate control lines (RC) were maintained. Correlated traits were measured in each of the 10 generations of selection. Realized (rG
R) and offspring-sire genetic correlations generally were in agreement. In HF and LF, 3–6 week postweaning gain (rG
R = 0.36±0.04) and feed intake (rG
R = 0.50±0.13) had positive correlated responses, but feed efficiency and feed intake/metabolic body size did not change. Positive correlated responses were found for subcutaneous fat pad percentage, body weight-adjusted subcutaneous fat pad weight and fat percentage in the hind carcass (rG
R's were 1.04±0.13, 0.93±0.13 and 0.90±0.08). In the hind carcass, fat-free dry (protein + ash) percentage showed a small negative correlated response, and fat-free dry weight did not change. In HL and LL, the correlated responses for the above traits were generally opposite to those observed in HF and LF. Litter size, percentage of infertile matings, and preweaning mortality showed no consistent correlated responses in any of the lines, but an index of fitness combining the three traits showed a decrease in all four selection treatments.Paper no. 11057 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, 27695-7601. The use of trade names in this publication does not imply endorsement by the North Carolina Agricultural Research Service of the products named, nor criticisms of similar ones not mentioned 相似文献
106.
Cladocera in space and time: Analysis of lake sediments 总被引:6,自引:6,他引:0
Wolfgang Hofmann 《Hydrobiologia》1987,145(1):315-321
Shells of Bosminidae and Chydoridae are quantitatively preserved in lake sediments. The chronological deposition of these
remains provides the means for longterm observation of these Cladocera, both in terms of species and communities.
Chydorid analysis, as based on subfossil assemblages, is an analysis of community and provides direct observation of community
dynamics over extended periods of time. It has proved to be a valuable method to obtain information on the influence of environmental
factors and time on community characteristics.
Morphological variation inBosmina (Eubosmina) has been followed for some thousand years. This is of special interest for the evaluation of taxonomic rank (species, forms)
if closely related taxa have co-existed.
Bosmina successions, as well as shifts in the chydorid fauna, are related to environmental change. Thus, cladoceran analysis of lake
sediments provides information on the developmental history of lakes and allows observation of the effects of longterm environmental
changes, such as climatic changes and eutrophication. 相似文献
107.
108.
109.
Food composition of Clarias gariepinus (= C. lazera) (Cypriniformes,Clariidae) in Lake Kinneret (Israel) 总被引:2,自引:2,他引:0
The feeding behaviour of the freshwater piscivore, Clarias gariepinus (C. lazera) (C & V 1840) was studied over two periods: 1973–1975 and 1981–1982, in Lake Kinneret (Israel). The total number of fish analysed was 264 and their sizes (SL) and weights varied between 238 and 830 mm (146 to 5728 g). More than fifty species of plants and animals from the plankton, benthos and nekton of Lake Kinneret were identified in the intestines of C. gariepinus. Preyed fish were the most abundant food component (81%) and constituted the highest biomass, with Mirogrex terraesanctae representing the majority (although other species were also found). The potential impact of piscivory in the Kinneret ecosystem is considered. 相似文献
110.
Summary Voltage-dependent K channels could be identified in on-cell and excised patch-clamp records on membranes of isolated plant cell vacuoles. The current through a membrane patch is dominated by a channel population with a conductance of about 121 pS in symmetrical 250mm KCl solution. The single channel adopts at least two conducting levels the 121-pS state being most frequently observed. The channel shows outward rectification, representing a cation flux into the vacuoles. The rectification appears to be caused by a vanishing open probability and a short channel lifetime at hyperpolarizing voltages. A selectivity ratio of potassium over sodium of about 6 was derived as an estimate. Occasionally, an additional population of K channels with a single-channel conductance of approximately 18 pS is observed. This channel type exhibits outward rectification as well. 相似文献