全文获取类型
收费全文 | 2901篇 |
免费 | 212篇 |
国内免费 | 4篇 |
专业分类
林业 | 136篇 |
农学 | 53篇 |
基础科学 | 10篇 |
339篇 | |
综合类 | 407篇 |
农作物 | 64篇 |
水产渔业 | 147篇 |
畜牧兽医 | 1710篇 |
园艺 | 71篇 |
植物保护 | 180篇 |
出版年
2023年 | 15篇 |
2022年 | 14篇 |
2021年 | 47篇 |
2020年 | 63篇 |
2019年 | 50篇 |
2018年 | 50篇 |
2017年 | 57篇 |
2016年 | 55篇 |
2015年 | 50篇 |
2014年 | 78篇 |
2013年 | 151篇 |
2012年 | 163篇 |
2011年 | 197篇 |
2010年 | 120篇 |
2009年 | 103篇 |
2008年 | 167篇 |
2007年 | 168篇 |
2006年 | 164篇 |
2005年 | 156篇 |
2004年 | 168篇 |
2003年 | 165篇 |
2002年 | 128篇 |
2001年 | 50篇 |
2000年 | 52篇 |
1999年 | 37篇 |
1998年 | 22篇 |
1997年 | 21篇 |
1996年 | 17篇 |
1995年 | 16篇 |
1994年 | 16篇 |
1993年 | 19篇 |
1992年 | 33篇 |
1991年 | 30篇 |
1990年 | 39篇 |
1989年 | 33篇 |
1988年 | 29篇 |
1987年 | 22篇 |
1985年 | 16篇 |
1984年 | 14篇 |
1983年 | 18篇 |
1979年 | 26篇 |
1978年 | 13篇 |
1977年 | 15篇 |
1974年 | 16篇 |
1973年 | 14篇 |
1972年 | 14篇 |
1971年 | 14篇 |
1970年 | 14篇 |
1969年 | 14篇 |
1967年 | 13篇 |
排序方式: 共有3117条查询结果,搜索用时 15 毫秒
991.
992.
Abnormal hybrid seedling development hinders attempts to introgress diverse germplasm in common bean (Phaseolus vulgaris L.) breeding programs. In this study, the hypothesis that DL1 and DL2 loci control abnormal seedling development both in Phaseolus vulgaris L. intraspecific hybrids and in P. vulgaris×P. acutifolius A. Gray interspecific hybrids was tested. Hybrids from crosses between six P. vulgaris lines (two Andean dl1 d1, DL2DL2, two Mesoamerican DL1DL1 dl2 dl2, and two facilitators dl1dl1, dl2dl2) and P. vulgaris cultivar Ex Rico 23 developed normally, indicating that Ex Rico 23 has the facilitator genotype dl1, dl1, dl2dl2. Previous studies have shown that Ex Rico 23 ×P. acutifolius results in abnormal seedlings, but ‘ICA Pijao’, also dl1, dl1 dl2dl2, ×P. acutifolius results in normal seedlings. Neither the Andean nor the Mesoamerican lines crossed successfully with P. acutifolius, but crosses between one of the two facilitators (G3807) and P. acutifolius resulted in normal seedlings. These results support the hypothesis that different loci control intra- and interspecific hybrid seedling development. 相似文献
993.
Tall fescue is an important outcrossing forage and turf grass species that requires vernalization to flower. A reproducible protocol was developed for vernalization of regenerated, transgenic and seed‐derived tall fescue plants. Following the vernalization scheme that involved gradual changes of temperature and daylength, seeds were routinely produced from vernalized plants under greenhouse conditions. Molecular analyses of progenies obtained from crosses between transgenic and seed‐derived plants revealed stable meiotic transmission of transgenes following Mendelian inheritance in transgenic tall fescue. 相似文献
994.
In order to explore for the most effective strategy for using forests to mitigate global climate change, we have constructed a simple model of C uptake during forest growth and the fate of this C when forests are harvested and used as fuel to replace fossil fuels. We suggest that trees are equally effective in preventing the accumulation of CO2 in the atmosphere if they remove a unit of C from the atmosphere or if they supply a sustainable source of energy that substitutes for a unit of C discharged by burning fossil fuels. The model shows that the most effective strategy for using forest land to minimize increases in atmospheric CO2 will depend on the current status of the land, the productivity that can be expected, the efficiency with which the forest harvest is used to substitute for fossil fuels, and the time perspective of the analysis. For forests with large standing biomass and low productivity the most effective strategy is to protect the existing forest. For land with little standing biomass and low productivity, the most effective strategy is to reforest or otherwise manage the land for forest growth and C storage. Where high productivity can be expected, the most effective strategy is to manage the forest for a harvestable crop and to use the harvest with maximum efficiency either for long-lived products or to substitute for fossil fuels. The longer the time perspective, the more likely that harvesting and replanting will result in net C benefits. 相似文献
995.
P M Scott T Panalaks S Kanhere W F Miles 《Journal of the Association of Official Analytical Chemists》1978,61(3):593-600
A simple cleanup procedure based on pH adjustments was used to obtain extracts of corn foods. The method gave good recoveries of zearalenone determined by thin layer chromatography (TLC) and high pressure liquid chromatography (HPLC). As little as 5 ng zearalenone was detected by TLC, using Fast Violet B Salt as the spray reagent; the lower limit of detection in cornflakes was about 20 microgram/kg. With HPLC on Spherisorb silica (5 micrometer) and detection by fluorescence at an excitation maximum of 310 nm as little as 5 microgram zearalenone/kg cornflakes could be determined. While the TLC method was also applicable to corn chips, cornmeal, popcorn, and frozen corn, an interference was observed in HPLC of the latter 3 products. This interference was separated from zearalenone by adding a second HPLC analytical column (Spherisorb ODS). Gas-liquid chromatography coupled with mass spectrometric single ion monitoring at high resolution, although of limited availability, was shown to be the most sensitive and selective method for determining zearalenone in corn foods. The natural occurrence of zearalenone in a sample of cornflakes (13-20 microgram/kg) was demonstrated by all 3 detection procedures. 相似文献
996.
Robert G Haight Brian Cypher Scott Phillips Hugh P Possingham 《Biological conservation》2004,117(1):61-72
Expanding habitat protection is a common strategy for species conservation. We present a model to optimize the expansion of reserves for disjunct populations of an endangered species. The objective is to maximize the expected number of surviving populations subject to budget and habitat constraints. The model accounts for benefits of reserve expansion in terms of likelihood of persistence of each population and monetary cost. Solving the model with incrementally higher budgets helps prioritize sites for expansion and produces a cost curve showing funds required for incremental increases in the objective. We applied the model to the problem of allocating funds among eight reserves for the endangered San Joaquin kit fox (Vulpes macrotis mutica) in California, USA. The priorities for reserve expansion were related to land cost and amount of already-protected habitat at each site. Western Kern and Ciervo-Panoche sites received highest priority because land costs were low and moderate amounts of already-protected habitat resulted in large reductions in extinction risk for small increments of habitat protection. The sensitivity analysis focused on the impacts of kit fox reproductive success and home range in non-native grassland sites. If grassland habitat is lower quality than brushland habitat resulting in higher annual variation in reproductive success or larger home ranges, then protecting habitat at the best grassland site (Ciervo-Panoche) is not cost-efficient relative to shrubland sites (Western Kern, Antelope Plain, Carrizo Plain). Finally, results suggested that lowest priority should be given to three relatively high-cost grassland sites (Camp Roberts, Contra Costa, and Western Madera) because protecting habitat at those sites would be expensive and have little effect on the expected number of surviving kit fox populations. 相似文献
997.
J. P. Shepard M. J. Mitchell T. J. Scott Y. M. Zhang D. J. Raynal 《Water, air, and soil pollution》1989,48(1-2):225-238
Inputs of wet and dry deposition were monitored at the Huntington Forest in the Adirondack Mountains of New York for two years in the open and beneath the canopy of a northern hardwood forest. In the open, ion flux estimates were similar using wet-only weekly (NADP protocol) and event collections, but bulk collections were higher for all ions except H+, which was much lower. These differences were due to the contribution of dry deposition and possible biotic alterations in bulk collectors. Dry deposition was estimated using air concentrations and ion-specific depositional velocities modeled with meteorological data, and contributed substantially to the input of all ions [H+ (45%), Na+ (24%), K+ (22%), NH4 + (12%), Ca2+ (58%), Mg2+(43%), NO3 ? (55%), Cl? (27%) and SO4 ?2 (26%)]. Dry input of base cations was dominated by coarse particles, whereas gaseous inputs were more important for S and NO3 ?. Atmospheric concentrations of SO2 and inputs of SO4 2? and H+ were lower at this site than sites closer to point sources of S gas emission. The importance of estimating atmospheric inputs was examined using examples of elemental budgets. For example, different estimates of the contribution of dry deposition of SO4 2? (9–21 meq m?2 y?1) resulted in conclusions ranging from no net retention to a net loss of this element. Such differences have important implications in assessing the current and future role of atmospheric inputs in affecting elemental cycling. 相似文献
998.
999.
1000.
Jin-Hyeob Kwak Sang-Sun Lim Scott X. Chang Kye-Han Lee Woo-Jung Choi 《Journal of Soils and Sediments》2011,11(5):709-721