全文获取类型
收费全文 | 6603篇 |
免费 | 291篇 |
国内免费 | 353篇 |
专业分类
林业 | 1930篇 |
农学 | 372篇 |
基础科学 | 325篇 |
557篇 | |
综合类 | 2515篇 |
农作物 | 396篇 |
水产渔业 | 39篇 |
畜牧兽医 | 367篇 |
园艺 | 578篇 |
植物保护 | 168篇 |
出版年
2024年 | 16篇 |
2023年 | 133篇 |
2022年 | 169篇 |
2021年 | 146篇 |
2020年 | 154篇 |
2019年 | 200篇 |
2018年 | 130篇 |
2017年 | 220篇 |
2016年 | 288篇 |
2015年 | 217篇 |
2014年 | 315篇 |
2013年 | 385篇 |
2012年 | 486篇 |
2011年 | 506篇 |
2010年 | 420篇 |
2009年 | 435篇 |
2008年 | 369篇 |
2007年 | 377篇 |
2006年 | 349篇 |
2005年 | 269篇 |
2004年 | 220篇 |
2003年 | 180篇 |
2002年 | 137篇 |
2001年 | 174篇 |
2000年 | 131篇 |
1999年 | 89篇 |
1998年 | 75篇 |
1997年 | 93篇 |
1996年 | 98篇 |
1995年 | 62篇 |
1994年 | 74篇 |
1993年 | 56篇 |
1992年 | 44篇 |
1991年 | 60篇 |
1990年 | 50篇 |
1989年 | 42篇 |
1988年 | 28篇 |
1987年 | 27篇 |
1986年 | 3篇 |
1985年 | 5篇 |
1984年 | 3篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1980年 | 1篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1955年 | 2篇 |
排序方式: 共有7247条查询结果,搜索用时 640 毫秒
21.
采用专门设计的生物材料试验机测定了茶树新梢的折断力、断裂挠度和弹性模量等力学指标,初步探明了新梢节位(嫩度)、截面积、品种和空气温、湿度对这些指标的影响。结果表明折断式采摘机构能有效地实现选择性采茶,可克服切割式采茶机“一刀切”的弊端。此外,作者根据大量的测试数据,提出了折断式采摘器的基本工作参数。 相似文献
22.
23.
Abstract. Usual residue-management options are to remove the residue, use it as mulch with or without undercutting or to incorporate it into the soil. While the role of surface mulch in evaporation has been widely studied, the information on the effect on evaporation of mulch with undercutting or residue incorporated into soil, particularly in relation to soil type and evaporativity (Eo) is lacking. We studied the effect of wheat straw used in various ways on the course of evaporation loss from soil columns with three soils at Ludhiana, India and one soil at Bushland, Texas, USA, under two Eo's Energy-limited evaporation rates under mulch (Eom) followed the soil-specific relation Eom/Eo= a e(bRes+cEo), where Res is residue rate t/ha and a, b and c are constants; Eo, is expressed in mm/d. In an effort to model the total evaporation (CE) during the energy-limited stage ‘U’ was obtained from appropriate CE versus time curves and (CE-U) was regressed over (t - ti)0.5 to obtain the slope ‘α’ (Ritchie 1972) for the soil-limited evaporation stage. The observed ‘U’ was independent of mulch rate and Eo but was strongly affected by soil type, Values of ‘α’ decreased with increase in mulch rate and decrease in Eo and coarseness of soil. The otherwise short lived benefit of evaporation reduction with mulch per se, which peaked after a few days was maintained when residue was mixed with soil at the stage when evaporation reduction reached a maximum; this benefit continued for several weeks. Cumulative evaporation values computed from ‘U’ and ‘α’ agreed closely with the observed values under straw mulch for loamy sand and clay loam soils and for ‘undercut’ and ‘residue mixed’ treatments on all soils regardless of Eo, and for all situations under small Eo. However, for sandy loam and silt loam soils under Eo of 10 mm/d, the modified square root of the time function of Jalota et al. (1988) gave a better fit. 相似文献
24.
对经营区边境伐区开设便道,还是开设手扶拖拉机道进行技术分析与经济分析,采用盈亏平衡分析法决策,并用实例论证其实用价值。 相似文献
25.
Although agronomically interesting, selection of creeping-rootedness inlucerne, that is, the ability to form adventitious shoots on horizontal,spreading roots, remains difficult because of the complex and still unclear geneticcontrol of this character, coupled with the possible occurrence of external factorsthat affect unpredictably its expression. Two experiments were undertaken onprogenies of creeping-rooted plants, to verify previous findings and inferences,and test novel hypotheses, concerning the effect of various plant factors on theexpression of creeping-rootedness. The ultimate aim was to provide information onthe most appropriate germplasm, procedures, or indirect selection criteria, that wouldimprove the efficiency of breeding for this character. The factors here examinedincluded genotype, age, kind of progeny, vigour, spreading ability, and undergroundmorphology. Some genotypes appeared to be good donors of the character, whereasothers did not. The proportion of creeping-rooted plants increased in allcases with the plant age. Selfing consistently depressed the expression ofthe character compared with clonal propagation or crossing. It is suggestedthat the effect of age and selfing on the penetrance of creeping-rootedness ismediated through their effect on plant vigour. Both aerial and root vigour showedindeed an effect per se on the presence of the character. Beside thevigour, possible indirect selection criteria were identified in the frequenciesof swollen zones and latent stem primordia on horizontal roots. Proportion ofcreeping-rooted plants in progenies after one year of growth appeared as a simple anduseful criterion to predict future trends. Consistent with previous conclusions, theresults provided evidence that a combination of family and individualselection may enhance the character expression. 相似文献
26.
Fulmer R Loeb WF Martin DP Gard EA 《Veterinary clinical pathology / American Society for Veterinary Clinical Pathology》1984,13(1):19-25
The intravenous administration of 0.75 gm glucose per kg and the measurement of serum glucose pretest and at 10, 20, 30, 60, 90 and 120 minutes constitute a satisfactory protocol for intravenous glucose tolerance testing of Rhesus (Macaca mulatto) and African Green (Cercopithecus aethiops) monkeys. No significant differences were noted between animals restrained with ketamine hydrochloride and those restrained with sodium pentobarbital, but the African Green males and females and the male Rhesus monkeys yielded significantly different results while being manually restrained. 相似文献
27.
28.
29.
30.