全文获取类型
收费全文 | 8066篇 |
免费 | 532篇 |
国内免费 | 704篇 |
专业分类
林业 | 1425篇 |
农学 | 698篇 |
基础科学 | 218篇 |
1274篇 | |
综合类 | 3188篇 |
农作物 | 662篇 |
水产渔业 | 613篇 |
畜牧兽医 | 564篇 |
园艺 | 271篇 |
植物保护 | 389篇 |
出版年
2024年 | 32篇 |
2023年 | 162篇 |
2022年 | 192篇 |
2021年 | 234篇 |
2020年 | 288篇 |
2019年 | 354篇 |
2018年 | 218篇 |
2017年 | 377篇 |
2016年 | 465篇 |
2015年 | 359篇 |
2014年 | 473篇 |
2013年 | 542篇 |
2012年 | 619篇 |
2011年 | 679篇 |
2010年 | 504篇 |
2009年 | 483篇 |
2008年 | 408篇 |
2007年 | 434篇 |
2006年 | 386篇 |
2005年 | 291篇 |
2004年 | 218篇 |
2003年 | 170篇 |
2002年 | 163篇 |
2001年 | 157篇 |
2000年 | 154篇 |
1999年 | 134篇 |
1998年 | 124篇 |
1997年 | 94篇 |
1996年 | 101篇 |
1995年 | 91篇 |
1994年 | 69篇 |
1993年 | 61篇 |
1992年 | 59篇 |
1991年 | 53篇 |
1990年 | 40篇 |
1989年 | 44篇 |
1988年 | 23篇 |
1987年 | 21篇 |
1986年 | 11篇 |
1985年 | 2篇 |
1984年 | 2篇 |
1983年 | 3篇 |
1982年 | 2篇 |
1981年 | 2篇 |
1980年 | 3篇 |
1976年 | 1篇 |
排序方式: 共有9302条查询结果,搜索用时 31 毫秒
111.
The morphological basis of resistance to the whiteflyBemisia tabaci Genn. (Aleyrodidae: Hemiptera) was studied. The plant characters examined were leaf area, thickness of leaf lamina, hair
density, hair length, angle of insertion of leaf hair, and density of gossypol glands. Hair density and leaf thickness were
positively correlated with the population ofB. tabaci, and a positive correlation was obtained between the adult whitefly population and gossypol glands on stem internodes. Cotton
genotype USA-22 (sparsely hairy) was found to be more tolerant toB. tabaci than was genotype USA-13 (velvety hairy). The use of thinner and glabrous leaved cotton varieties is suggested to minimize
the whitefly menace in cotton. 相似文献
112.
半荒漠地区灌木饲料林营造技术研究 总被引:5,自引:0,他引:5
孙德祥 《干旱区资源与环境》1995,9(2):74-79
本文对半荒漠地区灌木饲料林的树种选择、经营管理等问题进行了初步探讨,给出了花棒、杨柴、柠条、沙柳、毛条、毛柳、白沙蒿、油蒿8个半荒漠地区灌木饲料林树种的造林适宜立地类型、最适造林季节、最佳造林密度、最适收获期以及最佳平茬年龄。 相似文献
113.
114.
115.
Many chlorine-containing pesticides, for example 2-chloro-s-triazines, are of great concern both environmentally and toxicologically. As a result, ascertaining or predicting the fate and transport of these compounds in soils and water is of current interest. Transformation pathways for 2-chloro-s-triazines in the environment include dealkylation, dechlorination (hydrolysis), and ring cleavage. This study explored the feasibility of using computational chemistry, specifically the hybrid density functional theory method, B3LYP, to predict hydrolysis trends of atrazine (2-chloro-N4-ethyl-N6-isopropyl-1,3,5-triazine-2,4-diamine) and related 2-chloro-s-triazines to the corresponding 2-hydroxy-s-triazines. Gas-phase energetics are described on the basis of calculations performed at the B3LYP/6-311++G(d,p)//B3LYP/6-31G* level of theory. Calculated free energies of hydrolysis (delta h G298) are nearly the same for simazine (2-chloro-N4,N6-diethyl-1,3,5-triazine-2,4-diamine), atrazine, and propazine (2-chloro-N4,N6-di-isopropyl-1,3,5-triazine-2,4-diamine), suggesting that hydrolysis is not significantly affected by the side-chain amine-nitrogen alkyl substituents. High-energy barriers also suggest that the reactions are not likely to be observed in the gas phase. Aqueous solvation effects were examined by means of self-consistent reaction field methods (SCRF). Molecular structures were optimized at the B3LYP/6-31G* level using the Onsager model, and solvation energies were calculated at the B3LYP/6-311++G(d,p) level using the isodensity surface polarizable continuum model (IPCM). Although the extent of solvent stabilization was greater for cationic species than neutral ones, the full extent of solvation is underestimated, especially for the transition state structures. As a consequence, the calculated hydrolysis barrier for protonated atrazine is exaggerated compared with the experimentally determined one. Overall, the hydrolysis reactions follow a concerted nucleophilic aromatic substitution (SNAr) pathway. 相似文献
116.
油料向日葵根系生态与生理特性分析 总被引:2,自引:0,他引:2
长期以来对作物地上部分的结构功能和叶光系统进行了大量研究,对向日葵的根系发育规律的研究相对较少,盐渍化地区向日葵根系发育规律的研究就更少。为了有效地研究油料向日葵根系吸水过程和根系分布的关系,本文对向日葵根系生长发育规律和根系密度分布进行研究。 相似文献
117.
118.
119.
土壤容重对生姜生长及产量和品质的影响 总被引:1,自引:0,他引:1
以莱芜大姜为试材,研究了容重分别为1.49(CK)、1.36(T1)、1.20(T2)g·cm-3的3种土壤对生姜生长及产量、品质的影响。结果表明,容重较小的土壤,其孔隙度较大,最大持水量较高,有利于生姜植株的生长和产量的提高。生长末期T2的株高、茎粗、分枝数和叶片数分别比CK增加17.9%、14.0%、35.2%和36.1%,单株根、茎、叶、根茎鲜质量分别比CK提高43.3%、24.4%、28.1%和35.2%,收获时CK、T1、T2的产量分别为2534.1、3017.7和3326.8kg·(667m2)-1,处理间差异极显著。此外,生姜根茎干物质、淀粉及挥发油含量随土壤容重降低而降低,而蛋白质、游离氨基酸则相反,粗纤维、可溶性糖及VC含量则基本不受土壤容重的影响。 相似文献
120.
Agrobacterium-mediated transformation for both Melastoma malabathricum and Tibouchina semidecandra were optimized using green fluorescent protein (GFP) as a reporter. The binary vector pCAMBIA1304 harboring the modified green fluorescent protein (mgfp) gene driven by the CaMV 35S promoter was used. Parameters optimized were bacterial strain, bacterial concentration, pre-culture period, co-cultivation period, immersion time, acetosyringone concentration and wounding type. Results obtained were based on the percentage of GFP expression which was observed 3 days post-transformation. Agrobacterium tumefaciens strain LBA4404 and EHA105 at concentration 1 × 107 cfu ml−1 (OD600nm 0.8) showed the highest virulence on M. malabathricum and T. semidecandra, respectively. Four days of pre-culture and 2 days of co-cultivation were optimum for M. malabathricum transformation, while 3 days of pre-culture and co-cultivation for T. semidecandra. Result also showed that 60 min of immersion and addition of 200 μM acetosyringone gave the highest percentage of positive transformants for both M. malabathricum and T. semidecandra. Mild wounding also significantly increased the efficiency of M. malabathricum transformation. 相似文献