全文获取类型
收费全文 | 15497篇 |
免费 | 745篇 |
国内免费 | 1078篇 |
专业分类
林业 | 1529篇 |
农学 | 1225篇 |
基础科学 | 450篇 |
1771篇 | |
综合类 | 7668篇 |
农作物 | 904篇 |
水产渔业 | 476篇 |
畜牧兽医 | 1503篇 |
园艺 | 609篇 |
植物保护 | 1185篇 |
出版年
2024年 | 120篇 |
2023年 | 292篇 |
2022年 | 432篇 |
2021年 | 486篇 |
2020年 | 464篇 |
2019年 | 553篇 |
2018年 | 323篇 |
2017年 | 512篇 |
2016年 | 692篇 |
2015年 | 585篇 |
2014年 | 924篇 |
2013年 | 790篇 |
2012年 | 1237篇 |
2011年 | 1223篇 |
2010年 | 1000篇 |
2009年 | 1053篇 |
2008年 | 842篇 |
2007年 | 938篇 |
2006年 | 822篇 |
2005年 | 620篇 |
2004年 | 492篇 |
2003年 | 388篇 |
2002年 | 379篇 |
2001年 | 293篇 |
2000年 | 239篇 |
1999年 | 242篇 |
1998年 | 186篇 |
1997年 | 153篇 |
1996年 | 166篇 |
1995年 | 154篇 |
1994年 | 145篇 |
1993年 | 118篇 |
1992年 | 101篇 |
1991年 | 91篇 |
1990年 | 88篇 |
1989年 | 62篇 |
1988年 | 53篇 |
1987年 | 27篇 |
1986年 | 15篇 |
1985年 | 3篇 |
1984年 | 2篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1976年 | 2篇 |
1962年 | 1篇 |
1956年 | 1篇 |
1955年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
目的 探讨支架植入术治疗椎动脉起始部狭窄的安全性及有效性。方法 回顾性分析2009年1月至2014年1月在长沙市第一医院神经内科住院的80例椎动脉起始部狭窄患者进行支架成形术治疗的病例资料,并对其进行随访,观察其治疗的安全性及长期疗效。结果 80例患者成功植入80枚支架,技术成功率100%;放入支架前椎动脉狭窄率为(76.67±4.03)%,放入支架后椎动脉狭窄率为(13.70±1.88)%;80例患者最长随访7年,最短随访2年,失访1例,2例发生支架内再狭窄,再狭窄率3.75%。58例患者临床症状消失,21例临床症状缓解,1例临床症状加重,总有效率98.75%。结论 支架成形术治疗椎动脉起始部狭窄是一种非常安全及有效的方法。 相似文献
992.
Trees are the dominant species in agroforestry systems, profoundly affecting the performance of understory crops. Proximity to trees is a key factor in crop performance, but rather little information is available on the spatial distribution of yield and yield components of crop species under the influence of trees in agroforestry systems. Also, little information is available on how crop density may be exploited to optimize the yield in such systems. Here we studied the performance of cotton in jujube/cotton agroforestry. Field experiments were conducted in 2012 and 2013 in Hetian, Xinjiang, China. Cotton was grown at a row distance of 60 cm in three densities, 13.5, 18.0 and 22.5 plants m−2 in six m wide paths between tree lines in a jujube plantation. Plant density affected both cotton aboveground dry matter and yield significantly. The highest yield was attained at the intermediate density of 18.0 plants m−2 (20.0 plants m−2 corresponding in sole cotton), lower than the optimal density in sole cotton (25.0 plants m−2). Yield at the lower density was constrained by the low number of bolls per m2 as a direct consequence of the low density, whereas at the high plant density yield was constrained by a lower allocation of assimilates to cotton seed and lint, as a consequence of intraspecific and interspecific competitions. There were strong gradients in yield and yield components in relation to the distance from the tree rows. Leaf area and total dry matter of cotton in rows close to the tree lines were reduced, especially in the rows next to the trees. Moreover, biomass allocation to cotton fruits was reduced in these rows. Competitive influences from the trees on cotton performance extended two rows deep in a six-year old jujube stand, and even three rows deep in a seven-year old stand. Shading effects on cotton yield were compensated by increasing plant density as a result of greater boll numbers per unit ground area. Data from this study help guide the design of optimal plant density of cotton in jujube plantations and give insight in the spatial distribution and dynamics of competitive effects in agroforestry systems in general. 相似文献
993.
994.
Urban green spaces (UGSs) play an essential role in sustainable urban development and are closely related to public health and human well-being. Previous studies seldom consider the relationships between spatial patterns and equity for the heterogeneity of built-up residential types in the city, not to mention how to place the pragmatic design strategies to explore green injustice for vulnerable neighborhoods. To address this gap, our study adopts a cascade of three methodological stages: 1) applying an urban morphological spatial pattern analysis (MSPA) to compare urban UGS patterns and the UGS-adapted Gini coefficient to measure the spatial equity of UGS distributions in three local built-up areas, 2) employing the minimum cumulative resistance (MCR) model to determine the location of the newly added UGSs and adopting the ‘just green enough’ strategy into the redistribution of UGSs for the marginal neighborhood and at the same time to avoid inequality associated with green gentrification, and 3) exploring an integrated design and implementation method for the newly added UGSs that can accommodate both normal conditions and the possibility of future pandemics. The results indicate that there is a strong correlation between socioeconomic status and the distributional justice of UGSs among built-up types, while disadvantaged people living in the densest residential neighborhoods have lower UGS provisions. This paper demonstrates a multifunctional and pragmatic approach for realizing multiscale UGS optimization by exploring it at the subdivision and neighborhood scales. This implication can also help guide decision-makers and planners to understand the complex interplay between social vulnerabilities, gentrification, and urban greening, thus formulating effective policies and hands-on strategies to address green injustice that are not only crucial for this study region but also of great significance for developing countries in the Global South. 相似文献
995.
996.
采用苜蓿汁、韭菜汁和木醋液三种物质分别按三种不同浓度和三种不同施用方式对番茄早疫病的防治效果进行研究。研究结果表明,三种物质防治番茄早疫病,叶面 灌根处理方式最好。三种物质中木醋液20倍和40倍对番茄早疫病的防治效果最好,5d和20d都分别为71.36%和71.42%。 相似文献
997.
宿生棉耐冷相关性状的种间杂种优势及细胞质效应分析 总被引:1,自引:0,他引:1
探讨一二年生棉花耐冷相关性状的种间杂种优势和细胞质效应,为棉花耐冷杂交育种提供理论依据.采用随机区组设计,对由4个不同耐冷级别的棉花材料配制的4对正反交组合F1进行株高、茎粗、出苗至开花天数、露地越冬指数和茎表皮耐冷生理生化指标等的杂种优势和细胞质效应分析.结果表明:①越冬耐冷性相关性状的相关分析表明,露地越冬指数与茎表皮电导率、丙二醛(MDA)含量、出苗至开花天数显著相关,出苗至开花天数这一生育期指标与电导率、茎粗极显著相关.(②二年生陆海杂种F1的耐冷性具备超亲优势,一年生陆海杂种F1的耐冷性只具有中亲优势而不具备超亲优势,同时二年生材料的耐冷性强于一年生,说明陆海杂种F1二年生栽培更有优势.③一二年生棉花耐冷性的细胞质效应均不显著,但存在一定的核质互作效应,且在不同的正反交陆海杂种F1组合中具有多样化特征. 相似文献
998.
在陕西省关中土娄土区布置线辣椒、甘蓝、白菜、芹菜钾肥大田试验,研究不同蔬菜对钾肥的肥效反应。结果表明,增施钾肥可使蔬菜硝态氮含量降低11.8%~116.0%,维生素C含量增加0.4~147.4mg/kg;产量增加9.3%~33.8%;明显促进营养生长和生殖生长;400cm深土壤剖面硝态氮累积量减少22.7~48.0kg/hm2,尤其在80~220cm剖面减少最为明显。 相似文献
999.
1000.
摩擦耗能支撑减震技术是一种行之有效的减震措施,本文利用ANSYS分析了斜撑刚度对结构减震效果的影响,得出对于本文算例斜撑刚度与结构刚度的比值为2-6时,减震效果最好。 相似文献