首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   2884篇
  免费   708篇
  国内免费   172篇
  2024年   10篇
  2023年   100篇
  2022年   75篇
  2021年   147篇
  2020年   217篇
  2019年   224篇
  2018年   157篇
  2017年   213篇
  2016年   221篇
  2015年   224篇
  2014年   197篇
  2013年   257篇
  2012年   163篇
  2011年   169篇
  2010年   133篇
  2009年   183篇
  2008年   168篇
  2007年   161篇
  2006年   183篇
  2005年   135篇
  2004年   100篇
  2003年   80篇
  2002年   58篇
  2001年   38篇
  2000年   33篇
  1999年   34篇
  1998年   9篇
  1997年   11篇
  1996年   8篇
  1995年   8篇
  1994年   4篇
  1993年   10篇
  1992年   3篇
  1991年   4篇
  1990年   5篇
  1989年   1篇
  1988年   2篇
  1987年   2篇
  1986年   1篇
  1985年   3篇
  1984年   3篇
  1983年   2篇
  1982年   3篇
  1980年   2篇
  1979年   1篇
  1977年   1篇
  1958年   1篇
排序方式: 共有3764条查询结果,搜索用时 140 毫秒
121.
In addition to their impact on natural habitats, invasive alien plants can have a significant negative effect on agricultural systems and cause economic losses. Flood‐irrigated orchards in the Mediterranean Basin are vulnerable to the invasion of alien weeds, primarily because of the traditional management practices used in the orchards, which are characterized by high soil moisture during the dry summer period, nutrient availability and high levels of disturbance. This study sought to determine whether their biological traits can explain the success of alien weed species. To answer this question, 408 floristic relevés were conducted in 136 flood‐irrigated orchards on the Plains of Lleida (Catalonia, NE of Spain). Richness and cover of native and alien weeds were compared. Furthermore, a set of biological traits were compared between successful and non‐successful weeds for the whole data and separately between native and alien weeds using logistic regression and classification trees. In flood‐irrigated orchards, alien species covered most of their area, even though the richness of alien species was lower than that of the native species. The most important species were C4 species with seeds dispersed by water, and on the other hand, rosulate and caespitose‐reptant hemicryptophytes with long flowering period. Most of these traits fitted with those of the invasive alien weeds, which were mostly C4 species with seeds dispersed by water. Perennial life form characterized successful native weeds. In this study, we discuss how the traditional management of flood irrigation in fruit‐tree orchards favours invasive alien weeds that have specific traits, acting as a reservoir for the spread of alien weeds into other crops and surrounding riparian habitats. We also propose changing management practices in order to avoid the selection of alien weeds and to promote native species.  相似文献   
122.
123.
水分胁迫和杀真菌剂对黄顶菊生长和抗旱性的影响   总被引:1,自引:0,他引:1  
利用盆栽试验研究水分胁迫下AM真菌对黄顶菊生长和抗旱性的影响,揭示黄顶菊入侵过程中的微生物学机制。以苯菌灵为杀真菌剂,在土壤相对含水量为120%、80%、40%和20%条件下,分别设灭菌和不灭菌两种处理。结果表明,水分胁迫显著降低了黄顶菊株高、干重和主根长,而对AM真菌侵染率无显著影响。施用苯菌灵显著降低了菌根侵染率、叶片保水力、保护酶活性、可溶性糖和可溶性蛋白含量,提高了MDA含量。不灭菌处理下黄顶菊植株对土壤有效N和有效P的利用率较高,且植株全N、P含量显著高于灭菌处理,菌根贡献率随土壤相对含水量降低而逐渐提高,重度胁迫分别是渍水条件下的1.84和1.88倍。土壤水分状况和AM真菌的交互作用对黄顶菊生物量和生理指标影响显著。AM真菌共生能够促进黄顶菊根系对土壤水分和矿质营养吸收,改善植物代谢活动,提高抗旱性。实验结果为黄顶菊合理防控措施的制定提供了依据,同时作为丛枝菌根的基础性研究也具有重要的意义。  相似文献   
124.
An audit of the screening history of all new cervical cancer cases has been a requirement since April 2007. While NHS cervical screening programmes (NHSCSP) guidance requires that women diagnosed with cervical cancer are offered the findings of the audit, as yet there has been no research to investigate the psychological impact that meeting to discuss the findings might have on patients. This is in spite of the fact that cytological under‐call may play a role in as many as 20% of cervical cancer cases. This review draws on the literature concerning breaking bad news, discussing cancer and disclosing medical errors, in order to gain insight into both the negative and positive consequences that may accompany a cervical screening review meeting. We conclude that while patients are likely to experience some distress at disclosure, there are also likely to be positive aspects, such as greater trust and improved perception of care.  相似文献   
125.
Abstract

The larval parasitoid Asecodes hispinarum Bou?ek has been used to control the coconut hispine beetle, Brontispa longissima (Gestro) in Vietnam. Although A. hispinarum has succeeded in suppressing B. longissima in southern Vietnam, biocontrol has not succeeded in central Vietnam. One factor underlying this failure might be the high temperature during the hot season. In the present work, we examined whether A. hispinarum parasitises the beetles within the range of temperatures that occur during the hot season in central Vietnam and we evaluated the potential use of the pupal parasitoid Tetrastichus brontispae Ferrière. Development of the beetle and two parasitoids was investigated at two constant temperatures (28 and 30°C) and at fluctuating temperatures from 25 to 35°C (corresponding to temperatures during the hot season in central Vietnam). B. longissima reached adulthood in all the temperature treatments. More than 70% of A. hispinarum emerged from hosts at 28°C, but none emerged at 30°C or under fluctuating temperature conditions, indicating that A. hispinarum cannot be used as a biological control agent in central Vietnam. However, T. brontispae could reproduce the next generation at all temperatures. These results suggest that T. brontispae is a potential agent for continuous biological control of B. longissima in central Vietnam.  相似文献   
126.
For maintaining social and financial support for eradication programs of invasive species, quantitative assessment of recovery of native species or ecosystems is important because it provides a measurable parameter of success. However, setting a concrete goal for recovery is often difficult owing to lack of information prior to the introduction of invaders. Here, we present a novel approach to evaluate the achievement level of invasive predator management based on the carrying capacity of endangered species estimated using long‐term monitoring data. In Amami‐Oshima Island, Japan, where the eradication project of introduced small Indian mongoose is ongoing since 2000, we surveyed the population densities of four endangered species threatened by the mongoose (Amami rabbit, the Otton frog, Amami tip‐nosed frog, and Amami Ishikawa's frog) at four time points ranging from 2003 to 2011. We estimated the carrying capacities of these species using the logistic growth model combined with the effects of mongoose predation and environmental heterogeneity. All species showed clear tendencies toward increasing their density in line with decreased mongoose density, and they exhibited density‐dependent population growth. The estimated carrying capacities of three endangered species had small confidence intervals enough to measure recovery levels by the mongoose management. The population density of each endangered species has recovered to the level of the carrying capacity at about 20–40% of all sites, whereas no individuals were observed at more than 25% of all sites. We propose that the present approach involving appropriate monitoring data of native organism populations will be widely applicable to various eradication projects and provide unambiguous goals for management of invasive species.  相似文献   
127.
128.
目的:探讨小切口微创手术治疗新鲜跟腱断裂的一晦床价值。方法:选取我院新鲜闭合性跟腱断裂患者50例,随机分为实验组和对照组各25例。实验组行小切口手术,对照组行常规切口手术。术后对患者进行随访,采用美国足踝协会(AOFAS)推荐的评分标准对患者术后功能恢复情况进行评价,观察并记录完全恢复患者例数、完全恢复时间、小腿最大周长差和术后并发症发生情况。结果:实验组AOFAS评分为(98.6±9.7)分,痊愈率96.00%,痊愈时间(20.2±3.2)周,两侧小腿最大周长差为(0.79+0.68)cm,共有1例患者出现并发症,并发症发生率8%;而对照组的AOFAS评分为(91.4±11.5)分,痊愈率92.00%,痊愈时间(22.4±3.8)周,两侧小腿最大周长差为(0.91~0.76)cm;共有6例患者出现并发症,并发症的发生率为24%。两组患者的痊愈率、两侧小腿最大周长差比较差异无统计学意义(痊愈率:x2=-0.355,P=0.552;侧小腿最大周长差:t=O.588,P=0.559);而与对照组比较,实验组AOFAS评分明显升高,完全恢复时间明显缩短,术后并发症的发生率显著降低,差异均有统计学意义(AOFAS评:t=2.393,P=0.021;恢复时间:t=2.150,P=0.037;并发症发生率:xⅫ.153,P=0.042)。结论:小切口手术与常规切口手术治疗新鲜跟腱断裂的疗效相当,但小切口手术术后恢童时间曼短.并发症更少.临床价值相对更高.  相似文献   
129.
目的:分析并总结食管破裂的患者急诊住院后经过系统、科学、快速的护理手段和临床治疗,及使用特殊的治疗方法(借助电子胃镜行食管破裂的修补治疗)得到救治的过程中的护理体会.方法:入院后急检血常规、血型、凝血象、生化系列等相关检查;给予禁食水,重症监护,对症处理,营养支持,胸腔闭式引流术排出引流液,应用敏感抗生素等治疗方法;给予科学的严密护理(生命体征的监测护理、严密观察体温的变化及对症处理、胃管的护理、胸腔闭式引流管的护理、心里支持的护理).结果:通过上述方法,患者在无明显并发症发生的情况下,最终在较短的时间内顺利痊愈出院.结论:与常规开胸下行食管破裂修补术治疗食管破裂的患者相比,在电子胃镜下行食管破裂修补术拥有治疗时间短,恢复快,住院时间短,患者医疗费降低等优点.  相似文献   
130.
目的:探讨严重胫骨远端骨折的临床治疗方案。方法:2008年2月至2011年2月我科治疗的106例严重胫骨远端骨折患者,84例急诊应用封闭负压引流术(vacuum sealing drainage,VSD)联合内固定治疗胫骨远端骨折(观察组),22例延期应用内固定治疗胫骨远端骨折(对照组),比较两组的住院天数、住院费用、术后并发症、术后疼痛程度评分。结果:两组在住院天数、住院费用的比较差异无统计学意义(P〉0.05)。观察组术后出现2例伤口裂开,疼痛程度评分为:(2.31±1.37)分,对照组出现3例伤口感染并裂开,疼痛程度评分为:(4.09±2.34)分。术后并发症、术后疼痛程度评分的比较差异有统计学意义(P〈0.05)。结论:急诊应用VSD联合内固定治疗胫骨远端骨折不增加患者的治疗费用,减少患者的痛苦,在临床上值得推广应用。  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号