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   378篇
  免费   26篇
  国内免费   30篇
  2023年   2篇
  2022年   8篇
  2021年   17篇
  2020年   9篇
  2019年   11篇
  2018年   3篇
  2017年   7篇
  2016年   14篇
  2015年   15篇
  2014年   13篇
  2013年   20篇
  2012年   19篇
  2011年   19篇
  2010年   14篇
  2009年   21篇
  2008年   16篇
  2007年   17篇
  2006年   24篇
  2005年   12篇
  2004年   20篇
  2003年   18篇
  2002年   14篇
  2001年   5篇
  2000年   6篇
  1999年   7篇
  1998年   7篇
  1997年   11篇
  1996年   5篇
  1995年   4篇
  1994年   2篇
  1993年   6篇
  1992年   3篇
  1991年   5篇
  1989年   3篇
  1988年   6篇
  1987年   3篇
  1986年   4篇
  1985年   2篇
  1984年   4篇
  1983年   1篇
  1982年   2篇
  1980年   8篇
  1979年   9篇
  1978年   6篇
  1977年   7篇
  1976年   3篇
  1975年   1篇
  1973年   1篇
排序方式: 共有434条查询结果,搜索用时 15 毫秒
1.
In order to elucidate the factors affecting the genetic diversity of Quercus serrata in secondary forests in mountainous regions, we evaluated the level and distribution of genetic variation within and between 15 populations using seven microsatellite markers. The populations were at altitudes ranging from 140 to 1200 m in and around the Chichibu Mountains, central Japan.The expected heterozygosity (HE) ranged from 0.766 to 0.837. The two populations that exhibited the highest and the second highest values of HE are located beside a river and a lake, respectively. The two populations exhibiting the lowest and the second lowest values of HE are, in contrast, located on a summit and a ridge. The observed heterozygosity (HO) varied between 0.638 and 0.844, and the value of this variable was also higher for the populations beside water than those on summits or ridges. The soils at the waterside are wet, in contrast to those on ridges and summits, which tend to be shallow and subject to rapid desiccation. These results suggest that a lack of soil moisture is likely to inhibit the development and regeneration of Q. serrata, and that genetic diversity is reduced in arid areas. The genetic differentiation was low (FST=0.013) among the investigated populations, although all five populations in Yamanashi prefecture clustered together in an UPGMA tree. According to a multiple regression analysis, there was no significant isolation by distance among the populations along either the horizontal or vertical axes. Therefore, genetic variation within populations is affected by topography, but variation between populations is hardly affected by geographical factors. Furthermore, the results of this study suggest two conclusions. First, that altitude is not always a useful variable when estimating the genetic diversity of plant populations in mountainous regions. Second, that genetic diversity can vary even among the undifferentiated plant populations in small areas like the Chichibu Mountains.  相似文献   
2.
Macroalgae harbour specific microbial communities on their surface that have functions related to host health and defence. In this study, the bacterial biofilm of the marine brown alga Fucus spiralis was investigated using 16S rRNA gene amplicon-based analysis and isolation of bacteria. Rhodobacteraceae (Alphaproteobacteria) were the predominant family constituting 23% of the epibacterial community. At the genus level, Sulfitobacter, Loktanella, Octadecabacter and a previously undescribed cluster were most abundant, and together they comprised 89% of the Rhodobacteraceae. Supported by a specific PCR approach, 23 different Rhodobacteraceae-affiliated strains were isolated from the surface of F. spiralis, which belonged to 12 established and three new genera. For seven strains, closely related sequences were detected in the 16S rRNA gene dataset. Growth experiments with substrates known to be produced by Fucus spp. showed that all of them were consumed by at least three strains, and vitamin B12 was produced by 70% of the isolates. Since growth of F. spiralis depends on B12 supplementation, bacteria may provide the alga with this vitamin. Most strains produced siderophores, which can enhance algal growth under iron-deficient conditions. Inhibiting properties against other bacteria were only observed when F. spiralis material was present in the medium. Thus, the physiological properties of the isolates indicated adaption to an epiphytic lifestyle.  相似文献   
3.
4.
The display of personal photographs in hospital is a common practice that has yet to be rigorously examined. The photographs displayed are subject to interpretation by the viewer and may lead to misunderstandings or miscommunication if clarification of meaning is not sought. This paper explores a range of possible meanings that the display of photographs in hospital may hold, based on a case study of a 15 year old boy hospitalised with a life threatening illness. Further research is needed into the actual meanings attributed to the display of photographs in hospital by patients and family members.  相似文献   
5.
Fear is a concept comprising several dimensions, but the nature of these dimensions and the relationships between them remain elusive. To investigate these dimensions in birds, we have used two genetic lines of quail divergently selected on tonic immobility duration, a behavioural index of fear. These two lines differ in their behavioural response to some, but not all, fear-inducing situations. In the present study, we investigated the contribution of human intervention in the differentiation between the two lines. To do this, fear responses towards a novel object were compared between lines in three conditions: (1) in the home cage without any human intervention, (2) in the home cage after human handling and (3) after placement in a novel environment by human handling. Fear behaviour differed between lines after human handling, with or without placement in a novel environment, but presentation of a novel object in the home cage without any human intervention induced similar fear responses in the two lines of quail. These results lead us to suggest that in quail, human intervention evokes a dimension of fear that differs from that evoked by sudden presentation of a novel object, in that these two dimensions may be selected independently.  相似文献   
6.
全球环境基金(GEF)作为《生物多样性公约》财务机制的运行主体, 已在全球范围内实施了7个周期, 各国在执行GEF项目期间, 遇到了可持续性不强、项目设计方案复杂、期望过高等挑战。作为GEF的国际实施机构, 联合国开发计划署(UNDP)与中国政府合作, 针对各项挑战, 采取综合应对措施, 优化设计与实施的生物多样性项目取得了系列成就, 这些项目的实施是落实爱知生物多样性目标和《2011-2020年生物多样性战略计划》的有效实践并提供了成功范例。这些应对策略与成就包括: (1)撬动了中国政府5-10倍的配套资金用于生物多样性相关工作, 推动生物多样性在中国各级政府的主流化, 使得财务、制度和环境的可持续得以实现。项目贡献于国家宏观战略规划和五年计划的制订、完善法律法规、探索融资机制、推进政策导向等方面, 并将生物多样性纳入国家重要议程, 使生态文明的理念发挥政策引领作用。(2)项目从设计到执行, 充分征询利益相关者的意见, 目标的设定明智, 确保了项目的实施成效。(3)借鉴相关国家成功经验, 结合中国实际开展了诸多创新和示范。(4)提供培训、教育和广泛交流, 综合提升了生物多样性领域相关机构和人员的能力, 对于增强技术、管理、协调、协作等领域的软实力和巧实力发挥了桥梁和纽带作用。(5)利用各种窗口和平台, 广泛开展宣传推广、研讨交流等, 提高了公众对生物多样性的认知, 为长期践行可持续发展战略奠定群众基础。本文结合UNDP作为国际实施机构与相关部委和省级及地方政府共同实施的GEF生物多样性项目实践, 总结了项目的成就与经验, 期望为全球相关项目设计和实施以及2020年后全球生物多样性框架的制定提供借鉴, 以共同实现“到2050年与自然和谐相处”的愿景。  相似文献   
7.
污染环境中可再生资源的最优收获问题   总被引:10,自引:2,他引:8  
探讨了污染环境下双种群的最优收获问题.利用Pontryagin极大值原理得到一种最优分配方案——处理资源种群体内毒素的努力度与收获资源种群的努力度的分配方案,使经营者的经济收入达到最大,同时也得到次最优均衡解。  相似文献   
8.
绿尾虹雉生态学研究   总被引:16,自引:0,他引:16  
绿尾虹雉Lophophorus lhuysti生态和生物学研究是中国科学院重点科研课题——“中国珍稀濒危难类生态生物学”——部分工作。 本文为作者对绿尾虹雉所作的生态和生物学系统研究首次报道。绿尾虹雉主要牺息于海拔3,500—4,000米亚高山灌丛和草甸。杂食性鸟类,以植物性食物为主,亦食昆虫。营巢于岩洞或灌丛地面上,每窝产3—4,多达11枚卵。种群密度1.32—1.58只/km~2。目前数量急剧下降,处于濒危,急待保护。  相似文献   
9.
Saccharomyces cerevisiae, grown aerobically or anaerobically under conditions which induce a requirement for a sterol and an unsaturated fatty acid, synthesized approximately the same amounts of neutral lipid and intracellular low-density vesicles, although the neutral lipids in aerobically-grown cells contained more esterified sterol and less triacylglycerol than those in anaerobically-grown cells. Kluyveromyces fragilis synthesized much less neutral lipid and a smaller quantity of low-density vesicles than S. cerevisiae whether grown at 30°C (generation time 1.1 h) or 20°C (generation time 2.1 h). Both yeasts synthesized highly saturated triacylglycerols, relatively unsaturated phospholipids, and esterified sterols with an intermediate degree of unsaturation irrespective of the conditions under which they were grown. Free sterols in the yeasts were rich in ergosterol and 22(24)-dehydroergosterol, while the esterified sterol fractions were richer in zymosterol.  相似文献   
10.
毛宗铮   《广西植物》1989,9(1):1-11
我国银杉自1955年发现以来,陆续发现了它的新分布,至今已知在四省、区7个分布区的30多个分布点上,自然生长着银杉3200余株。它们是:广西花坪林区(越城岭南部)6个点有银杉1040抹,其中高5米以上的68株,1~5米的49株,1米以下的923株,最高21.1米,最大胸径83厘米;广西大瑶山4个点有银杉143株,其中高10米以上的94株,1~3米的3株,1米以下的46株,最高30米,最大胸径79.2厘米;湖南罗汉洞(越城岭北部)2个点有银杉58株,5米以上的有45株,最高18米,最大胸径43厘米;湖南罗霄山脉八面山有银杉707株,高1米以上的334株,最高24米,最大胸径46厘米;四川南川金佛山(大娄山东段北部)6个点有银杉527株,其中成年树400余株,最高17米,最大胸径50厘米;百枝山有银杉52株,最高8米,最大胸径27厘米;贵州道真沙河林区(大娄山东段南部)3处12个点有银杉729株,其中高5米以上的105株,2~5米的209株,1~2米的111株,1米以下的304株,最高18.8米,最大胸径48厘米;贵州桐梓白菁有银杉10株,高4~5米的2株,1~3米的6株,1米以下的2株。 银杉分布区范围自北纬24°5′~29°13′14″,东经107°10′~113°40′。其垂直分布自海拔940~1840米,金佛山银杉分布最高,为1600~1840米;罗汉洞银杉分布最低,为940~1060米。分布区内,年均温8.2°~17℃,绝对最低温  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