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   723篇
  免费   29篇
  国内免费   73篇
  2024年   3篇
  2023年   13篇
  2022年   23篇
  2021年   18篇
  2020年   27篇
  2019年   21篇
  2018年   27篇
  2017年   30篇
  2016年   20篇
  2015年   24篇
  2014年   23篇
  2013年   33篇
  2012年   34篇
  2011年   35篇
  2010年   24篇
  2009年   32篇
  2008年   48篇
  2007年   53篇
  2006年   29篇
  2005年   42篇
  2004年   39篇
  2003年   22篇
  2002年   20篇
  2001年   6篇
  2000年   20篇
  1999年   12篇
  1998年   17篇
  1997年   11篇
  1996年   19篇
  1995年   8篇
  1994年   13篇
  1993年   7篇
  1992年   10篇
  1991年   11篇
  1990年   7篇
  1989年   10篇
  1988年   5篇
  1987年   4篇
  1986年   6篇
  1985年   5篇
  1984年   5篇
  1982年   1篇
  1981年   1篇
  1979年   1篇
  1978年   1篇
  1977年   1篇
  1972年   1篇
  1969年   1篇
  1968年   1篇
  1963年   1篇
排序方式: 共有825条查询结果,搜索用时 15 毫秒
51.
The collection of biological samples through non-invasive techniques represents one way of monitoring in vivo physiological changes associated with reproductive activity. Such techniques are particularly important for the study of animal species in the wild.The goals of this study were 1) to evaluate fecal progestogen (P), estrogen (E), and androgen (A) by means of radioimmunoassays, in male and female wild boars culled in the Piedmont, Italy area; 2) to compare them with plasmatic concentrations and the animals’ reproductive status; and 3) to assess variations in reproductive seasonality between two populations of wild boars living in a mountainous vs. a plain habitat in Piedmont.The results demonstrate a positive correlation between fecal and plasmatic steroid concentrations (r = 0.46, 0.58, and 0.45 for plasma P4 and P, E2 and E, and T and A; P < 0.05). Moreover, high fecal levels of both P and E (>170 ng/g and >100 pg/g respectively) were found in 70.6% of pregnant sows and in none of the non-pregnant animals, thus supporting the use of this technique for detecting pregnancy status in wild boar.Similar birth patterns were displayed by the mountain and plain populations, but births peaked significantly only in the mountain population, in the spring (46%, P < 0.05, vs. other seasons). A corresponding autumnal peak of plasma testosterone concentrations in males was displayed only by the mountain population (7.4 vs. < 2.0 ng/mL in the other seasons, P < 0.05).The correlation between fecal and plasmatic steroid concentrations obtained in this study supports the applicability of this non-invasive sampling technique for monitoring reproductive status in wild boar, thus enabling a more informed and correct management of the species.  相似文献   
52.
53.
A simple tissue culture protocol was developed for efficient plant regeneration from young inflorescence-derived calli in wild barley, Hordeum brevisubulatum (Trin.) Link, an important pasturage grass. Genetic and epigenetic instabilities in the regenerated plants (regenerants) were assessed by three molecular markers AFLP, S-SAP and MSAP. Two pools of calli derived from young inflorescences of a single donor plant and 44 randomly chosen regenerants were subjected to AFLP analysis. Results showed that 74 out of 793 scored bands were polymorphic among the studied samples, giving rise to a genetic variation frequency of 9.3%. The number of variant bands as compared to the donor plant varied greatly among the regenerants, with a small number of regenerants accumulated a large number of variant bands (maximum 55), while the majority of regenerants showed only 2–3 variant bands. A subset of regenerants together with the two pools of calli were selected for S-SAP and MSAP analysis to detect possible retrotranspositional activity of a prominent retroelement family, BARE-1, in the genomes of Hordem species, and possible alterations in cytosine methylation. S-SAP analysis showed that of the 768 scored bands, 151 were polymorphic among the analyzed samples, giving rise to a genetic variation frequency of 19.7%, albeit no evidence for retrotranspositional event was obtained based on locus-specific PCR amplifications. MSAP analysis revealed that tissue culture has caused cytosine methylation alterations in both level and pattern compared with the donor plant. Sequencing of selected variant bands indicated that both protein-coding genes and transposon/retrotransposons were underlying the genetic and epigenetic variations. Correlation analysis of the genetic and epigenetic instabilities indicated that there existed a significant correlation between MSAP and S-SAP (r = 0.8118, 1,000 permutations, P < 0.05), whereas the correlation between MSAP and AFLP (r = 0.1048) is not statistically significant. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. Xiaoling Li and Xiaoming Yu contributed equally to this work.  相似文献   
54.
鄂尔多斯高原蒙古族食用野生植物传统知识的研究   总被引:2,自引:0,他引:2  
运用民族植物学"5W+1H"提问法,结合访谈方法和野外调查进行编目,研究了鄂尔多斯高原蒙古族食用野生植物的传统知识。结果:1)鄂尔多斯蒙古族传统食用植物22科32属50种,其中野生粮用植物14种、野生蔬菜13种、野果植物18种、野果酒植物3种、野生调味植物5种、野生薰净植物2种、催化乳制品植物1种。2)鄂尔多斯蒙古族野生食用植物传统知识包括7个方面,其中用植物薰净和消毒奶食器具的传统知识、用冷蒿催化加速油脂成分从奶子中分离的传统知识是蒙古族植物饮食文化的创新。具有鲜明的民族特色、简单、快捷的"果实+奶食"酿果酒文化,能够说明蒙古族祖先在生产实践中掌握了开发和利用植物资源的更高水平的技术和文化。食用芨芨草颖果、选用中间锦鸡儿韧皮部和白沙蒿的果实做面食粘合剂的传统知识是区域特色的文化。3)因生态环境条件的不同,山区、沙区、盐渍区和丘陵区蒙古族牧民的传统食用植物知识表现出多样性,反映了生态环境条件的多样性。4)鄂尔多斯蒙古族野生食用植物传统知识丰富了蒙古族饮食文化,对蒙古族文化的发展做出了贡献。开展搜集、研究、保护和科学分析鄂尔多斯蒙古族植物传统知识的工作,将对社会经济的可持续发展产生十分积极的直接影响。  相似文献   
55.
西双版纳傣族利用野生蔬菜种类变化及原因分析   总被引:1,自引:0,他引:1  
运用民族植物学的方法,选择西双版纳自然及社会经济发展不同的三个傣族村寨曼安、曼伞和曼广囡为研究对象,调查当地村民利用野生蔬菜情况,并探讨利用种类变化的原因。三个傣族村寨村民利用的野生蔬菜有228种,分属于75个科,其中曼安村寨147种,曼伞村寨144种,曼广囡村寨105种。通过Multivariate方差分析(P<0.05)表明:不同村寨的村民平均每人提及的野生蔬菜物种数存在显著差异;年轻组平均每人提及的野生蔬菜物种数明显低于年长组,不同性别之间差异不显著;野生蔬菜知识的流失在男性之间比女性更为严重。最后对影响野生蔬菜利用的因素,野生蔬菜利用变化的原因以及传统野生蔬菜知识的流失进行了分析,就如何保护传统知识进行了探讨。  相似文献   
56.
Seven populations of Hordelymus europaeus and four populations of Leymus arenarius from Poland were subjected to examination of 36 morphological characters. This study showed that both species are relatively uniform and that morphological variation of their populations represents a continuum. Of those, three populations of either species were selected for analysis with molecular markers – RAPDs and AFLPs. These populations differed with respect of geographical location as well as syntaxa and habitat. RAPD-PCR was performed for individual plants and clearly grouped them according to the population origin. For either H. europaeus or L. arenarius - the studied populations differed in degree of their intrinsic variation while none of them as a whole was significantly different from the remaining ones. In AFLP analysis the studied populations were represented by DNA pools of several individual plants. Also this approach allowed discrimination among the population samples of both H. europaeus and L. arenarius. Both RAPDs and AFLPs were accordant in indication that H. europaeus exceeds L. arenarius with respect to variation accumulated at the DNA level. It is the sixth paper of the series: Biodiversity of wild Triticeae (Poaceae) in Poland. The first is: M. Mizianty (2005). Variability and structure of natural populations of Elymus caninus (L.) L. based on morphology. Pl. Syst. Evol. 251: 199-216. The second: M. Mizianty, and M. Szklarczyk (2005). Systematic significance of Elymus caninus morphological characters revealed by AFLP analysis. In: L. Frey (ed.) Biology of grasses. W. Szafer Institute of Botany, Polish Academy of Sciences. Kraków, pp 9–21. The third: M. Mizianty et al. (2006). Variability and structure of natural populations of Elymus caninus (L.) L. and their possible relationship with Hordelymus europaeus (L.) Jess. ex Harz revealed by AFLP analysis. Pl. Syst. Evol. 256: 193–200. The fourth: M. Mizianty (2006). Variability and structure of natural populations of Hordeum murinum L. based on morphology. Pl. Syst. Evol. 261: 139–150. The fifth: B. Paszko Variability and structure of natural populations of Brachypodium pinnatum and B. sylvaticum based on morphology. Acta Soc. Bot. Pol. (in press).  相似文献   
57.
Plant communities around natural CO2 springs have been exposed to elevated CO2 levels over many generations and give us a unique opportunity to investigate the effects of long-term elevated CO2 levels on wild plants. We searched for natural CO2 springs in cool temperate climate regions in Japan and found three springs that were suitable for studying long-term responses of plants to elevated levels of CO2: Ryuzin-numa, Yuno-kawa and Nyuu. At these CO2 springs, the surrounding air was at high CO2 concentration with no toxic gas emissions throughout the growth season, and there was natural vegetation around the springs. At each site, high-CO2 (HC) and low-CO2 (LC) plots were established, and three dominant species at the shrub layers were used for physiological analyses. Although the microenvironments were different among the springs, dicotyledonous species growing at the HC plots tended to have more starch and less nitrogen per unit dry mass in the leaves than those growing at the LC plots. In contrast, monocotyledonous species growing in the HC and LC plots had similar starch and nitrogen concentrations. Photosynthetic rates at the mean growth CO2 concentration were higher in HC plants than LC plants, but photosynthetic rates at a common CO2 concentration were lower in HC plants. Efficiency of water and nitrogen use of leaves at growth CO2 concentration was greatly increased in HC plants. These results suggest that natural plants growing in elevated CO2 levels under cool temperate climate conditions have down-regulated their photosynthetic capacity but that they increased photosynthetic rates and resource use efficiencies due to the direct effect of elevated CO2 concentration.  相似文献   
58.
A scabies outbreak, with unusually high mortality and morbidity, occurred in a poorly managed goat flock in north-eastern Italy. The source of infection was traced back to contact with scabietic chamois (Rupicapra rupicapra) within a recognised sylvatic focus of the disease. Spill-over of scabies from chamois to goats is a matter of great concern to resource managers. Currently scabies-free populations of wild Caprinae in the central and western Alps may eventually be exposed to infected goats transported over long distances. Sanitation of the studied flock was obtained following three applications of pour-on moxidectin at an approximate dose of 500 mcg/kg. Follow-up included titration of circulating anti-Sarcoptes antibodies in a labelled avidin–biotin ELISA test. Two operators developed pseudoscabies after handling the infested flock.  相似文献   
59.
Full analysis of eight seed samples collected in the 1960's excavations at Neolithic Çatalhöyük East, Turkey, is presented. Detailed investigation of the composition and context of the samples suggests that the Neolithic population collected, processed and stored seeds from Capsella sp. and Descurainia sp. (wild crucifers) for food use. In addition seeds of Vicia/Lathyrus sp. (wild vetch), Helianthemum spp. and Taeniatherum caput-medusae mixed with Eremopyrum type (grasses) were also found, some of which may have been used for food or other purposes. The analysis demonstrates that wild seed exploitation was a regular part of subsistence practice alongside the economic staple of crop production, and again demonstrates how diverse plant use practices were at the site.  相似文献   
60.
为了解浙江清凉峰国家级自然保护区内野猪(Sus scrofa)的分布及活动节律,2018年3月至2019年2月,我们利用红外相机技术对其进行了网格化监测。在千顷塘和龙塘山区域总计布设109个红外相机监测点,共计调查39 240个相机日。结果表明,在千顷塘区域,野猪倾向于在落叶阔叶林(拍摄率5.33 ±5.64)和针阔混交林(3.75 ±3.46)活动,而在龙塘山区域,则倾向于针阔混交林(4.32 ±5.21);千顷塘与龙塘山区域野猪各月间的日活动差异指数α均存在极显著差异(P < 0.01),冬季的α值高于其他季节;昼行性指数β的平均值分别为0.870和0.768,显著高于理论值13/24(P < 0.01),表明清凉峰的野猪种群具有明显的昼行性特征,且在冬季的活动时间分配较之其他季节更不均匀。通过核密度估计法分析不同季节野猪的日活动节律发现,保护区内野猪春季有晨昏两个活动高峰(08:00-10:00、17:00-19:00),冬季活动高峰出现在午后(13:00-17:00),其他季节无明显活动高峰。本研究有助于加深对野猪华南亚种生态习性的认知,为保护区的野生动物管理工作提供科学依据。  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