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   209篇
  免费   15篇
  国内免费   31篇
  2024年   2篇
  2023年   6篇
  2022年   8篇
  2021年   5篇
  2020年   7篇
  2019年   4篇
  2018年   8篇
  2017年   8篇
  2016年   14篇
  2015年   7篇
  2014年   13篇
  2013年   13篇
  2012年   12篇
  2011年   13篇
  2010年   3篇
  2009年   10篇
  2008年   11篇
  2007年   13篇
  2006年   20篇
  2005年   5篇
  2004年   4篇
  2003年   8篇
  2002年   7篇
  2001年   5篇
  2000年   3篇
  1999年   2篇
  1998年   2篇
  1997年   6篇
  1996年   1篇
  1995年   2篇
  1994年   5篇
  1993年   2篇
  1992年   1篇
  1991年   2篇
  1990年   2篇
  1989年   3篇
  1987年   2篇
  1986年   1篇
  1985年   3篇
  1984年   2篇
  1982年   2篇
  1980年   1篇
  1979年   2篇
  1978年   1篇
  1976年   2篇
  1963年   2篇
排序方式: 共有255条查询结果,搜索用时 78 毫秒
71.
During an investigation of type collections in The Natural History Museum, London, made in India during the nineteenth century, the syntypes of Cypris subglobosa Sowerby have been re-discovered. This species is shown to belong to the genus Paracypretta and to be confined to the Upper Cretaceous, and possibly Palaeocene, non-marine intertrappean deposits of the Indian Deccan Volcanic Province. The numerous and widespread Recent and Quaternary records of this taxon are of a separate species that belongs to the genus Cypris . The examination of comparative material from a number of localities in India reveals the presence of two other contemporary species of Paracypretta P. jonesi Bhatia and Rana and P. elizabethae sp. nov., which is formally described herein.  相似文献   
72.
A total of 1280 (670 females and 610 males) Squatina guggenheim , an angel shark endemic to the south-west Atlantic, was caught during five research cruises (1995, 1997 and 1998) carried out in the Argentine-Uruguayan Common Fishing Zone (AUCFZ) at depths of 3·5–266 m. All angel sharks captured were sexed and measured. Of the 947 examined for stomach contents, 556 (58·7%) contained food and 391 (41·3%) were empty. Numerically, bony fishes were the principal dietary component (89·7%), followed by crustaceans (4·8%) and molluscs (4·4%). The most important prey species were Engraulis anchoita (58·3%), shrimps (4·4%) and Ilex argentinus (2·3%). Male and female diets were not found to differ significantly ( P  > 0·01), however, the diets of different size classes showed significant differences ( P  ≤ 0·01) (group 1, 23–44, group 2, 45–74 and group 3, 75–91 cm L T). Low rates of cannibalism were observed. The spatial and seasonal variability in the trophodynamics of S. guggenheim is discussed.  相似文献   
73.
The present study investigates the chemical composition, antioxidant and antimicrobial bioactivities of essential oil and hexane extract from Citrus limon leaves. The isolation of essential oil was carried out using the Clevenger apparatus. The percentage yield of essential oil and hexane extract from Citrus limon leaves was 0.59 and 0.50 %, respectively. 2,2-Diphenyl-1-picrylhydrazyl (DPPH) scavenging assay highlighted that Citrus limon leaves essential oil (CLEO) and hexane extract exhibited the significant antioxidant potential of 69.64 and 67.55 %, respectively, compared to the BHT standard. Similarly, a significant inhibition in linoleic acid peroxidation was recorded in both CLEO (81.93 %) and hexane extract (50.34 %). Characterization of chemical constituents in CLEO and extract was executed using GC/MS, where Limonene was detected as a major compound in CLEO (60.52 %) and hexane extract (73.62 %). The haemolytic activity ranged from 2.46 to 5.75 % revealing negligible cytotoxicity of CLEO and hexane extract. In silico studies agree with the in vitro antimicrobial studies, where vinimalol, taraxasterol, and moretenol present in CLEO showed strong interactions/inhibition against dihydroorotase and DNA gyrase from E. coli, and the tyrosyl-tRNA synthetase and DNA gyrase from S. aureus. Based on the current data, it may be concluded that both CLEO and hexane extract possessed significant bioactivities, such as antimicrobial and antioxidant activity, with minimal cytotoxicity.  相似文献   
74.
Summary The Sasca zone situated in the innermost part of the Getic Domain from the South Carpathians comprises mainly Triassic deposits of Scythian-Anisian (?Ladinian) age that can be ascribed to four different members forming the Sasca Formation. Three of the members consist of carbonate deposits. Their study permitted a brief characterization of the main microfacies types, and especially in the Valea Susara Limestone Member the identification of a relatively rich association of foraminifers and calcareous algae. The assemblage withMeandrospira dinarica, Pilammina densa, Oligoporella pilosa andPoncetella hexaster identified in these limestones indicates a Middle Anisian age (Pelsonian-Lowermost Illyrian). Difficulties arise in differentiating between the forms belonging to theOligoporella-Physoporella group for which a taxonomic revision is necessary. The morphologic characteristics of the three varieties ofDiplopora subtilis allow a splitting into different species.Teutloporella peniculiformis Ott, 1963 is regarded as anomen nudum.  相似文献   
75.
二叠纪丑中虫科放射虫中有一类顶锥部分向腹边弯曲的放射虫,Ishiga称为Follicucullusbipartitus-F.charveti组合带,由于其形状奇特,层位稳定,已成为二叠纪较好的标准带化石,并与类Lepidolinakumaensis带对比。目前这类化石仅在东南亚地区的西南日本、菲律宾巴拉望地区和中国广西钦州地区发现表明,这些地区的放射虫当时很可能是在同一个洋盆中沉积的,只是由于它们各自的沉积历史不一样,以及后来的板块活动,造成了原始块体的离散、飘移,形成目前这种分布格局。这个放射虫动物群包括6科11属21种(比较种、未定种),其中有新种1个。这个动物群的时代要早于晚二叠世。  相似文献   
76.
Wu B  Isobe K  Ishii R 《Mycorrhiza》2004,14(6):391-395
Arbuscular mycorrhizal (AM) colonization was observed on four plant species in primary successional volcanic deserts on the Southeast slope of Mount Fuji. The AM colonization of the dominant species, Polygonum cuspidatum, contradicts the conclusion that Polygonaceae are often regarded as being non-mycorrhizal species. The secondary dominant species, Polygonum weyrichii var. alpinum, formed no mycorrhizas. The roots of Cirsium purpuratum, Clematis stans and Campanula punctata ssp. hondoensis, showed a higher percentage of AM colonization than P. cuspidatum. AM colonization and spore density in the rhizosphere soil of P. cuspidatum significantly decreased as elevation increased. AM colonization in roots of Cirsium purpuratum and Clematis stans also tended to decrease with increased altitudes. Cirsium purpuratum and Campanula punctata ssp. hondoensis formed single structural types of Arum- and Paris-type, respectively, whereas P. cuspidatum and Clematis stans formed both Arum- and Paris-type morphologies.  相似文献   
77.
A 14C-dated high-resolution palaeoenvironmental record from a mire in southern Chile is used to reconstruct the Late-glacial and Holocene vegetation history of the Magellanic rain forest. The Late-glacial environment after around 15400–13500 cal b.p. was dominated by Gunnera magellanica, Nothofagus species (dombeyi type) and Gramineae (Poaceae) indicating an open parkland with cool and damp climatic conditions. At the end of the Late-glacial there was an increase in G. magellanica and a decline in Nothofagus dombeyi type. This ecological signal is interpreted as a result of a re-advance of the Gran Campo Nevado icefield, caused by either Younger Dryas cooling or a latitudinal shift of the southern Westerlies. After around 11250–10750 cal b.p. Nothofagus species, Drimys winteri and Embothrium coccineum expanded, indicating development of the Magellanic rain forest. At 4254±120 cal b.p. a tephra layer was deposited by the eruption of the Mt. Burney volcano leading to a long-term decline of the Nothofagus forest. A primary succession was then initiated, lasting for over 800 years before pre-eruption vegetation patterns redeveloped. In summary, our results indicate the extreme sensitivity of the Magellanic rain forest to climatic or volcanic impacts and the slow recovery of a mature forest after environmental changes.  相似文献   
78.
Pérez  Francisco L. 《Plant Ecology》2001,157(2):215-233
The geoecological effects of organic matter addition to soil byHawaiian silverswords (Argyroxiphium sandwicense DC.,Asteraceae) in Haleakala's crater (Maui) are examined. Sets of 12 surfacesoil samples were gathered in three positions: bare soils; under the crowncanopy of adjacent live plants; and below dead, withered rosettes. Silverswordstanding litter and live foliage were also collected. Several physical andchemical properties were investigated. Bare soils were coarse and exceptionallylow in chemical nutrients and water-holding capacity. Litter addition belowsilverswords altered all properties except particle-size distribution; soilsunder dead rosettes became modified more strongly than those below live plants.While rosettes grow, standing litter is tightly preserved, thus only smallamounts of organic matter are added to soil. When plants die, a substantialinflux of litter is released in a short period (7 to 9 years average); thisraises the C and N content, C/N ratios, exchangeable nutrient levels (Ca, Mg,K, P) and cation exchange capacity of soil. Physical properties, such as color,bulk density and porosity are also greatly affected; as a result, soilwater-retention at field capacity nearly doubles under withering plants. Soiltemperatures (5 cm depth) at noon were 18.1°C lower beneath silversword canopies(18.5°C mean) than in contiguous bare soils(36.6°C). This cooling may result in lower evaporationand further conservation of soil moisture, thus increased seedgermination. Nutrient-retention strategies may have evolved in the semelparousArgyroxiphium to help it cope with the harsh,drought-prone soils and unpredictable climate of Haleakala. Silverswordflowering results in a tremendous seed output, but the plant needs to ensurethat some seeds – which have limited dispersal capability – willgerminate, and thus provides a nutrient-rich, water-retaining substrate moresuitable for seedling establishment than bare soils. Through itsself-regulating influence on underlying soils,Argyroxiphium affects both soil formation and its ownregeneration in Haleakala.  相似文献   
79.
赵兰坡  杨学明  刘伟 《应用生态学报》2003,14(10):1677-1680
通过电位滴定曲线探讨了长白山和五大连池火山灰土壤剖面的电荷零点(ZPC)特性。结果表明,两火山灰土壤既有一定量的可变电荷,又有一定量的永久电荷,其土壤胶体是可变电荷胶体和永久电荷胶体的混合体系,两土壤剖面各层ZPC均位于滴定曲线的酸方一侧,并有随深度增加而增大的趋势,但ZPC的位移(σi)则有随剖面深度的增加而减小的趋势,两剖面表层土壤ZPC均较国外的火山灰土壤低,显著高于中国西北地区的黄土质土壤和东北地区的黑土,ZPC和σi值的变化,既受亚洲大陆起源的广域风尘堆积的影响,又与土壤有机质积累有关,二者的综合作用是两地火山灰土壤ZPC降低和σi值增大的主要原因。  相似文献   
80.
Middle/late Devonian and early Carboniferous metasedimentary sequences in the northernmost region (Porto-Espinho-Tomar) of the Ossa-Morena Zone (Portuguese Iberian Variscan Massif) contain black shales of very low to low-grade metamorphism. These metasedimentary rocks form a discrete NNW-SSE structure within a major shear zone (Porto-Tomar-Ferreira do Alentejo) and remain subparallel to the observed regional major structures (folding, thrusts or overthrusts). These black shales are overhanged and then imbricated in an upper Proterozoic metamorphic substratum. A multi-disciplinary study of these metasedimentary rocks from the Espinho-Tomar region has tectonostratigraphy, palynology, organic petrology and clay mineralogy combined methods. This approach provides new insights into the tectonic evolution and geological framework of Palaeozoic basement of the Iberian Variscides. Palaeoenvironmental and tectonostratigraphic implications on the Iberian geodynamic framework are discussed.  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