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   172篇
  免费   19篇
  2022年   7篇
  2021年   6篇
  2020年   5篇
  2018年   5篇
  2017年   3篇
  2016年   4篇
  2015年   7篇
  2014年   6篇
  2013年   15篇
  2012年   7篇
  2011年   7篇
  2010年   14篇
  2009年   6篇
  2008年   5篇
  2007年   4篇
  2006年   2篇
  2005年   1篇
  2004年   6篇
  2003年   4篇
  2001年   3篇
  1999年   5篇
  1998年   5篇
  1997年   5篇
  1996年   4篇
  1995年   3篇
  1993年   2篇
  1992年   2篇
  1991年   3篇
  1990年   2篇
  1988年   2篇
  1987年   1篇
  1985年   1篇
  1982年   2篇
  1980年   2篇
  1978年   1篇
  1977年   1篇
  1975年   2篇
  1974年   2篇
  1973年   1篇
  1970年   1篇
  1968年   2篇
  1958年   5篇
  1957年   1篇
  1956年   2篇
  1955年   2篇
  1954年   2篇
  1953年   4篇
  1952年   4篇
  1951年   2篇
  1949年   1篇
排序方式: 共有191条查询结果,搜索用时 156 毫秒
51.
Jach, R., Machaniec, E. & Uchman, A. 2011: The trace fossil Nummipera eocenica from the Tatra Mountains, Poland: morphology and palaeoenvironmental implications. Lethaia, Vol. 45, pp. 342–355. The tubular trace fossil Nummipera eocenica Hölder 1989 occurs in a single stratigraphical horizon in Eocene nummulitic limestones of the Tatra Mountains, Poland. The wall of N. eocenica is built of Discocyclina and Nummulities (larger foraminifera) tests, very rarely of the Ditrupa (Polychaeta) tube fragments, bivalve shell fragments, echinoid spines and coralline algae. Morphotype are distinguished on the basis of wall composition and structure. Morphotype A is dominated by fusiform Discocyclina tests, which were preferentially selected by the trace makers for construction of a well‐constructed and resistant wall. Morphotype B contains more robust tests of Nummulites, while morphotype C is dominated by saddle‐shaped tests of Discocyclina. Nummipera eocenica was produced during a period of seafloor stabilization caused by a deepening. The succession of the morphotypes B, A reflects diminishing energy and increasing water depth. Probably morphotype C represents even lower energy environment than morphotype A. The trace fossil is interpreted as a domichnion, which wall was constructed for protection. The trace maker can be considered between polychaetes and crustaceans; however, comparisons to the closest recent analogues, the polychaete Diopatra cuprea or alpheid shrimps, are not satisfactory. □Bartonian, burrow, Carpathians, large foraminifera, trace fossils.  相似文献   
52.
Abstract: Among the new dental remains from the late Early Eocene of Chambi (Kasserine area, Tunisia) is a large‐sized upper molar of a new bat species, Witwatia sigei nov. sp. (Chiroptera, Vespertilionoidea, Philisidae), described herein. The locality of Chambi has revealed evidence for an early appearance of two modern microchiropteran superfamilies in Africa: Dizzya exsultans, a Philisidae, which is considered to be an archaic Vespertilionoidea, and an indeterminate Rhinolophoidea. In addition to D. exsultans, the new species, W. sigei, is the second representative of the Philisidae in this locality. W. sigei extends back to the late Early Eocene the occurrence of the genus Witwatia, which was previously only reported from the early Late Eocene of the Fayum (BQ‐2, Egypt). By analogy with the largest extant microbats, the large size of Witwatia suggests a tendency to the opportunistic diet of this taxon, thereby contrasting with the strict insectivory characterizing primitive bats found in other continents in the same epoch.  相似文献   
53.
Cell-penetrating peptides (CPPs) are membrane permeable vectors recognized for their intrinsic ability to gain access to the cell interior. The hydrophobic counter-anion, pyrenebutyrate, enhances cellular uptake of oligoarginine CPPs. To elucidate CPP uptake mechanisms, the effect of pyrenebutyrate on well-recognized CPPs with varying hydrophobicity and arginine content is investigated. The cellular CPP uptake and CPP-mediated oligonucleotide delivery is analyzed by fluorescence activated cell sorting, confocal microscopy, and a cell-based splice-switching assay. The splice-switching oligonucleotide is a mixmer of 2′-O-methyl RNA and locked nucleic acids delivered as a non-covalent complex with 10-fold molar CPP excess. CPP-induced membrane perturbation on large unilamellar vesicles is investigated in calcein release experiments. We observed that pyrenebutyrate facilitates cellular uptake and translocation of oligonucleotide mediated by oligoarginine nonamer while limited effect of pyrenebutyrate on more hydrophobic CPPs was observed. By combining the different experimental results we conclude that the pathway for cellular uptake of oligoarginine is dominated by direct membrane translocation, whereas the pathway for oligoarginine-mediated oligonucleotide translocation is dominated by endocytosis. Both mechanisms are promoted by pyrenebutyrate and we suggest that pyrenebutyrate has different sites of action for the two uptake and translocation mechanisms.  相似文献   
54.
Ginger is used as one of the important ingredients in traditional as well as modern medicine besides as a spice. It boosts immunity and is a rich source of many biologically active substances and minerals. Although it is a medicinally important crop, its productivity is, however, affected due to poor nutrient management and therefore it requires an adequate supply of nutrients in the form of inorganic fertilizers or organic manuring, or a mixture of both. In this context, the present study was aimed to investigate the effect of mineral fertilizers on the content of mineral elements in the ginger rhizome, on soil enzyme activity, and soil properties. Lysimeter experiments were conducted at the Institute of Genetics and Plant Experimental Biology, Kibray, Tashkent region, Uzbekistan. The experiment comprised of four treatments T1 – Control, T2 - N75P50K50 kg/ha, T3 - and T4 - N100P75K75 + B3Zn6Fe6 kg/ha. The results showed that the application of N125P100K100 kg/ha increased rhizome K content by 49%, P content by 20%, and Na content by 58% as compared to control without fertilizer. While the application of N100P75K75 + B3Zn6Fe6 kg/ha showed a significant enhancement in rhizome K, Ca, P, Mg, Na, Fe, Mn, Zn, Cu, Cr, Mo, and Si contents over the control. This treatment also improved active P content by 29%, total P content by 80%, total K content 16%, and N content by 33% content, and the activities of urease, invertase, and catalase activities as compared to control of without mineral fertilizer and control respectively. Thus the application of NPK + BZnFe at the rate of 100:75:75:3:6:6 kg/ha helps in improving macroelements and microelements in the ginger rhizome and activities of soil enzymes that helps in mineral nutrition of the rhizome.  相似文献   
55.
56.
57.
Protein-tyrosine phosphatases (PTPases) play key roles in regulating tyrosine phosphorylation levels in cells, yet the identity of their substrates remains limited. We report here on the identification of PTPases capable of dephosphorylating the phosphorylated immune tyrosine-based activation motifs present in the T cell receptor zeta subunit. To characterize these PTPases, we purified enzyme activities directed against the phosphorylated T cell receptor zeta subunit by a combination of anion and cation chromatography procedures. A novel ELISA-based PTPase assay was developed to rapidly screen protein fractions for enzyme activity following the various chromatography steps. We present data that SHP-1 and PTPH1 are present in highly enriched protein fractions that exhibit PTPase activities toward a tyrosine-phosphorylated TCR zeta substrate (specific activity ranging from 0.23 to 40 pmol/min/microg). We also used a protein-tyrosine phosphatase substrate-trapping library comprising the catalytic domains of 47 distinct protein-tyrosine phosphatases, representing almost all the tyrosine phosphatases identified in the human genome. PTPH1 was the predominant phosphatase capable of complexing phospho-zeta. Subsequent transfection assays indicated that SHP-1 and PTPH1 are the two principal PTPases capable of regulating the phosphorylation state of the TCR zeta ITAMs, with PTPH1 directly dephosphorylating zeta. This is the first reported demonstration that PTPH1 is a candidate PTPase capable of interacting with and dephosphorylating TCR zeta.  相似文献   
58.
The leaf temperature of six annual and six perennial plant species was monitored during spring and summer on a sand dune ecosystem in the delta Mediterranean coast of Egypt. During winter, leaves of all tested perennial species attained temperatures higher than the air temperature at night and shortly after sunrise, with maximum leaf–air temperature differences reaching up to 8°C. The lowest differences were less than 1°C. Around noon, the leaves of several species attained temperatures lower than that of the air whereas others showed higher temperatures. The opposite was true during summer, when leaf temperatures were lower than air temperature. The maximum leaf–air temperature differences occurred after midnight towards sunrise and reached up to 10°C. The lowest differences were found around noon and were of less than 5°C. The annual plant species have more pronounced variations than perennials in their leaf temperatures during the night and for most of the day. The leaves were heated or cooled a few degrees above or below the air temperature. The results are discussed in relation to the morphological characters of the leaves. The variation in leaf temperature at different times of the day was significantly related to leaf morphology, specific leaf area, thickness, volume, leaf area index and the surrounding environment.  相似文献   
59.
60.
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