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   173篇
  免费   0篇
  国内免费   1篇
丛书文集   1篇
现状及发展   62篇
研究方法   24篇
综合类   76篇
自然研究   11篇
  2018年   2篇
  2017年   1篇
  2012年   6篇
  2011年   17篇
  2010年   3篇
  2009年   2篇
  2008年   6篇
  2007年   2篇
  2006年   7篇
  2005年   6篇
  2004年   9篇
  2003年   4篇
  2002年   4篇
  2001年   6篇
  2000年   5篇
  1999年   7篇
  1993年   1篇
  1992年   3篇
  1991年   3篇
  1990年   4篇
  1989年   1篇
  1988年   2篇
  1987年   2篇
  1986年   3篇
  1985年   1篇
  1984年   1篇
  1983年   3篇
  1982年   4篇
  1981年   2篇
  1979年   6篇
  1978年   4篇
  1977年   4篇
  1976年   6篇
  1975年   6篇
  1974年   2篇
  1973年   2篇
  1972年   4篇
  1971年   3篇
  1970年   6篇
  1969年   4篇
  1968年   3篇
  1966年   1篇
  1965年   1篇
  1964年   2篇
  1959年   2篇
  1955年   1篇
排序方式: 共有174条查询结果,搜索用时 15 毫秒
41.
Larval and juvenile stages of many fishes require nursery habitats that provide optimal conditions for growth. Loss or degradation of these habitats limits recruitment, causing population and species declines. Least chub ( Iotichthys phlegethontis ), an endemic cyprinid in the Bonneville Basin, is currently restricted to a few spring complexes in Utah. This species utilizes the warm shallow spring margins as spawning and rearing habitat throughout the summer. This study was conducted to determine effects of temperature on survival and growth of age-0 least chub to understand the importance of temperature in selection of spring margins as rearing habitat. Age-0 least chub were exposed to 5 temperatures (14°C, 17°C, 21°C, 24°C, and 27°C) for 112 days. Growth rates varied significantly with temperature except at 17°C and 27°C, and growth rates were highest at 21°C ( P < 0.001). Maximum growth rate for age-0 least chub, estimated from a 2nd-order polynomial regression, would occur at 22.3°C. These thermal requirements indicate the importance of warm rearing habitats in producing strong year classes and viable populations. However, warm spring margins also enable western mosquitofish, a nonnative competitor and predator of least chub, to thrive in these spring habitats. Broad thermal limits of the least chub suggest that it could persist at cooler temperatures, which would reduce the viability of western mosquitofish populations.  相似文献   
42.
Most tumors exhibit increased glucose metabolism to lactate, however, the extent to which glucose-derived metabolic fluxes are used for alternative processes is poorly understood. Using a metabolomics approach with isotope labeling, we found that in some cancer cells a relatively large amount of glycolytic carbon is diverted into serine and glycine metabolism through phosphoglycerate dehydrogenase (PHGDH). An analysis of human cancers showed that PHGDH is recurrently amplified in a genomic region of focal copy number gain most commonly found in melanoma. Decreasing PHGDH expression impaired proliferation in amplified cell lines. Increased expression was also associated with breast cancer subtypes, and ectopic expression of PHGDH in mammary epithelial cells disrupted acinar morphogenesis and induced other phenotypic alterations that may predispose cells to transformation. Our findings show that the diversion of glycolytic flux into a specific alternate pathway can be selected during tumor development and may contribute to the pathogenesis of human cancer.  相似文献   
43.
Summary Annual rings of 140–160-year-old beeches (Fagus sylvatica) from St. Ingbert, Saarland (FRG) were prepared and analyzed for 14 metals by atomic absorption spectroscopy. According to the chronological variations of their concentrations, the elements could be divided into three groups: 1) Metals without any tendency for chronological changes. This was established for Na, K, Cu, Cr, Co, Ni, Pb, and Cd. 2) Metals with a recent decrease of their concentrations, appropriate for Ca, Mg, Mn, and Zn. 3) Metals with a recent tendency to increase, e.g. Fe and Al. These variations are discussed in connection with the industrial history of the Saarland region and the influence of acid immissions which may alter the soil and thereby the trace element metabolism of the trees with consequences for the vitality of the plants.  相似文献   
44.
Zusammenfassung Kollodiumpartikel wurden mit Extrakten aus menschlichen Organen, wie Leber, Nieren und Myocard, sensibilisiert und damit Sera von Personen getestet, die an Krankheiten mit autoimmunisierenden Vorgängen in der Pathogenese litten. Die Ergebnisse dieser Versuche wurden mit denjenigen von Kontrollsera verglichen.  相似文献   
45.
Summary Patients with iron deficiency whether uncomplicated or associated with other types of anemias, had plasma catecholamine levels which were significantly increased above normal controls. Patients with a variety of other anemias had no significant increase in catecholamine levels. Plasma catecholamine levels in uncomplicated iron deficient patients approached normal values as early as 3 h following oral FeSO4.Acknowledgment. Funded in part by an NIH grant No. R01 HL 19933.  相似文献   
46.
A severe potentiating effects of methyl-ethyl-ketone (MEK) on the peripheral and central neurotoxicity of n-hexane could be demonstrated in a chronic inhalation study in rats.  相似文献   
47.
Résumé On a contrôlé des femmes enceintes, atteintes d'éclampsie et, chez un groupe d'entre elles, la présence d'autoanticorps contre le placenta, le foie, les reins et le myocarde. On a constaté un taux significativement plus éleyé d'autoanticorps chez les femmes dont la grossesse a été compliquée d'éclampsie.  相似文献   
48.
49.
Summary In rats, CH 29-717 inhibits basal and physiologically or chemically stimulated prolactin secretion. It is more potent than the standard bromocriptine.  相似文献   
50.
Normal 0 false false false EN-US X-NONE X-NONE MicrosoftInternetExplorer4 st1\:*{behavior:url(#ieooui) } /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-qformat:yes; mso-style-parent:""; mso-padding-alt:0in 5.4pt 0in 5.4pt; mso-para-margin:0in; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:11.0pt; font-family:"Calibri","sans-serif"; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-fareast-font-family:"Times New Roman"; mso-fareast-theme-font:minor-fareast; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin; mso-bidi-font-family:"Times New Roman"; mso-bidi-theme-font:minor-bidi;} Historical accounts and matched photographs indicate sharp decline of once-abundant Kochia americana in eastern Great Basin vegetation since the early 1900s, most of the decline by the late 1950s. Exclosure data show further decline from 1957 to 1973, then some increase between 1970 and 1973 and 1976 and 1981. Utah sheep numbers, at maximum from 1925 to 1940 and declining steadily to the 1970s, may have induced the long-term changes. Black-tailed jackrabbits ( Lepus californicus ), could not have induced vegetation decline, but could have added to livestock pressure and abetted the trend. In 1972, rabbits near a cyclic high were indirectly estimated to completely utilize K. americana in Curlew Valley, northwestern Utah. In 1976&ndash;1977 at rabbit low, direct measurements show 4%&ndash;18% of plants browsed by late summer, about 30%&ndash;50% of herbage removed from browsed plants. The latter rose to 45%&ndash;82% by end of winter. Late-summer percent browsed may have risen slightly (11%&ndash;21%) in 1980&ndash;1981 at next rabbit high. Increase in K. americana density from 1973 to 1976, then a slight decrease from 1976 to 1980, suggests fluctuating K. americana abundance induced by rabbit browsing, superimposed on long-term K. americana decline and recovery.  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