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   69篇
  免费   0篇
  2016年   1篇
  2011年   2篇
  2010年   4篇
  2009年   3篇
  2008年   2篇
  2007年   1篇
  2005年   1篇
  2004年   1篇
  2000年   1篇
  1993年   4篇
  1992年   2篇
  1991年   2篇
  1990年   1篇
  1989年   2篇
  1987年   1篇
  1986年   1篇
  1985年   1篇
  1984年   2篇
  1983年   1篇
  1982年   2篇
  1980年   1篇
  1977年   1篇
  1974年   1篇
  1971年   1篇
  1970年   1篇
  1969年   1篇
  1966年   1篇
  1959年   1篇
  1958年   1篇
  1957年   2篇
  1956年   4篇
  1955年   1篇
  1954年   2篇
  1953年   4篇
  1952年   6篇
  1951年   1篇
  1950年   3篇
  1949年   2篇
排序方式: 共有69条查询结果,搜索用时 343 毫秒
51.
The capability of Time of Flight–Secondary Ion Mass Spectrometry (ToF‐SIMS) of analysing molecular archaeal biomarkers in geobiological samples was tested and demonstrated. Using a bismuth cluster primary ion source, isopranyl glycerol di‐ and tetraether core lipids were detected in small amounts of total organic extracts from methanotrophic microbial mats, simultaneously and without further chemical treatment and chromatographic separation. ToF‐SIMS was also employed to track the distribution of fossilized ether lipids in a massive carbonate (aragonite) microbialite that precipitated as a result of the microbial anaerobic oxidation of methane. An unambiguous signal was obtained when analysing a freshly broken rock surface (base of a microdrill core). Though some limitation occurred due to µm‐topographical effects (sample roughness), it was possible to display the abundance of high molecular weight (C86) of tetraethers exposed in particular regions of the rock surface. ‘Molecular mapping’ revealed that a part of these molecules was encased within the rock fabric in a cluster‐like distribution that might trace the arrangement of the calcifying microbial colonies in the once active mat system. The results reveal promising perspectives of ToF‐SIMS for (i) the quasi‐nondestructive analysis of lipids in extremely small geobiological samples at low concentrations; (ii) resolving the spatial distribution of these compounds on a µm2‐ to cm2‐scale; and (iii) the more exact assignment of lipid biomarkers to their biological source.  相似文献   
52.
53.
Abstract. Homologies among traditional morphological characters in the Membracoidea ( sensu lato ) are reassessed and the phylogenetic relationships among higher membracoid taxa are explored, incorporating new morphological evidence from nymphs and adults. Weighted and unweighted parsimony analyses of a matrix of sixty–three characters and thirty-nine OTUs representing the families Aetalionidae, Cicadellidae, Melizoderidae and Membracidae, and an outgroup (superfamily Cercopoidea) yielded various topologies that were largely congruent but presented alternative hypotheses of relationships among the Membracidae. These analyses indicate that the superfamily consists of the following clades: Cicadellidae + (Melizoderidae + (Aetalionidae + Membracidae)). The family Membracidae, traditionally characterized by the presence of a posterior pronotal process, apparently gave rise to Nicomia Stål and other genera that lack this process.  相似文献   
54.
55.
1. Precise models for the phenology of different species are essential for predicting the potential effects of any temporal mismatch of life cycles with environmental parameters under different climate change scenarios. Here we investigated the effects of ambient water temperature on the onset and synchrony of emergence for a widespread European riverine dragonfly, Gomphus vulgatissimus .
2. Long-term field data on the annual emergence from two rivers in northern Germany, and additional data from a laboratory experiment with different temperature regimes, were used to develop a model that predicted the onset of emergence by using mainly the temperature sum (degree days) as a parameter.
3. Model predictions of the onset of emergence fitted the observations well and could be transferred between localities. This was particularly so when weighting early winter temperature data by using a day length and a temperature-response function, implying potential additional control mechanisms for the onset of emergence.
4. We simulated effects of different winter temperature regimes on the emergence curves in order to predict the effects of climate change. These indicated an acceleration of emergence by 6–7 days per 1 °C temperature increase, which is corroborated by the laboratory data and is in the upper range of data published for other dragonflies.  相似文献   
56.
A comparative study was made of erythrocyte counts and weights of the newborn at term. Three groups were used: Cases in which the cord was clamped at once, those in which the cord was allowed to pulsate five minutes, and those in which the cord and placental blood was stripped into the baby. Standards and procedure were set up so that there would be a minimum of error. Evidence was elicited showing that babies in the "pulsating" and the "stripped" groups received a significant amount of blood which was beneficial. The amount varied, but when the stripping method was used, the term baby received about 100 cc. of blood.Babies receiving this blood had higher erythrocyte counts, higher hemoglobin values, higher initial weights, less weight loss, and less rapid loss of weight. It is believed the additional blood supplied is of benefit especially to prematures and to those infants who are in any degree of shock following long labors, difficult deliveries, abruptio placenta, placenta previa, or compression of the cord. The added blood benefits the baby by combating the initial shock, by aiding in filling the capillary bed of the expanding lungs, by increasing iron reserve, by lessening demand upon blood-forming organs (especially in prematures), by protecting the breakdown of body proteins and by aiding the transition from one source of oxygen to another. Five minutes, as a rule, is not long enough to wait for pulsation if the baby is to receive its quota of available blood. Stripping of cord and placental blood into the infant is not a harmful procedure when done gently and is particularly useful in cases where the condition of the mother or child is such that it is inadvisable to wait for the uterus to force the blood physiologically into the child. The additional blood does not cause icterus. The pulsating of the umbilical cord plays only a minor role in the process by which the baby receives blood after the second stage of labor. The pressure of the uterine contractions upon a blood-filled placenta, forcing blood through the umbilical vein into the child, plays the major role. Pitocin and/or ergot preparations would aid in this process. Anemic mothers have a tendency toward having anemic babies. Venous pressure experiments using a phlebaumanometer showed pressures before and after stripping, the force of uterine contractions on the umbilical vein pressure and the variation in pressure with crying and at rest. The giving of 100 cc. of blood by stripping affects the venous pressure of the child very little, if done slowly. If the stripping is done rapidly, there is a transient rise with a return to normal within a few minutes.A survey of 1,900 diplomates of the American Board of Obstetrics and Gynecology showed that there is wide variation in the management of cord and placental blood. Of 455 specialists who stripped the cord, five thought there was some increase in icterus, and one reported cardiovascular distress.  相似文献   
57.
The free sugars and polysaccharides extractable from coffeeseeds during germination in light or darkness increase greatlywhen these seeds are treated with gibberellic acid (GA3) whichinhibits germination. The major changes are observed in thecontent of free mannose and mannosecontaining polysaccharides.The levels of polysaccharidases are also enhanced by GA3 treatment.The possible role of these events in the inhibition of coffeeseed germination by GA3 is discussed.  相似文献   
58.
Phylogenetic relationships among major lineages of the leafhopper subfamily Iassinae were explored by analysing a dataset of 91 discrete morphological characters and DNA sequence data from nuclear 28S rDNA and histone H3 genes and mitochondrial 12S rDNA. Bayesian, maximum‐likelihood and maximum parsimony analyses yielded similar tree topologies that were well resolved with strong branch support except at the base of the tree, resulting in equivocal support for inclusion of Bythoniini as a tribe of Iassinae but strong support for the monophyly of Iassinae (excluding Bythoniini) and most previously recognized iassine tribes. Divergence times for recovered nodes were estimated using a Bayesian relaxed clock method with two fossil calibration points. The results suggest that the deepest divergences coincided with Gondwanan vicariant events but that more recent divergences resulted from long‐range dispersal and colonization. Biogeographical analyses suggest that the group most likely has a Neotropical origin. The following changes to the taxonomic classification are proposed: establishment of three new tribes, Batracomorphini trib.n. (based on type genus Batracomorphus Lewis), Hoplojassini trib.n. (based on type genus Hoplojassus Dietrich and including one other South American genus), Lipokrisnini trib.n. (based on type genus Lipokrisna Freytag and including two other endemic Caribbean genera); Krisnini is redefined to include only the Old World genera Krisna and Gessius; Iassini is redefined to include only the type genus and four endemic Afrotropical genera; Bascarrhinus Fowler and Platyhynna Berg, recently treated as genera incertae sedis, are placed in Hyalojassini; Thalattoscopus Kirkaldy is added to the previously monobasic tribe Trocnadini. Iassinae now includes 12 tribes, all of which appear to be monophyletic. Revised morphological diagnoses of the subfamily and each of the included tribes are provided and a key to tribes is also given. This published work has been registered in ZooBank, http://zoobank.org/urn:lsid:zoobank.org:pub:41295B68‐2DAB‐4C4F‐B260‐F7C054922173 .  相似文献   
59.
60.
Kinnback, A., Mellor, R. B. and Werner, D. 1987. Alpha-mannosidaseII isoenzyme in the peribacteroid space of Glycine max rootnodules.—J. exp. Bot. 38: 1373–1377. Three isoenzymes of alpha-mannosidase from soybean tissue wereseparated by isoelectric focussing. The vacuomal isoenzymesI and II accounted typically for over 90% of the activity inextracts from roots whether or not they were infected with Bradyrhizobiumjaponicum, tissue culture and effective nodules. Ineffectivenodules, on the other hand, contained up to 62% of the typicallyextracellular located isoenzyme III. The compartment betweenthe symbiotic partners (the peribacteroid space) contained onlyisoenzyme II. The bearing of isoenzyme distribution upon modelsof peribacteroid membrane biogenesis is discussed. Key words: Aipha-mannosidase, Bradyrhizobium japonicum, Glycine max, membrane flow/recycling, peribacteroid space, symbiosis  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