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   2112篇
  免费   167篇
  2279篇
  2015年   32篇
  2014年   49篇
  2013年   65篇
  2012年   70篇
  2011年   61篇
  2010年   66篇
  2009年   46篇
  2008年   61篇
  2007年   87篇
  2006年   58篇
  2005年   54篇
  2004年   54篇
  2003年   47篇
  2002年   55篇
  2001年   40篇
  2000年   50篇
  1999年   43篇
  1998年   24篇
  1997年   39篇
  1996年   24篇
  1995年   26篇
  1994年   27篇
  1993年   32篇
  1992年   32篇
  1991年   51篇
  1990年   53篇
  1988年   25篇
  1987年   35篇
  1986年   32篇
  1985年   24篇
  1984年   33篇
  1982年   27篇
  1980年   28篇
  1979年   26篇
  1978年   25篇
  1976年   24篇
  1975年   27篇
  1974年   40篇
  1973年   42篇
  1972年   39篇
  1971年   28篇
  1970年   26篇
  1968年   26篇
  1965年   44篇
  1963年   36篇
  1962年   26篇
  1961年   36篇
  1960年   23篇
  1959年   29篇
  1958年   29篇
排序方式: 共有2279条查询结果,搜索用时 0 毫秒
1.
Three different types of outgrowths develop from epidermal cells of excised juvenile leaves of Microgramma vacciniifolia: aposporous gametophytes, intermediates, and regenerated sporophytic plantlets. The gametophytes and intermediates arise from derivatives of epidermal cell divisions which are developed to the exterior of the leaf surface, whereas the sporophytic regenerants originate from derivatives produced by cell divisions to the interior of the leaf. Anatomical observations of excised leaves grown in vitro demonstrate that only the epidermal cells are stimulated to divide and give rise to the various types of outgrowths. Incorporation of tritiated thymidine by the nuclei of leaf epidermal cells gives further evidence for the metabolic activity of these cells.  相似文献   
2.
Five albino female rats which had been trained to run a maze were exposed to 3 nanosecond pulses of electromagnetic energy which had a density of 600,000 v/m2. A disturbance of the ability of the animals to perform this recently learned task was observed.The effect was found to be reversible within a period of 30 min. Possible mechanisms by which the decision making strategy of the animals was temporarily disrupted are considered. The parallelism between the conditions of this experiment and those attending the exposure of animals to lightning bolts during electrical storms is pointed out.
Zusammenfassung Fünf weibliche Albinoratten, die darauf trainiert waren, in einem Labyrinth zu laufen, wurden 3 Nanosekunden-Impulsen elektromagnetischer Energie ausgesetzt, die eine Energie von 600.000 v/m2 hatten. Es wurde eine Störung der Fähigkeit der Tiere beobachtet, diese kürzlich gelernte Aufgabe zu bewältigen. Die Wirkung war innerhalb von 30 Min. reversibel. Die verschiedenen Mechanismen, durch die die Fähigkeit der Tiere, ihre Aufgabe zu lösen, zeitweise unterbunden wurde, werden diskutiert. Auf die Parallele zwischen den Bedingungen dieses Experiments und denen bei Entladungen während elektrischer Stürme wird hingewiesen.

Resume On a exposé cinq femelles de rats albinos à des impulsions électromagnétiques de 3 manosecondes et d'une énergie de 600.000 v/m2. Ces bêtes avaient au préalable été entraînées à se mouvoir dans un labyrinthe.On a alors pu constater que les bêtes ainsi exposées perdaient la faculté de se retrouver dans le labyrinthe. L'effet de l'exposition est cependant réversible après 30 minutes.On discute les divers mécanismes par lesquels la faculté qu'ont les bêtes d'effectuer certaines tâches est momentanément supprimée. On mentionne le parallélisme existant entre les conditions de ces expériences et celles que provoquent les décharges qui se produisent pendant les orages.
  相似文献   
3.
A [3H]-PBAN (pheromone biosynthesis-activating neuropeptide) analog was synthesized, and binding of the radioligand to a specific PBAN-antiserum was achieved. The inhibition of binding of the radioligand by unlabeled PBAN, several PBAN analogs, and other competitors was studied and a specific radio-immunoassay was developed. Using this radioimmunoassay we found PBAN-like immunoreactivity in methanol extracts of hemolymph and neural tissues from females. Higher levels of PBAN-like immunoreactivity in extracts of brain-suboesophageal ganglion complexes, corpora cardiaca, thoracic ganglia, and abdominal ganglia were observed during the 4-5th h scotophase when compared to the PBAN-like immunoactivity levels during the 6-11th h photophase. On the other hand, the concentrations of PBAN-like immunoreactivity, in the terminal abdominal ganglion were higher during the photophase relative to minimal levels observed during the scotophase, indicating an accumulation before the onset of pheromone production. These differences in concentrations of PBAN were also reflected in the stimulation of in vitro pheromone glands, whereby significant stimulations were obtained by scotophase and photophase brain extracts, scotophase thoracic ganglia extracts, and photophase terminal abdominal ganglia extracts. No detectable levels of PBAN were found in hemolymph extracts during the sampling periods.  相似文献   
4.
5.
6.
Abstract: We developed a rapid and sensitive radioimmunohistochemical method for the quantification of tyrosine hydroxylase (TH) at both the anatomical and cellular level. Coronal tissue sections from fresh-frozen rat brains were incubated in the presence of a TH monoclonal antibody. The reaction was revealed with a 35S-labeled secondary antibody. TH content was quantified in catecholaminergic brain areas by measuring optical density on autoradiographic films or silver grain density on autoradiographic emulsion-coated sections. Regional TH concentrations determined in the locus ceruleus (LC), substantia nigra pars compacta (SNC), and ventral tegmental area (VTA) were significantly increased by 45% after reserpine treatment in the LC but unchanged in the SNC and VTA. Microscopic examination of TH radioimmunolabeling showed a heavy accumulation of silver grains over catecholaminergic cell bodies. In the LC, grain density per cell was heterogeneous and higher in the ventral than in the dorsal part of the structure. After reserpine treatment, TH levels were significantly increased (57%) in the neurons of the LC but not in those of the SNC or VTA. The data support the validity of this radioimmunohistochemical method as a tool for quantifying TH protein at the cellular level and they confirm that TH protein content is differentially regulated in noradrenergic and dopaminergic neurons in response to reserpine.  相似文献   
7.
Summary Chromosomal analysis of lymphocytes and fibroblasts in a slightly retarded 7-month-old female infant with some minor malformations showed a mosaicism 45,XX,G-/46,XX,Gr. The ring chromosome was identified as No. 21 by banding technique.
Zusammenfassung Chromosomenuntersuchungen an Lymphocyten und Fibroblasten bei einem 7 Monate alten Mädchen mit leichter statomotorischer Entwicklungsverzögerung und degenerativen Stigmata zeigten ein Mosaik 45,XX,G-/46,XX,Gr. Durch die Bandentechnik mit Trypsinbehandlung konnte das Ringchromosom als Nr.21 identifiziert werden.
  相似文献   
8.
Anammox bacteria belonging to the phylum Planctomycetes are responsible for N removal through NH4+ oxidation coupled with NO2 reduction. Microbial diversity and ecology of anammox bacteria have not yet been fully revealed due to limitations of 16S rRNA analysis. The hydrazine oxidase gene in cluster 1 (hereafter hzoA/hzoB) was suggested as a proper genetic marker due to its high expression and ubiquitous presence in anammox bacteria. We conducted a comparative analysis of 16S rRNA and hzoA/hzoB genes to reveal anammox bacterial diversity and distribution in various aquatic environments. Phylogenetic analyses of 16S rRNA and hzoA/hzoB genes showed the dominance of Scalindua organisms in marine ecosystems, but there was no congruence of 16S rRNA and hzoA/hzoB gene phylogenies among the freshwater anammox bacteria associated with Brocadia sp., Jettenia sp., and Anammoxoglobus sp. Higher diversity of anammox bacteria was revealed based on hzoA/hzoB genes than 16S rRNA genes in the examined environments. Multiple regression analysis showed that salinity had significant influence on differential distribution and diversity of anammox bacteria in different ecosystems. Thus, molecular detection and resulting phylogeny of the hzoA/hzoB gene generated a better understanding of anammox bacterial diversity and their ecological distribution in various aquatic ecosystems.  相似文献   
9.
With more than 160 years of contrasting fertilizer regimes, the Broadbalk winter wheat experiment represents a unique experimental resource for studying the effects of long-term fertilizer application on microbial population diversity. Using DGGE and clone library analysis, we report here on eubacterial species diversity (16S rRNA gene) and diversity within two sets of gene products associated with microbial N acquisition: NifH (nitrogen fixation) and AmtB (ammonium transport). Comparisons were made within and between soils treated with mineral N fertilizer, farmyard manure or receiving no fertilizer. Analysis of 16S rRNA gene DGGE profiles showed no clear patterns to qualitatively distinguish bacterial community structure between the three different treatments (P > 0.05), with all samples containing a range of eubacterial taxa similar to those that are characteristic of soil bacteria reported elsewhere. Intra-plot heterogeneity was high and of a similar magnitude to that between treatments. This lack of qualitative between plot differences was echoed in the representative sequences of 16S rRNA, nifH, and amtB genes in the various samples. Taken together, both phylogenetic and functional gene analyses showed bacterial communities in the Broadbalk-trial soil were very stable and relatively non-responsive to long-term management of balanced fertilizer inputs.  相似文献   
10.
  总被引:9,自引:1,他引:9  
Although the effect of vaginal tampons on the microbial flora during menstruation has recently been studied by several investigators, quantitative effects attributable to particular tampon fibers have received less attention. The purposes of the present study were (i) to determine and then to compare the effects of polyacrylate rayon tampons and viscose rayon tampons on the normal vaginal flora, (ii) to compare quantitative bacterial counts obtained from these tampons with those obtained from concomitant vaginal swabs, and (iii) to determine whether either of these tampon types alters the vaginal microflora when compared with the microflora in the same women using all-cotton tampons or external catamenial pads. Tampon and swab samples were obtained at predetermined times from 18 women for an average of seven menstrual cycles. Samples consisting of swabs from women wearing menstrual pads were compared with swab and tampon samples taken at predetermined times during the menstrual cycle from women using cotton, polyacrylate rayon, or viscose rayon tampons. Samples were analyzed for total aerobic, facultative, and anaerobic bacterial counts. Statistical evaluation of the results indicated that, on the whole, total bacterial counts decreased during menstruation and that the numbers of bacteria in tampons tended to be lower than those in swab samples taken at the same time. The tampon type had little effect on the vaginal microflora.  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