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   406篇
  免费   6篇
  412篇
  2017年   3篇
  2016年   2篇
  2015年   2篇
  2014年   6篇
  2013年   10篇
  2012年   10篇
  2011年   22篇
  2010年   30篇
  2009年   27篇
  2008年   22篇
  2007年   24篇
  2006年   15篇
  2005年   21篇
  2004年   11篇
  2003年   9篇
  2001年   5篇
  2000年   2篇
  1999年   9篇
  1998年   9篇
  1997年   8篇
  1996年   21篇
  1995年   9篇
  1994年   14篇
  1993年   12篇
  1992年   9篇
  1991年   7篇
  1990年   8篇
  1989年   13篇
  1988年   5篇
  1987年   5篇
  1986年   5篇
  1985年   8篇
  1983年   1篇
  1982年   2篇
  1981年   1篇
  1980年   2篇
  1979年   4篇
  1978年   4篇
  1977年   7篇
  1976年   6篇
  1975年   1篇
  1974年   2篇
  1973年   4篇
  1972年   2篇
  1971年   5篇
  1969年   2篇
  1968年   1篇
  1967年   2篇
  1962年   1篇
  1947年   1篇
排序方式: 共有412条查询结果,搜索用时 15 毫秒
91.
Summary

In organisms containing sperm storage organs, sperm competition constitutes an important sexual selection factor. Sexual conflicts and unilateral manipulation of the reproductive physiology of the mating partner are common in the copulation behaviour of such organisms. In hermaphrodites, such conflicts escalate even more because of the identical interests of all individuals and the unique possibility to manipulate the degree of “femaleness” of the partner. This often results in bizarre reproductive tactics and mating behaviours. The simultaneous hermaphrodite Lumbricus terrestris uses specialized setae to pierce the partner's skin and inject a gland product during copulation. This happens in addition to, and is spatially distinct from, reciprocal sperm transfer. For the identification of bioactive compounds in the transferred secretion, comparative proteomic studies were performed, applying mass spectrometry and two-dimensional gel electrophoresis. Although initially targeting a substance with so-called “allohormone” characteristics, ubiquitin, a highly conserved 76 amino acid protein, could be identified as one of the predominant compounds. This contributes to recent findings concerning general roles of ubiquitin in reproductive events. One possible mechanism proposed, namely, that of labelling sperm for phagocytotic removal, is discussed.  相似文献   
92.
In embryos of the white mutant axolotl, prospective pigment cells are unable to migrate from the neural crest (NC) due to a deficiency in the subepidermal extracellular matrix (ECM). This raises the question of the molecular nature of this functional defect. Some PGs can inhibit cell migration on ECM molecules in vitro, and an excess of this class of molecules in the migratory pathways of neural crest cells might cause the restricted migration of prospective pigment cells seen in the white mutant embryo. In the present study, we use several monoclonal antibodies against epitopes on keratan sulphate (KS) and chondroitin sulphate (CS) and LM immunofluorescence to examine the distribution of these glycosaminoglycans at initial (stage 30) and advanced (stage 35) stages of neural crest cell migration. Most KS epitopes are more widely distributed in the white mutant than in the wild type embryo, whereas CS epitopes show very similar distributions in mutant and wild type embryos. This is confirmed quantitatively by immunoblotting: certain KS epitopes are more abundant in the white mutant. TEM immunogold staining reveals that KS as well as CS are present both in the basal lamina and in the interstitial ECM in both types of embryos. It remains to be investigated whether the abundance of certain KS epitopes in the white mutant embryo might contribute to the deficiency in supporting pigment cell migration shown by its ECM.  相似文献   
93.
The section of habitat used by particular bloodsucking insects when seeking bloodmeals may influence the spectrum of hosts to which they have access and consequently the diseases they transmit. The vertical distribution of ornithophilic bloodsucking Diptera (Culicidae, Simuliidae and Ceratopogonidae) was studied using bird‐baited traps set at both ground and tree canopy levels. In total, 1240 mosquito females of eight species, 1201 biting midge females of 11 species, and 218 blackfly females of two species were captured during 2003–2005. Culex pipiens (L.) (Diptera: Culicidae) was found to prefer ground‐level habitats, whereas Anopheles plumbeus (Stephens) (Diptera: Culicidae), biting midges [Culicoides spp. (Diptera: Ceratopogonidae)] and Eusimulium angustipes (Edwards) (Diptera: Simuliidae) preferred the canopy. The results of this study with regard to Cx. pipiens behaviour differ from those of most previous studies and may indicate different spatial feeding preferences in geographically separate populations. The occurrence of E. angustipes in the canopy is concordant with its role in the transmission of avian trypanosomes. These findings may be important for surveillance programmes focusing on ornithophilic Diptera which transmit various pathogenic agents.  相似文献   
94.
Abstract: Circumorbital dermal bones are found in most groups of early vertebrates that have dermal bony plates on the head. Taxonomic distribution of dermal sclerotic plates on the eye itself is less clear, partly because the eyeball is rarely preserved and sometimes because sclerotic bones have been misinterpreted as circumorbital bones. Based on the examination of climatiid Climatius plus mesacanthid, cheiracanthid and acanthodid acanthodiform acanthodians, we conclude that most, if not all, acanthodiforms and climatiids had sclerotic rings. Presence and number of these elements should be included as a character in phylogenetic analyses of early jawed vertebrates.  相似文献   
95.
Encephalitozoon hellem is a new human microsporidian isolated from corneal biopsies and conjunctival scrapings of three AIDS patients and cultured in Madin Darby canine kidney (MDCK) cells. Encephalitozoon hellem and Encephalitozoon cuniculi display different protein profiles with sodium dodecyl sulfate-polyacrylamide gel electrophoresis and unique antibody binding patterns with murine antisera against Western blots of each organism. Developmental stages of E. hellem in culture are similar to E. cuniculi. Meronts are 1.3–2.7 μm in diameter, develop within a parasitophorous vacuole adjacent to the vacuolar membrane, divide by binary fission, and contain one or two discrete nuclei. Sporonts measure 2 × 3 μm, separate from the vacuolar membrane, and have a thickened outer membrane. Sporoblasts display a tri-layered wall and possess the earliest recognized polar filaments. Mature spores measure 1 × 1.5 μm and are more electron-dense than other stages. Each spore contains a single nucleus, a polar tubule with four to nine coils, thin electron-dense exospore and thick, electron-lucent endospore. Although E. hellem and E. cuniculi differ biochemically and immunologically, their fine structure and development are indistinguishable.  相似文献   
96.
97.
Changes in the transpiration rate of intact spring barley plants, cv. “Slovensky dunajsky trh”, were studied separately in the light and in the dark under controlled temperature and illumination, after the infection withErysiphe graminis DC, during an 8 day period of the development of the fungus. In the first stage of pathogenesis, the fungus diminishes water output from the host plants in the light. An opposite phenomenon can be observed in the dark; water output from infected plants in the dark increases sharply mainly in the stage of advanced fructification. Thus, the fungus considerably diminishes the ratio of water output from the host plants in the light to that in the dark.  相似文献   
98.
Broadbean plants (Vicia faba L.) were submitted to three differentlevels of steady state N limitation. Relative addition ratesof N were 0.06, 0.1 and 0.14d-1. Plants were harvested at fiveevenly distributed times over a 2d period. Shoot growth correspondedwell with the imposed treatment. Root growth, relative to shootgrowth, was highest at the 0.06d-1treatment. Diurnal patternsof soluble sugars, amino acids and starch were analysed. Onaverage, soluble sugar levels were highest in the plants ofthe 0.06d-1treatment whereas average free amino acid levelswere highest in the 0.14d-1treatment. Shoot growth increasedas the concentration of shoot amino acids increased. No suchcorrelation however could be found between root growth and freesugar levels in the root. Broadbean; Vicia faba L.; exponential addition; N limitation; free sugar; amino acids; diurnal cycle; functional equilibrium; starch  相似文献   
99.
100.
Susceptibility was evaluated of host and non-host plants to three pathogenic Botrytis species: the generalist B. cinerea and the specialists B. elliptica (lily) and B. tulipae (tulip). B. tulipae was, unexpectedly, able to infect plant species other than tulip, and to a similar extent as B. cinerea. To study host and non-host interactions in more detail, the three Botrytis species were inoculated on Arabidopsis wild-types and 23 mutant genotypes. Disease development was monitored macroscopically by quantifying the lesion area and microscopically by bright-field and fluorescence microscopy following histochemical staining. B. cinerea and B. tulipae were very similar in their ability to infect the tested Arabidopsis genotypes, whereas B. elliptica caused disease only on a few Arabidopsis mutant genotypes. Arabidopsis mutants with a delayed or reduced cell death response were generally more resistant to Botrytis infection, whereas mutants in which cell death was accelerated were more susceptible. Differences in susceptibility between genotypes were generally gradual. Only the camalexin-deficient mutant pad 3 was fully susceptible to all three Botrytis species. Cellular changes were monitored during compatible and incompatible interactions. The formation of papillae, the presence of lysosome-like vesicles and the intracellular accumulation of H2O2 and nitric oxide were visualized in the infection zones using fluorescent probes. Based on histology and responses of Arabidopsis mutants, a model is proposed in which resistance against Botrytis , besides the production of camalexin, depends on the balance between cell death and survival.  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