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   1619篇
  免费   98篇
  1717篇
  2022年   17篇
  2021年   25篇
  2020年   17篇
  2019年   33篇
  2018年   36篇
  2017年   36篇
  2016年   45篇
  2015年   54篇
  2014年   62篇
  2013年   91篇
  2012年   114篇
  2011年   106篇
  2010年   77篇
  2009年   42篇
  2008年   72篇
  2007年   68篇
  2006年   76篇
  2005年   63篇
  2004年   56篇
  2003年   57篇
  2002年   71篇
  2001年   21篇
  1999年   13篇
  1998年   18篇
  1997年   14篇
  1996年   22篇
  1994年   15篇
  1993年   14篇
  1992年   11篇
  1991年   14篇
  1990年   18篇
  1989年   15篇
  1988年   12篇
  1987年   14篇
  1986年   9篇
  1985年   12篇
  1984年   14篇
  1983年   15篇
  1982年   23篇
  1981年   17篇
  1980年   13篇
  1979年   11篇
  1978年   12篇
  1976年   9篇
  1975年   14篇
  1974年   17篇
  1973年   18篇
  1972年   9篇
  1968年   9篇
  1967年   10篇
排序方式: 共有1717条查询结果,搜索用时 15 毫秒
991.
By means of the artificially induced heterochronic developmental deviations represented by local prothetelies and metathetelies it has been possible to investigate the individual developmental fates of ontogenetically different tissues, such as larval, pupal, and adult epidermal cells, in one and the same body and under the identical concentration of juvenile hormone (JH) in the haemolymph.In contrast to the widely accepted hormonal theories which claim that the kind of morphogenesis is determined by large, intermediate, and low titres of JH, the heterochronic character of the tissues never developed into a uniform population of homomorphic epidermal cells. Instead, in the presence of effective amounts of JH the heterochronic pattern has been fully preserved and carried on into the next developmental instar. Moreover, in the absence of the effective JH amounts the ontogenetically different tissues, such as larval and pupal epidermal cells, simultaneously undergo their respective morphogenetic developments, i.e. larval-pupal and pupal-adult morphogenesis in the same hormonal milieu. It is concluded that the selective factor in determination of the kind of morphogenetical changes is not an altered JH titre but the extant, previously attained degree of ontogenetic structural differentiation. It has been demonstrated that JH can temporarily and reversibly inhibit the morphogenetic progress at quite different ontogenetic levels but it cannot cause a ‘reversal of metamorphosis’ at any of these levels.Under specific experimental conditions the larval epidermal cells can undergo pupal and adult morphogenesis without secreting the pupal cuticle. However, the pupal morphogenetic interstage, whether with the cuticle or without the pupal cuticle, constitutes an obligatory developmental step. Further, it appears that an absence of JH may represent an important condition but not a real cause of insect metamorphosis, as presumed in some other hormonal concepts. Thus, chromosomal duplications or cellular divisions in the absence of JH have not committed the cells to morphogenesis unless provided by an additional stimulus of endogenous prothoracic gland hormone or exogenous ecdysterone. An important factor in understanding the hormonal control of insect morphogenesis is the critical timing of the respective morphogenetic steps. This corresponds closely with the duration of the pharate phases in insect development. Possible hormonal mechanisms concerned in the regulation of morphogenesis in endopterygote insects have been outlined.  相似文献   
992.
993.
Bures S  Weidinger K 《Oecologia》2003,137(4):634-641
Calcium availability may limit the reproductive output of birds and snail shells are considered to be the main source of calcium in many passerine species. This study of collared (Ficedula albicollis) and pied (F. hypoleuca) flycatchers evaluates calcium intake of a natural diet in Central Europe, and sex differences in the utilization of experimentally supplemented sources of calcium during the entire breeding period in aviary birds. The study provides the first evidence that successful reproduction of these species depends on the availability of woodlice (Isopoda) and millipedes (Diplopoda). Each of these two components provided about 3 times more calcium than the snail shells contained in a natural nestling diet. The breeding performance of aviary birds was poor when only snail shells and the fragments of eggshells were provided in food, i.e., irregular laying, smaller clutches, eggshell defects (25 of 53 eggs), and eggs dried-up during incubation. In contrast, no defective eggshell or dried-up eggs were found and the overall breeding performance increased 2–3 times when woodlice were added to the food. Females increased their intake of woodlice during both the pre-laying and laying periods, and both sexes did so during the nestling period. Both sexes took more woodlice in the evening than in the morning, independent of the nesting stage. Intake by females was low until 4 days before laying the first egg, then increased to the highest level, dropping immediately after laying the last egg. Intake of woodlice by both sexes increased steadily from hatching until the nestling age of about 10–12 days decreasing thereafter, which corresponds with the period of rapid skeletal growth. In contrast, the intake of mealworms increased until the nestling age of 13–14 days leveling off thereafter which corresponds with the growth curve of nestling body mass.Due to an error in the citation line, this revised PDF (published in December 2003) deviates from the printed version, and is the correct and authoritative version of the paper.  相似文献   
994.
995.
In augmentative biological control (ABC), invertebrate and microbial organisms are seasonally released in large numbers to reduce pests. Today it is applied on more than 30 million ha worldwide. Europe is the largest commercial market for invertebrate biological control agents, while North America has the largest sales of microbials. A strong growth in use of ABC, particularly of microbial agents, is taking place in Latin America, followed by Asia. The current popularity of ABC is due to (1) its inherent positive characteristics (healthier for farm workers and persons living in farming communities, no harvesting interval or waiting period after release of agents, sustainable as there is no development of resistance against arthropod natural enemies, no phytotoxic damage to plants, better yields and a healthier product, reduced pesticide residues [well below the legal Maximum Residue Levels (MRLs)], (2) professionalism of the biological control industry (inexpensive large scale mass production, proper quality control, efficient packaging, distribution and release methods, and availability of many (>440 species) control agents for numerous pests), (3) a number of recent successes showing how biological control can save agricultural production when pesticides fail or are not available, (4) several non-governmental organizations (NGOs), consumers, and retailers demanding pesticide residues far below the legal MRLs, and (5) policy developments in several regions of the world aimed at reduction and replacement of synthetic pesticides by more sustainable methods of pest management. We are convinced, however, that ABC can be applied on a much larger area than it is today. We plead in the short term for a pragmatic form of agriculture that is adaptable, non-dogmatic and combines the sustainability gain from all types of agriculture and pest management methods. We then propose to move to “conscious agriculture”, which involves participation of all stakeholders in the production and consumer chain, and respects the environment and resource availability for future generations. Were “conscious agriculture” to be considered a serious alternative to conventional farming, ABC would face an even brighter future.  相似文献   
996.
We studied the effects of differences in root growth and nutrient pool on the competitive ability of Festuca ovina (short grass), Arrhenatherum elatius and Calamagrostis epigejos (tall grasses) grown in monocultures and in mixtures of homogeneous and heterogeneous environments during two growing seasons. Analysis of variance revealed a significant effect of plant species on nutrient concentrations in above-ground biomass and of substrate type on contents of N, K, Ca, Mg in biomass. The ANOVA also confirmed the significant effect of competitive environment on the concentration of N, K in above-ground biomass. In heterogeneous environments, both tall grasses (in competition with F. ovina) were able to produce more roots in the nutrient-rich patches and to accumulate more nitrogen in plant tissues, which was associated with higher yield of their above-ground biomass. Thus, the relative competitive ability for nutrients of both tall grasses was higher than that of F. ovina. This competitive ability of A. elatius to C. epigejos increased in heterogeneous treatments.  相似文献   
997.
Recently we reported 6-(2-hydroxy-3-methylbenzylamino)purine (PI-55) as the first molecule to antagonize cytokinin activity at the receptor level. Here we report the synthesis and in vitro biological testing of eleven BAP derivatives substituted in the benzyl ring and in the C2, N7 and N9 positions of the purine moiety. The ability of the compounds to interact with Arabidopsis cytokinin receptors AHK3 and CRE1/AHK4 was tested in bacterial receptor and in live-cell binding assays, and in an Arabidopsis ARR5:GUS (Arabidopsis response regulator 5) reporter gene assay. Cytokinin activity of the compounds was determined in classical cytokinin biotests (tobacco callus, wheat leaf senescence and Amaranthus bioassays). 6-(2,5-Dihydroxybenzylamino)purine (LGR-991) was identified as a cytokinin receptor antagonist. At the molecular level LGR-991 blocks the cytokinin receptor CRE1/AHK4 with the same potency as PI-55. Moreover, LGR-991 acts as a competitive inhibitor of AHK3, and importantly shows reduced agonistic effects in comparison to PI-55 in the ARR5:GUS reporter gene assay and in cytokinin bioassays. LGR-991 causes more rapid germination of Arabidopsis seeds and increases hypocotyl length of dark-grown seedlings, which are characteristics of plants with a reduced cytokinin status. LGR-991 exhibits a structural motive that might lead to preparation of cytokinin antagonists with a broader specificity and reduced agonistic properties.  相似文献   
998.
Stimulation of the Escherichia coli ATPase activity by urea and trypsin shows that there is a latent ATPase activity in particles and in a crude coupling factor of E. coli. Moreover, crude coupling factor, completely dissociated by treatment with 7 M urea, can inhibit the ATPase activity of the crude coupling factor. It is suggested that the latency of the ATPase activity of the coupling factor is due to the presence of a protein, the ATPase-inhibitor.  相似文献   
999.
Alcoholysis of blackcurrant oil mediated by Pseudomonas fluorescens lipase performed at 30°C in ethanol (96%, v/v) used both as a solvent and as a reactant. After 16 h, 95% of triacylglycerols present in the oil was converted into a mixture consisting of fatty acid ethyl esters, free fatty acids, monoacylglycerols and diacylglycerols. The highest amount of fatty acid ethyl esters (52%) was achieved after 8 h.  相似文献   
1000.
A complex trisaccharide β-d-GalpNAcA-(1 → 4)-β-d-GlcpNAc-(1 → 4)-d-ManpNAc (3) was prepared in a good yield (35%) in a transglycosylation reaction catalyzed by β-N-acetylhexosaminidase from Talaromyces flavus using p-nitrophenyl 2-acetamido-2-deoxy-β-d-galacto-hexodialdo-1,5-pyranoside (1) as a donor followed by the in situ oxidation of the aldehyde functionality by NaClO2. The disaccharide β-d-GlcpNAc-(1 → 4)-d-ManpNAc (2) was used as galactosyl acceptor. A disaccharide β-d-GalpNAcA-(1 → 4)-d-GlcpNAc (4; 39%) originated as a by-product in the reaction. Oligosaccharides comprising a carboxy moiety at C-6 are shown to be very efficient ligands to natural killer cell activation receptors, particularly to human receptor CD69. Thus, oxidized trisaccharide 3 is the best-known oligosaccharidic ligand to this receptor, with IC50 = 2.5 × 10−9 M. The presented method of introducing a β-d-GalpNAcA moiety into carbohydrate structures is versatile and can be applied in the synthesis of other complex oligosaccharides.  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