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   3465篇
  免费   301篇
  国内免费   307篇
  2024年   10篇
  2023年   39篇
  2022年   59篇
  2021年   91篇
  2020年   96篇
  2019年   145篇
  2018年   143篇
  2017年   96篇
  2016年   123篇
  2015年   130篇
  2014年   218篇
  2013年   258篇
  2012年   138篇
  2011年   155篇
  2010年   125篇
  2009年   191篇
  2008年   193篇
  2007年   166篇
  2006年   158篇
  2005年   158篇
  2004年   126篇
  2003年   122篇
  2002年   124篇
  2001年   79篇
  2000年   69篇
  1999年   70篇
  1998年   76篇
  1997年   55篇
  1996年   63篇
  1995年   47篇
  1994年   66篇
  1993年   39篇
  1992年   47篇
  1991年   35篇
  1990年   37篇
  1989年   39篇
  1988年   29篇
  1987年   35篇
  1986年   15篇
  1985年   29篇
  1984年   37篇
  1983年   27篇
  1982年   27篇
  1981年   27篇
  1980年   17篇
  1979年   15篇
  1978年   9篇
  1977年   4篇
  1975年   5篇
  1973年   4篇
排序方式: 共有4073条查询结果,搜索用时 15 毫秒
41.
Acceptor proteins for poly(adenosine diphosphoribosyl)ation were determined in resting human lymphocytes, in lymphocytes with N-methyl-N′-nitro-N-nitrosoguanidine-induced DNA damage and in lymphocytes stimulated to proliferate by phytohemagglutinin. Kinetic studies showed that the increase in ADP-ribosylation which occurred in response to N-methyl-N′-nitro-N-nitrosoguanidine (MNNG) treatment was greater in magnitude but more transient in duration than that which occurred in phytohemagglutinin-stimulated cells. Gel electrophoretic analyses revealed that MNNG treatment and phytohemagglutinin stimulation both caused an increase in ADP-ribosylation of poly(ADP-ribose) polymerase and core histones. In MNNG-treated cells, an increase in ADP-ribosylation of histone H1 was also observed. In contrast, phytohemagglutinin-stimulated cells showed no increase in ADP-ribosylation of histone H1. In MNNG-treated cells there was also ADP-ribosylation of a protein of molecular weight 62 000, while in phytohemagglutinin-stimulated cells there was a marked increase in ADP-ribosylation of a protein of molecular weight 96000. MNNG treatment of phytohemagglutinin-stimulated cells produced a pattern of ADP-ribosylation that appeared to be due to the combined effects of the individual treatments. 3-Aminobenzamide effectively inhibited ADP-ribosylation under all treatment conditions.  相似文献   
42.
Critique of Wynne-Edwards' views on population regulation and sociality suppose a population of discrete, mutually exclusive groups essential to his thought. Yet both his past and present work focus on continually distributed, philopatric populations; his critics have argued the untenability of a position never his own. Wynne-Edwardsian ‘group selection’ focuses on local population productivity under philopatry. A ‘group’ is a local confluence of genotypes which need not be reified, and group selection consists of the differential replication (hence heritability) of the local social environment in which a genotype is embedded. Differential productivity contingent on social environment can eliminate some relational structures on genotypes in favor of others, creating an expanding wave of population productivity as in Wright's shifting balance metaphor. Such a process is inherent in the evolution of reciprocity, where cooperators must cluster to successfully invade a population of defectors. Regulation of resource exploitation in continuously distributed populations may be modeled as overlapping n-person Prisoner's Dilemmas, where each individual participates in several distinct commons and defection represents local over-exploitation of resources.  相似文献   
43.
The functions and social consequences of infant-adult male interaction in a captive group of lowland gorillas were evaluated. The two males repeatedly attempted to interact with the infant, the infant sometimes attempted to interact with the males, and the mother usually interrupted interactions between male and infant. Infant-directed actions by the two males frequently showed their interest in the infant and infrequently showed care giving toward her; their other infant-directed actions occurred near the time of excitement or of playful actions between adults. Male-directed actions by the infant frequently showed its interest in one male and infrequently showed care seeking from him. All infant-adult male interactions but one occurred in the less stimulating of the gorillas' two enclosures. Boredom and the mother's frequent thwarting of contact between a male and infant are suggested as influences on the males' interest in the infant. The data suggest that availability of interesting objects alleviated the boredom of captivity for males and thus dissuaded them from seeking stimulation which sometimes resulted in harmful behaviors toward the infant.  相似文献   
44.
Eighteen graded non-metric characters and 30 measurements of the mandible and lower dentition were used to investigate the grouping of African species of the genera Crocidura, Suncus and Sylvisorex. Seventy-seven taxa (species, subspecies and synonyms), including four European species, were represented by one to nine mandibles (147 mandibles in all), and the means of four Pleistocene African species were also included. The metric data were subjected to principal component and canonical variate analyses, and the non-metric data to principal component analysis. Grouping was detected by comparing the results of the three analyses. Six groups are recognized within the genus Crocidura , typified by C. fumosa, C. turba, C. russula, C. fuscomurina, C. hirta and C. flavescens. Sylvisorex is divisible into two groups, typified by S. lunaris and S. granti. The African species of Suncus (but not the introduced species, S. murinus ) form a single group, which is linked to the Sylvisorex granti group. Suncus is closely related to, or convergent with, the Crocidura fuscomurina group. The first non-metric principal component, the second metric principal component and the second canonical variate are significantly correlated. It is considered that these together represent a general trend of evolutionary advance. Phylogenetic relations are discussed in the light of this; Crocidura may be diphylefic.  相似文献   
45.
Abstract. South African mountain fynbos has been severely invaded by trees and shrubs introduced from other mediterranean-climate regions. Management of these invasions should involve controlling current invaders and screening future introductions. Invasion windows are described and functional groups are defined for pines based on life history attributes important for invasion in the fire-prone mountain fynbos. The most successful invasive pines here (Pinus halepensis, P. pinaster and P. radiata) are fire-resilient and have small seeds, low seed-wing loadings, short juvenile periods, moderate to high degrees of serotiny and relatively poor fire-tolerance as adults. Other species with these attributes, especially from mediterranean-climate regions, wouldbe high-risk introductions. Taxa in other functional groups have not become major weeds even with widespread man-aided dissemination. Experience with pine invaders was used to define functional groups in western Australian Banksia species (Proteaceae), shrubs and trees which include taxa with similar attributes to fynbos invaders (e.g. Hakea and Pinus spp.). Banksias have only recently been introduced to the Cape, and are likely to be increasingly cultivated for the cut flower market. Tall serotinous shrubs with many small seeds per plant, short juvenile periods and low fire tolerance were identified as high risk introductions. This group includes thicket-forming species which maintain very large viable seed banks, e.g. Banksia burdettii, B. hookeriana and B. leptophylla. Low sprouting shrubs with few large seeds per plant and long juvenile periods are unlikely to become invasive in mountain fynbos. The approach of defining functional groups based on life history attributes and invasion windows is valuable for predicting the probability of invasive success. Chance interactions suchas an opportunistic dispersal mutualism between Pinus pinea and an introduced squirrel sometimes confound these predictions and underscore the idiosyncracies inherent in biological invasions.  相似文献   
46.
Morphological and hybridization experiments were performed on Caloglossa leprieurii (Montagne) J. Agardh collected from Japan, Singapore and Australia in order to evaluate taxonomic characters of this species. Within C. leprieurii at least four mating groups were recognized from the Indo-Pacific region. These mating groups could be characterized by the blade width at the inter-node and the cell-row numbers on the opposite side derived from the first axial cell at the main axis, though these properties showed a certain variability even in the same plant under both field and culture conditions. The phylogenetic relationship and geographic distribution of the four mating groups are discussed.  相似文献   
47.
The three-dimensional structure ofDolichos biflorus seed lectin has been constructed using five legume lectins for which high resolution crystal structures were available. The validity of the resulting model has been thoroughly investigated. Final structure optimization was conducted for the lectin complexed with GalNAc, providing thereby the first three-dimensional structure of lectin/GalNAc complex. The role of theN-acetyl group was clearly evidenced by the occurrence of a strong hydrogen bond between the protein and the carbonyl oxygen of the carbohydrate and by hydrophobic interaction between the methyl group and aromatic amino acids. Since the lectin specificity is maximum for the Forssman disaccharide GalNAc(1–3)GalNAc-O-Me and the blood group A trisaccharide GalNAc(1–3)[Fuc(1–2)]Gal-O-Me, the complexes with these oligosaccharides have been also modelled.  相似文献   
48.
The carbohydrate binding properties of theDolichos biflorus seed lectin and DB58, a vegetative tissue lectin from this plant, were compared using two types of solid phase assays. Both lectins bind to hog blood group A + H substance covalently coupled to Sepharose 4B and this binding can be inhibited with free blood group A + H substance. However, the binding of the seed lectin is inhibited byD-GalNAc whereas DB58 binding was not inhbited by any monosaccharide tested, thus suggesting that its carbohydrate combining site may be more extensive than that of the seed lectin. The activities of these two lectins also differ from one another in ability to recognize blood group A + H substance adsorbed on to plastic and in the effects of salt and urea on their carbohydrate binding activities. Neither lectin showed glycosidase activity with p-nitrophenyl -D-GalNAc or p-nitrophenyl -D-GalNAc.  相似文献   
49.
含金地质体表生带中主要微生物类群的研究   总被引:4,自引:0,他引:4  
通过对16个含金矿区主要微生物类群的分布、生态结构的研究,发现不同类型的金矿具有不同的微生物类群.金矿矿石与金矿酸水中的自养硫细菌和异养菌分布具有明显差异,酸性水中含自养硫细菌较多,含异养菌极少;金矿氧化带中的异氧菌一般多于自氧硫细菌,其优势菌以芽孢杆菌属中蜡状芽孢杆菌为主.研究结果揭示了金矿中存在的主要微生物类群,对阐明表生金的形成、成岩和蚀变作用具有一定的意义,对发掘生物湿法冶金菌种资源具有重要作用.  相似文献   
50.
The shrews of the Sorex araneus group have undergone a spectacular chromosome evolution. The karyotype of Sorex granarius is generally considered ancestral to those of Sorex coronatus and S. araneus. However, a sequence of 777 base pairs of the cytochrome b gene of the mitochondrial DNA (mtDNA) produces a quite different picture: S. granarius is closely related to the populations of S. araneus from the Pyrenees and from the northwestern Alps, whereas S. coronatus and S. araneus from Italy and the southern Alps represent two well-separated lineages. It is suggested that mtDNA and chromosomal evolution are in this case largely independant processes. Whereas mtDNA haplotypes are closely linked to the geographical history of the populations, chromosomal mutations were probably transmitted from one population to another. Available data suggest that the impressive chromosome polymorphism of this group is quite a recent phenomenon.  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