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   1164篇
  免费   21篇
  国内免费   1篇
  1186篇
  2016年   6篇
  2013年   19篇
  2012年   30篇
  2011年   46篇
  2010年   60篇
  2009年   51篇
  2008年   59篇
  2007年   67篇
  2006年   44篇
  2005年   47篇
  2004年   24篇
  2003年   22篇
  2002年   7篇
  2001年   11篇
  2000年   10篇
  1999年   15篇
  1998年   22篇
  1997年   34篇
  1996年   26篇
  1995年   30篇
  1994年   22篇
  1993年   18篇
  1992年   31篇
  1991年   28篇
  1990年   28篇
  1989年   22篇
  1988年   23篇
  1987年   19篇
  1986年   12篇
  1985年   19篇
  1984年   11篇
  1983年   16篇
  1982年   16篇
  1981年   8篇
  1980年   25篇
  1979年   13篇
  1978年   12篇
  1977年   14篇
  1976年   13篇
  1975年   9篇
  1974年   15篇
  1973年   18篇
  1972年   27篇
  1971年   31篇
  1965年   7篇
  1958年   5篇
  1957年   6篇
  1954年   5篇
  1953年   5篇
  1952年   11篇
排序方式: 共有1186条查询结果,搜索用时 0 毫秒
131.
Seasonal changes in the distribution and composition of common seal haul–out groups were followed in a study area in Orkney, Scotland. A marking programme was also undertaken, using both conventional and radio–tags, to study individual movements between sites and seasonal changes in site–use. Certain haul–out sites were used only in the breeding season, while others were used during the winter. Seals were seen at one site all year round and at another during only the pre–pupping and moult period. On one island where two sites were used during the summer, there were significant differences in the sex ratio of groups at the two sites: at one site males predominated and few pups were seen; on another, nearby, mothers and pups were regularly seen, although the site was also used by males. There was also evidence for segregation of the sexes outside the breeding season. Repeated observations of marked seals showed that seals used several different haul–out sites throughout the year, and that the seasonal changes in abundance at different sites resulted from individual changes in site–use. These changes in site–use are discussed in relation to feeding movements, breeding requirements and the physical characteristics of different sites.  相似文献   
132.
133.
SYNOPSIS. Ultracentrifugal and electrophoretic experiments arereported on the subunit composition of myosin from skeletalmuscle of a benthic fish, Coryphaenoides species. Coryphaenoidesmyosin undergoes extensive association in concentrated KGI solutionsat neutral pH, but sedimentation equilibrium experiments indicatethe presence of a small fraction (3%) of monomeric myosin withmolecular weight approximately 440,000. At pH 11, some of theaggregated myosin is dissociated, and monomeric myosin is itselfdissociated into a heavy component (410,000 mol wt) and a lightcomponent (14,000 mol wt) that comprises 5–7% of the protein.The lialkali component of Coryphaenoides myosin yields a singlepredominant band on cellulose acetate electrophoresis and SDS-ureaelectrophoresis in 9% acrylamide gel. The stoichiometric evidenceindicates that Coryphaenoides myosin contains two heavy chains(205,000 mol wt) and two light chains (14,000 mol wt) that areequivalent with respect to net electrostatic charge and molecularweight. Preparations of myosin obtained by direct extractionfrom muscle mince and by dissociation of actomyosin extractedfrom muscle mince also contain 5% of a 47,000 mol wt componentpresumably actin), traces of 34–36,000 mol wt component,and about 5.7% of low molecular weight material (10,000–15,000)that probably represents contaminant protein, although the possibilityof denatured nivosin subunits cannot be excluded.  相似文献   
134.
SYNOPSIS. We report results of a three year comparative laboratorystudy of kin recognition abilities in Arctic ground squirrels(Spermophilus parryii) and Belding's ground squirrels (S. beldingi),and a field investigation of kin recognition in S. beldingi.Our laboratory work shows that in both species, preweaned pupsreared together, whether they are biological siblings or cross-fostered(unrelated) nestmates, are equally aggressive in subsequentpaired arena tests. Thus, pups that share a natal nest are treatedlike siblings. Among pups reared apart, sister-sister pairsare less aggressive in arena tests than are pairs of nonkinfemales, whereas relatedness does not affect male-male or male-femaleaggression. Thus both relatedness and rearing environment mediaterecognition among female S. parryii and S. beldingi. In free-livingBelding's ground squirrels at Tioga Pass, California, dam-offspringand sister-sister recognition apparently first occur at weaning,coincident with aboveground emergence of juveniles. Most intriguing(electrophoretically identified) littermate full-sisters andmaternal half-sisters, which result from multiple mating byfemales, seem to treat each other differently despite havingshared a natal nest. The full-sisters are less agonistic andmore cooperative than the half-sisters. In interpreting theselaboratory and field results, we explore four proximal mechanismsby which kin might be identified, including one in which recognitionis based on (learned) phenotypic similarity to an individual'snestmates or itself (phenotype matching). Our data and thoseof several recent investigators of recognition in other taxaimplicate both association with relatives and phenotype matchingin the ontogeny of kin recognition.  相似文献   
135.
136.
Sensitivity analysis for matching with multiple controls   总被引:1,自引:0,他引:1  
ROSENBAUM  PAUL R. 《Biometrika》1988,75(3):577-581
  相似文献   
137.
Morris, P. J., Linsley, R. M. & Cottrell. J. F. 1991 01 15: A Middle Devonian symbiotic relationship involving a gastropod, a trepostomatous bryozoan. and an inferred sceondary occupant. Lethaia , Vol. 24. pp. 55–67. Oslo. ISSN 0024–1164.
The high-spired gastropod Palaeozygopleuru (Loxonematacea) of the Hamilton Group (Middle Devonian) in New York State is often entrusted by the trepostomatous bryozoan Leptotrypella (Hetero-trypidae). The form of this enerustation leads us to infer a secondary occupant that dwelled in empty shells of Palaeozygopleura while the bryozoan grew upon them. Encrusted specimens usually have an open aperture, thin or no enerustation on the apertural side of the gastropod shell. and thick encrustation on the abapertural side. While the aperture is invariably open. the columellar and parictal lips of the aperture, and the apertural face of the first whorl are usually encrusted. While the gastropod is alive, this area rests upon the dorsal surface of the foot and the remainder of the shell rests upon the substratc. These encrustation patterns suggest that an occupant of the shell. other than the gastropexl. prevented overgrowth of the aperture and oriented the shell aperture-down. This allowed ahapertural growth of the bryozoan. The presence of thin encrustation on the apertural side of the shell is problematical. It requires either the presence of a secondary occupant capable of holding a thinly encrusted shell off the substrate, or the ability of the juvenile bryozoan colony to extend onto portions of the shell that were in contact with the substrate. The presence of a secondary occupant. such as a sipunculan worm. capable of lifting the shell, but usually resting it aperture-down on the substrate seems the most suitable explanation for the nature of the encrustation. Gastropoda. bryozoa. symbiosis. Devoniun. hermit crab, Sipuncula  相似文献   
138.
Abstract The sedimentation of amyloplasts in inverted statocytes of the flower stalk of dandelion (Taraxacum officinale Weber) was followed using light microscopy. Much variety in sedimentation pattern was evident with complete sedimentation occurring in as little as 3–4 min to as much as a few h and individual amyloplasts showing barely perceptible sedimentation or sedimenting at speeds of up to 40 μm min?1. It was also apparent that amyloplasts were subjected to considerable movement after they had sedimented. Large vacuoles and cytoplasmic streaming in statocytes were confirmed and are considered to greatly influence amy-loplast sedimentation pattern. The implications of these observations for the starch-statolith hypothesis of geotropism are discussed.  相似文献   
139.
Several genes critical to the regulation of melanin production in mammals have recently been cloned and characterized. They map to the albino, brown, and slaty loci in mice, and encode proteins with similar structures and features, but with distinct catalytic capacities. The albino locus encodes tyrosinase, an enzyme with three distinct catalytic activities—tyrosine hydroxylase, 3,4-dihydroxyphenylalanine (DOPA) oxidase and DHI (5,6-dihydroxyindole) oxidase. The brown locus encodes TRP-l (tyrosinase-related protein-I), which has the same, but greatly reduced, catalytic potential. The slaty locus encodes TRP-2, another tyrosinase related-protein, which has DOPAchrome tautomerase activity. In this study we have examined the enzymatic interactions of these proteins, and their regulation by a novel melanogenic inhibitor. We observed that tyrosinase activity is more stable in the presence of TRP-l and/or TRP-2, but that the catalytic function of TRP-2 is not affected by the presence of TRP-1 or tyrosinase. Other factors also may influence melanogenesis and a unique melanogenic inhibitor suppresses tyrosinase and DOPAchrome tautomerase activities, but does not affect the spontaneous rate of DOPAchrome decarboxylation to DHI. The results demonstrate the catalytic functions of these proteins and how they stably interact within a melanogenic complex in the melanosome to regulate the quantity and quality of melanin synthesized by the melanocyte.  相似文献   
140.
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