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   2880篇
  免费   38篇
  国内免费   3篇
  2015年   20篇
  2014年   21篇
  2013年   71篇
  2012年   72篇
  2011年   117篇
  2010年   167篇
  2009年   166篇
  2008年   155篇
  2007年   190篇
  2006年   105篇
  2005年   115篇
  2004年   52篇
  2003年   32篇
  2002年   27篇
  2001年   27篇
  2000年   23篇
  1999年   33篇
  1998年   61篇
  1997年   81篇
  1996年   68篇
  1995年   57篇
  1994年   54篇
  1993年   61篇
  1992年   55篇
  1991年   57篇
  1990年   60篇
  1989年   64篇
  1988年   50篇
  1987年   60篇
  1986年   56篇
  1985年   49篇
  1984年   35篇
  1983年   41篇
  1982年   24篇
  1981年   38篇
  1980年   37篇
  1979年   49篇
  1978年   32篇
  1977年   24篇
  1976年   17篇
  1975年   32篇
  1974年   34篇
  1973年   41篇
  1972年   58篇
  1971年   36篇
  1970年   15篇
  1969年   12篇
  1968年   11篇
  1954年   11篇
  1951年   11篇
排序方式: 共有2921条查询结果,搜索用时 15 毫秒
971.
A STUDY OF TEXTURE-FLAVOR INTERACTIONS USING FREE-CHOICE PROFILING   总被引:1,自引:0,他引:1  
Interactions between perceptions of texture and volatile flavors were assessed by free-choice profiling using gelatin desserts as a model product. Gels were prepared with all combinations of 2 levels of gelatin, and 2 concentrations of 2 different flavors. A trained sensory panel profiled each sample for aroma, flavor, and oral and manual texture. Generalized Procrustes Analysis was carried out separately for each sensory modality. The results indicate that while gelatin concentration and flavor type both modified perception of flavor, flavor concentration did not. Gelatin content influenced instrumental, oral, and manual evaluations of texture; however, the type and level of flavoring had little influence on oral perception of texture.  相似文献   
972.
The moult of Barred Warblers Sylvia nisoria was studied during three winter seasons in southeastern Kenya at a southward passage site (Ngulia) and a wintering site (Mtito Andei). Most Barred Warblers migrating through Ngulia in November had yet to commence winter moult. These birds probably moulted subsequently in winter in northern Tanzania. In December, birds were found in heavy moult at Mtito Andei, and some of these birds were known to stay throughout the winter. By contrast, most birds reaching southeastern Kenya from late December onwards had already completed part or all of their winter moult, presumably at stopover sites in northern and eastern Kenya or in Ethiopia. Thus, winter moult in Barred Warblers takes place mainly in late November and December, either just before or soon after the final leg of autumn migration. In general, first-year birds renewed all tertials and tail feathers, about three to five secondaries per wing and commonly also the outer one to four large primaries per wing. Adults renewed all tertials and tail feathers, almost all secondaries and only occasionally an outer primary. The replacement of relatively fresh juvenile secondaries during the birds' first winter implies that the split moult pattern of this species (secondaries, tertials and tail moulted in winter; primaries and tertials in summer) is endogenously controlled.  相似文献   
973.
Mass accumulations of belemnite rostra ('belemnite battlefields') are common in Mesozoic sediments, and accumulations of belemnoids are also known from older rocks. Many Recent teuthid species suffer mortality immediately after spawning, and some authors have suggested that belemnite accumulations record a similar phenomenon. Conversely, it is clear that many belemnite battlefields actually formed in an environment of net sediment loss, with current alignment and winnowing. A proposed model for the production of belemnite battlefields involves five possible pathways: post-spawning mortality, catastrophic mass mortality, predation concentration, stratigraphical condensation, and resedimentation. Although accumulations that have not been reworked are rare, it is possible to recognise the generating agents of belemnite accumulations through their intrinsic features. A genetic classification of belemnite battlefields, based on the identified pathways, is tested through field examples in Britain and Antarctica, as well as through published examples. □ Belemnites, fossil accumulations, palaeoecology, Mesozoic, Antarctica, Yorkshire.  相似文献   
974.
The folded and twisted calcareous ribbon, forming both the ascending and descending lamellae of the loop of Waltonia inconspicua (Sowerby), is a two-layered structure consisting of a wedge of regularly stacked secondary layer fibres that overlie a thin layer of non-fibrous calcite (herein termed brachiotest). On one surface, that facing into the mantle cavity, secondary fibrous mosaic predominates, but smooth, finely banded brachiotest occurs as a narrow marginal lip upon which secondary layer fibres proliferate and progressively overlap. This growing edge of the ribbon is secreted by long, folded epithelial cells with digitate extensions to their apical plasmalemmas, which are distinguishable from the cuboidal epithelium-secreting fibres and their membranous sheaths. The other surface, facing the body cavity and the brachial coelom, consists entirely of roughened brachiotest exhibiting prominent banding that is aligned parallel to the growing loop edge. This surface is overlain by microfilamentar epithelium acting as a holdfast for the connective tissue frame of the lophophore. The other edge of the ribbon consists of truncated sections of both secondary-layer fibres and brachiotest which bear signs of resorption consistent with the degenerated state of the associated epithelium. Growth of the Waltonia loop is controlled by these localized processes of secretion and resorption of the fibrous and brachiotest layers and is typical of all terebratulides so far studied. The brachiotest is not homologous with the non-fibrous primary shell secreted at the valve margin. □ Brachiopoda, Articulata, Terebratulida, ultrastructure, lophophore, loop.  相似文献   
975.
The Phytophagous Insects Data Base (PIDB) was used to summarize information about 6933 species of British insects/mites and their food-plant families. Total species in 183 insect families on 127 vascular plant families were correlated with numbers of plant species in each family in Britain and Europe, accounting for 41.8% of the variation. Families with trees had more insects, while ferns and aquatic, uncommon and ephemeral plants, particularly orchids, had fewer. Seven individual insect families were analysed separately: Aphididae were more closely correlated with plant species numbers than were the more polyphagous Geometridae. Agromyzidae were uncommon on families with trees. Of insect species 75.8% fed on only one plant family and 10.1% were recorded on two families. Species feeding on three families or more were considered to be polyphagous. In the 30 largest insect families, eight had species feeding on only one plant family, while 12 had more than 20% of their species polyphagous. Proportions of family-restricted insects were high on Pinaceae, Gramineae and Compositae, but low on Corylaceae, Primulaceae and Tiliaceae. More pest species were polyphagous. Polyphagy was related to large larvae, larvae overwintering, wood-feeding, ephemeral food sources, poor dispersal abilities and omnivory. Some polyphagous species might be actively evolving in exploitation of vacant niches, leading to later progressive specialization to food-plants.  相似文献   
976.
MIDDLETON, D. J., A systematic survey of leaf and stem anatomical characters in the genus Gaultheria and related genera (Ericaceae). Leaf and stem anatomical characters of 123 species within the Gaultheria group of genera of the tribe Andromedeae are surveyed. A number of characters show considerable variation within a species which limits their taxonomic use. Other characters such as marginal sclerenchyma and, to a lesser extent, the hypodermis, the presence of adaxial stomata, the presence of free fibres and the pith type were of more taxonomic interest. These characters are discussed in relation to the generic and infrageneric classification of the group.  相似文献   
977.
Ontogenetic changes in femoral morphology and locomotion were analysed in the iguanodontian dinosaur Dryosaurus lettowvorbecki using cross-sectional data and applying principles of beam theory. The results presented here suggest that locomotor ontogeny in D. lettowvorbecki was more complicated than has generally been recognized. The percentage cortical area (a measure of the relative amount of bone) increases abruptly over a relatively short period during early ontogeny and then remains uniform during subsequent increases in body size. Modifications in cross-sectional shape also occur with increasing size, as demonstrated by differences in second moment of area ratios. The patterns of change in these properties indicate that the orientation of mechanical loadings acting on the femur of D. lettowvorbecki differed at various stages of growth and development. It is suggested that the alterations in femoral architecture described here reflect a shift from quadrupedality to bipedality early in the ontogeny of this animal.  相似文献   
978.
The head of Anomochilus weberi combines features seen in living uropeltines and scolecophidians, two clades of fossorial snakes that appear to have the most specialized and, at the same time, the most divergent modifications of the head. However, the weakly supported premaxilla of Anomochilus departs from both scolecophidian and uropeltine modes of reinforcing the anterior tip of the snout, suggesting that Anomochilus is a less specialized burrower. Its skull also has a number of features unusual among snakes, including a unique buttress on the anterior ends of the septomaxillae, an ectopterygoid reduced to a splint that touches neither maxilla nor pterygoid, a short maxillary tooth row oriented at 45° to the long axis of the skull, and a braincase and snout complex that are uniformly wide. The features of the upper jaw are predicted to confer behavioural and mechanical attributes intermediate between those of typhlopid scolecophidians and uropeltines.  相似文献   
979.
980.
The existing definition of spatangoid fascioles as a narrow band of minute tubercles (miliaries) is inadequate. One reason is that many intermediates between the presence of indisputable fascioles and their total absence are known. Although fascioles are widely used in the classification of spatangoids, diversity of fasciole tuberculation has largely been ignored. We examine fascioles in about 100 spatangoid species, focusing on the earliest manifestations of fascioles (both developmentally and phylogenetically) and on their variable tuberculation. Qualitative observations are complemented by quantitative analyses (ANOVA and PCA) of 21 species. Three types of fascioles are defined according to tubercle pattern: protofascioles for localized concentrations of miliaries; parafascioles for bands of miliaries with a progressive transition to the surrounding tuberculation; orthofascioles for strongly differentiated, well-circumscribed bands of miliaries. Ontogenetic, architectural, ecological, and taxonomic variations of these three patterns are qualitatively and quantitatively investigated. We attempt to delineate plausible homologies, origins, and subsequent evolution of the different kinds of fascioles.  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