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   272篇
  免费   15篇
  2015年   4篇
  2013年   5篇
  2012年   3篇
  2011年   7篇
  2010年   5篇
  2009年   3篇
  2008年   8篇
  2007年   9篇
  2005年   4篇
  2004年   5篇
  2002年   4篇
  2000年   3篇
  1998年   5篇
  1997年   4篇
  1996年   5篇
  1994年   6篇
  1993年   3篇
  1992年   5篇
  1989年   7篇
  1987年   8篇
  1986年   4篇
  1985年   6篇
  1983年   5篇
  1980年   7篇
  1979年   4篇
  1978年   5篇
  1977年   6篇
  1976年   8篇
  1975年   3篇
  1974年   4篇
  1972年   3篇
  1971年   3篇
  1970年   2篇
  1968年   2篇
  1967年   3篇
  1966年   3篇
  1962年   2篇
  1959年   2篇
  1958年   11篇
  1957年   11篇
  1956年   5篇
  1955年   9篇
  1954年   9篇
  1953年   8篇
  1952年   9篇
  1951年   5篇
  1950年   5篇
  1949年   4篇
  1948年   3篇
  1945年   3篇
排序方式: 共有287条查询结果,搜索用时 15 毫秒
281.
Environmental processes govern demography, species movements, community turnover and diversification and yet in many respects these dynamics are still poorly understood at high latitudes. We investigate the combined effects of climate change and geography through time for a widespread Holarctic shrew, Sorex tundrensis. We include a comprehensive suite of closely related outgroup taxa and three independent loci to explore phylogeographic structure and historical demography. We then explore the implications of these findings for other members of boreal communities. The tundra shrew and its sister species, the Tien Shan shrew (Sorex asper), exhibit strong geographic population structure across Siberia and into Beringia illustrating local centres of endemism that correspond to Late Pleistocene refugia. Ecological niche predictions for both current and historical distributions indicate a model of persistence through time despite dramatic climate change. Species tree estimation under a coalescent process suggests that isolation between populations has been maintained across timeframes deeper than the periodicity of Pleistocene glacial cycling. That some species such as the tundra shrew have a history of persistence largely independent of changing climate, whereas other boreal species shifted their ranges in response to climate change, highlights the dynamic processes of community assembly at high latitudes.  相似文献   
282.
283.
The lily beetle Lilioceris lilii (Scopoli) (Coleoptera: Chrysomelidae) feeds on Lilium, Fritillaria and Cardiocrinum plants and is a serious pest in gardens and amenity plantings in parts of Northern Europe and North America. In the present study, the odour‐mediated behaviour of L. lilii is investigated by behavioural bioassays using a linear‐track olfactometer. The behavioural responses of L. lilii to hosts and conspecifics are, at least in part, odour‐mediated and the responses differ with respect to the physiological (reproductive) state of the adult beetle (i.e. pre‐ or post‐diapause). Significantly more diapaused female L. lilii move into air streams containing the odour of intact host plants than into clean air, and move into air streams containing odour of host plants and beetles combined in preference to odour from manually‐damaged host plants. Diapaused females also move into air streams containing odours from intact plants over those from larval‐infested plants. Pre‐diapause males move into the air streams of intact plants rather than L. lilii‐infested plants. Pre‐diapause females show no significant response in any experiment. The data indicate that the odour‐mediated responses of L. lilii are consistent with those known for other chrysomelids that produce a male aggregation pheromone to which reproductive individuals of both sexes respond.  相似文献   
284.
Factors controlling urease levels in sterile cultures of jackbeancotyledons were investigated. Urea, thiurea, not increased ratesof growth favour higher urease activity. Higher urease levelsin the urea-treated set were attributed to substrate protectionand/or induction of urease Lower urease activity in the absenceof thiourea may be due to inhibition of urease by metabolicallyproduced ammonia and increased protose activity. Utilizationof ammonia in protein synthesis, lower protease activity, andpossible induction of urease by thiourea in fast-growing tissuesof cotyledons may lead to higher urease activity. High temperaturelow pH, and the presence of EDTA, hydroxyurea, or actinomyhcinD cause a faster decrease in urease units.  相似文献   
285.
SYNOPSIS. Blood smears from 133 Rio Grande wild turkeys Meleagridis gallopavo intermedia live-trapped in the Welder Wildlife Refuge in south Texas, were examined for blood parasites. All of the 63 immature turkeys and 63% of the 70 adult turkeys were infected with Haemoproteus meleagridis. The infections were light, half the cases having less than 5 infected cells per 10,000 red blood cells. Infection with H. meleagridis caused significant enlargement of the host RBC. Other parasites specifically searched for but not found were Leucocytozoon, Trypanosoma, Plasmodium and microfilariae.  相似文献   
286.
External structures on the erect parts of zooids of Aetea havebeen demonstrated to be brood chambers by observation of release,settlement and metamorphosis of larvae from the chambers. Theancestrula is smaller than, but very similar to succeeding zooidsin the primary zone of astogenetic change, which do not showtubular connections. Sections through brood chambers and zooidsshow that part of the brood chamber wall may be slightly calcified.Brood chambers appear to be products of the external zooid walland not diverticula derived from the tentacle sheath.  相似文献   
287.
Abstract.  The Saturniidae, or wild silkmoths, number approximately 1861 species in 162 genera and nine subfamilies including Cercophaninae and Oxyteninae. They include some of the largest and most spectacular of all Lepidoptera, such as the moon or luna moths, atlas moths, emperor moths, and many others. Saturniids have been important as sources of wild silk and/or human food in a number of cultures, and as models for comparative studies of genetics, development, physiology, and ecology. Seeking to improve the phylogenetic framework for such studies, we estimated relationships across Saturniidae, sampling all nine subfamilies plus all five tribes of Saturniinae. Seventy-five exemplars (45 Saturniidae plus 30 bombycoid outgroups) were sequenced for four protein-coding nuclear gene regions (5625 bp total), namely CAD (the fusion protein carbamoylphosphate synthetase/aspartate transcarbamylase/dihydroorotase), DDC (dopa decarboxylase), period, and wingless. The data, analyzed by parsimony and likelihood, gave a strongly resolved phylogeny at all levels. Relationships among subfamilies largely mirrored the pre-cladistic hypothesis of Michener, albeit with significant exceptions, and there was definitive support for the morphology-based proposal that Ludiinae form a tribe (Micragonini) within Saturniinae. In the latter subfamily, the African tribe Urotini was shown to be paraphyletic with respect to Bunaeini and Micragonini, also in accord with recent morphological findings. Relationships within the New World subfamilies Arsenurinae, Ceratocampinae and Hemileucinae nearly always accord with previous morphology-based phylogenies when both are clearly resolved. Within Hemileucinae, Hemileucini are paraphyletic with respect to the monotypic Polythysanini. A preliminary biogeographical analysis supports ancestral restriction to the New World, followed by dispersal and/or vicariance splitting most of the family into a largely New World versus a largely Old World clade.  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