全文获取类型
收费全文 | 501篇 |
免费 | 5篇 |
国内免费 | 1篇 |
专业分类
507篇 |
出版年
2017年 | 2篇 |
2014年 | 8篇 |
2013年 | 11篇 |
2012年 | 15篇 |
2011年 | 25篇 |
2010年 | 35篇 |
2009年 | 41篇 |
2008年 | 44篇 |
2007年 | 46篇 |
2006年 | 32篇 |
2005年 | 20篇 |
2004年 | 11篇 |
2003年 | 10篇 |
2002年 | 3篇 |
2001年 | 5篇 |
2000年 | 6篇 |
1999年 | 7篇 |
1998年 | 17篇 |
1997年 | 14篇 |
1996年 | 8篇 |
1995年 | 6篇 |
1994年 | 5篇 |
1993年 | 4篇 |
1992年 | 8篇 |
1991年 | 9篇 |
1990年 | 11篇 |
1989年 | 7篇 |
1987年 | 4篇 |
1985年 | 10篇 |
1983年 | 4篇 |
1982年 | 4篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 5篇 |
1978年 | 4篇 |
1977年 | 2篇 |
1976年 | 3篇 |
1975年 | 9篇 |
1974年 | 3篇 |
1971年 | 6篇 |
1959年 | 3篇 |
1958年 | 3篇 |
1955年 | 3篇 |
1954年 | 3篇 |
1951年 | 2篇 |
1950年 | 6篇 |
1949年 | 2篇 |
1941年 | 1篇 |
1938年 | 1篇 |
1929年 | 1篇 |
排序方式: 共有507条查询结果,搜索用时 8 毫秒
91.
PIERIK RONALD; VERKERKE WOUTER; VOESENEK RENS .A. C. J.; BLOM KEES .W. P. M.; VISSER ERIC J. W. 《Annals of botany》1999,84(6):755-762
In commercial growth of horticultural plants in greenhouses,high financial losses are being suffered due to the so-calledthick root syndrome (TRS), a phenomenon characterized by severedeterioration of the root system. The early symptoms of TRSin cucumber (Cucumis sativus L.) are strongly curving rootsthat are swollen and superficially damaged. Research focusedon ethylene, as both the culture practice of cucumber and theknown effects of ethylene on root growth point to a possiblerole for this phytohormone in TRS. Ethylene induced root curvatureand swelling as well as damage of the epidermal layer and outercortex of roots of cucumber plants similar to TRS symptoms.Formation of root hairs was stimulated and root elongation wasalso severely inhibited by exogenous ethylene. However, basedon experiments with the ethylene inhibitor 相似文献
92.
93.
94.
Diurnal regulation of photosynthesis in understory saplings 总被引:6,自引:1,他引:5
Photosynthetic rates of plants grown in natural systems exhibit diurnal patterns often characterized by an afternoon decline, even when measured under constant light and temperature conditions. Since we thought changes in the carbohydrate status could cause this pattern through feedback from starch and sucrose synthesis, we studied the natural fluctuations in photosynthesis rates of plants grown at 36 and 56 Pa CO2 at a FACE (free-air-CO2 -enrichment) research site. Light-saturated photosynthesis varied by 40% during the day and was independent of the light-limited quantum yield of photosynthesis, which varied little through the day. Photosynthesis did not correspond with xylem water potential or leaf carbohydrate build-up, but rather with diurnal changes in air vapor-pressure deficit and light. The afternoon decline in photosynthesis also corresponded with decreased stomatal conductance and decreased Rubisco carboxylation efficiency which in turn allowed leaf-airspace CO2 partial pressure to remain constant. Growth at elevated CO2 did not affect the afternoon decline in photosynthesis, but did stimulate early-morning photosynthesis rates relative to the rest of the day. Plants grown at 56 Pa CO2 had higher light-limited quantum yields than those at 36 Pa CO2 but, there was no growth–CO2 effect on quantum yield when measured at 2 kPa O2 . Therefore, understory plants have a high light-limited quantum yield that does not vary through the day. Thus, the major diurnal changes in photosynthesis occur under light-saturated conditions which may help understory saplings maximize their sunfleck-use-efficiency. 相似文献
95.
B. OVERGAARD NIELSEN 《Ecological Entomology》1978,3(3):193-201
Abstract. 1. The position and zonation of mines and galls along beech leaves were recorded and analysed by means of an R X C test of independence using the G-test; in a few cases analysis of variance was applied.
2. Ten arthropod species, of which a few were congeneric, were studied. All species were recorded from all leaf sections; however, in all species except one, significant preferences for certain zones were demonstrated. High frequencies of leaf mines and galls were recorded from the middle and basal leaf sections. Apparently, the leaf apex is an unimportant microhabitat for stationary beech phyllophages, except the eriophyid Aceric stenaspis stenaspis and final leaf mining stages of the beech weevil Rhynchaenus fagi; intra-leaf differences are discussed in relation to leaf grazing by mobile, chewing phyllophagous insects.
3. The position of some abundant phyllophages in space and time is discussed. In old beech stands feeding activity is largely concentrated in the low canopy. Presumably, differences in feeding technique and position of feeding tracks of beech phyllophages on the leaf contribute to food resource partition in this canopy layer. In three congeneric eriophyid species niche diversification occurred. Further, in two cecidomyiid species phenologjcal differentiation may contribute to subdivision of food resources. 相似文献
2. Ten arthropod species, of which a few were congeneric, were studied. All species were recorded from all leaf sections; however, in all species except one, significant preferences for certain zones were demonstrated. High frequencies of leaf mines and galls were recorded from the middle and basal leaf sections. Apparently, the leaf apex is an unimportant microhabitat for stationary beech phyllophages, except the eriophyid Aceric stenaspis stenaspis and final leaf mining stages of the beech weevil Rhynchaenus fagi; intra-leaf differences are discussed in relation to leaf grazing by mobile, chewing phyllophagous insects.
3. The position of some abundant phyllophages in space and time is discussed. In old beech stands feeding activity is largely concentrated in the low canopy. Presumably, differences in feeding technique and position of feeding tracks of beech phyllophages on the leaf contribute to food resource partition in this canopy layer. In three congeneric eriophyid species niche diversification occurred. Further, in two cecidomyiid species phenologjcal differentiation may contribute to subdivision of food resources. 相似文献
96.
97.
Bayesian clustering algorithms ascertaining spatial population structure: a new computer program and a comparison study 总被引:1,自引:0,他引:1
CHIBIAO CHEN ERIC DURAND FLORENCE FORBES OLIVIER FRANOIS 《Molecular ecology resources》2007,7(5):747-756
On the basis of simulated data, this study compares the relative performances of the Bayesian clustering computer programs structure , geneland , geneclust and a new program named tess . While these four programs can detect population genetic structure from multilocus genotypes, only the last three ones include simultaneous analysis from geographical data. The programs are compared with respect to their abilities to infer the number of populations, to estimate membership probabilities, and to detect genetic discontinuities and clinal variation. The results suggest that combining analyses using tess and structure offers a convenient way to address inference of spatial population structure. 相似文献
98.
CHRISTOPHER S. ROSENBERRY ERIC S. LONG HEATHER M. HASSEL-FINNEGAN VINCENT P. BUONACCORSI DUANE R. DIEFENBACH BRET D. WALLINGFORD 《The Journal of wildlife management》2009,73(3):357-361
ABSTRACT Behavioral studies of white-tailed deer (Odocoileus virginianus) often assign mother-offspring relationships based on common capture of juveniles with adult deer, assuming that fawns associate closely with mothers. We tested this assumption using genetic parentage to assess mother-offspring relationships within capture groups based on data from 10 polymorphic microsatellite loci. At the 80% confidence level, we assigned maternity to 43% and 51% of juveniles captured with an adult female in 2 respective study areas. Capture with their mother did not differ by sex of juveniles in either study area, and limiting our analysis to capture groups that most represent family groups (i.e., one ad F with 1–3 juv) did not increase maternity assignment (35%). Our results indicate that common capture may be a poor indicator of mother-offspring relationships in many field settings. We recommend genetic verification of family relationships. 相似文献
99.
Satellite-marked waterfowl reveal migratory connection between H5N1 outbreak areas in China and Mongolia 总被引:1,自引:0,他引:1
DIANN J. PROSSER JOHN Y. TAKEKAWA SCOTT H. NEWMAN BAOPING YAN DAVID C. DOUGLAS YUANSHENG HOU ZHI XING DEHAI ZHANG TIANXIAN LI YONGDONG LI DELONG ZHAO WILLIAM M. PERRY & ERIC C. PALM 《Ibis》2009,151(3):568-576
The role of wild birds in the spread of highly pathogenic avian influenza H5N1 has been greatly debated and remains an unresolved question. However, analyses to determine involvement of wild birds have been hindered by the lack of basic information on their movements in central Asia. Thus, we initiated a programme to document migrations of waterfowl in Asian flyways to inform hypotheses of H5N1 transmission. As part of this work, we studied migration of waterfowl from Qinghai Lake, China, site of the 2005 H5N1 outbreak in wild birds. We examined the null hypothesis that no direct migratory connection existed between Qinghai Lake and H5N1 outbreak areas in central Mongolia, as suggested by some H5N1 phylogeny studies. We captured individuals in 2007 from two of the species that died in the Qinghai Lake outbreaks and marked them with GPS satellite transmitters: Bar-headed Geese Anser indicus ( n = 14) and Ruddy Shelduck Tadorna ferruginea ( n = 11). Three of 25 marked birds (one Goose and two Shelducks) migrated to breeding grounds near H5N1 outbreak areas in Mongolia. Our results describe a previously unknown migratory link between the two regions and offer new critical information on migratory movements in the region. 相似文献
100.
MICHAEL N. DAWSON RICHARD K. GROSBERG YOEL E. STUART ERIC SANFORD 《Molecular ecology》2010,19(8):1585-1605
As range shifts coincident with climate change have become increasingly well documented, efforts to describe the causes of range boundaries have increased. Three mechanisms—genetic impoverishment, migration load, or a physical barrier to dispersal—are well described theoretically, but the data needed to distinguish among them have rarely been collected. We describe the distribution, abundance, genetic variation, and environment of Tetraclita rubescens, an intertidal barnacle that expanded its northern range limit by several hundreds of kilometres from San Francisco, CA, USA, since the 1970s. We compare geographic variation in abundance with abiotic and biotic patterns, including sea surface temperatures and the distributions of 387 co‐occurring species, and describe genetic variation in cytochrome c oxidase subunit I, mitochondrial noncoding region, and nine microsatellite loci from 27 locations between Bahia Magdalena (California Baja Sur, Mexico) and Cape Mendocino (CA, USA). We find very high gene flow, high genetic diversity, and a gradient in physical environmental variation coincident with the range limit. We infer that the primary cause of the northern range boundary in T. rubescens is migration load arising from flow of maladapted alleles into peripheral locations and that environmental change, which could have reduced selection against genotypes immigrating into the newly colonized portion of the range, is the most likely cause of the observed range expansion. Because environmental change could similarly affect all taxa in a region whose distributional limits are established by migration load, these mechanisms may be common causes of range boundaries and largely synchronous multi‐species range expansions. 相似文献