全文获取类型
收费全文 | 6133篇 |
免费 | 1427篇 |
国内免费 | 3726篇 |
出版年
2024年 | 7篇 |
2023年 | 376篇 |
2022年 | 332篇 |
2021年 | 480篇 |
2020年 | 710篇 |
2019年 | 760篇 |
2018年 | 596篇 |
2017年 | 578篇 |
2016年 | 605篇 |
2015年 | 506篇 |
2014年 | 538篇 |
2013年 | 566篇 |
2012年 | 470篇 |
2011年 | 439篇 |
2010年 | 398篇 |
2009年 | 419篇 |
2008年 | 397篇 |
2007年 | 442篇 |
2006年 | 360篇 |
2005年 | 342篇 |
2004年 | 256篇 |
2003年 | 261篇 |
2002年 | 197篇 |
2001年 | 203篇 |
2000年 | 176篇 |
1999年 | 125篇 |
1998年 | 109篇 |
1997年 | 86篇 |
1996年 | 68篇 |
1995年 | 78篇 |
1994年 | 63篇 |
1993年 | 31篇 |
1992年 | 41篇 |
1991年 | 42篇 |
1990年 | 38篇 |
1989年 | 33篇 |
1988年 | 21篇 |
1987年 | 13篇 |
1986年 | 9篇 |
1985年 | 14篇 |
1984年 | 12篇 |
1983年 | 8篇 |
1982年 | 37篇 |
1981年 | 10篇 |
1980年 | 7篇 |
1978年 | 4篇 |
1977年 | 4篇 |
1976年 | 5篇 |
1975年 | 4篇 |
1958年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
61.
Species boundaries,phylogenetic relationships,and ecological differentiation inAnthriscus (Apiaceae)
Krzysztof Spalik 《Plant Systematics and Evolution》1996,199(1-2):17-32
Species boundaries and phylogenetic relationships of 17 taxa ofAnthriscus (Apiaceae), with special emphasis on the critical sect.Cacosciadium, were explored using morphological data with principal component analysis, phenetics, and phylogenetics. The analyses did not provide satisfactory resolution of taxa from sect.Cacosciadium and only four species were retained. The total number of species was reduced to nine. Sect.Cacosciadium is distinguished by only two synapomorphies while sects.Anthriscus andCaroides are better supported. Present geographic and ecological variation suggests that the radiation ofAnthriscus occurred through divergence of peripheral isolated populations adapting to different habitats: high montane meadows and screes, shady climax forests, and seasonally dry habitats at lower altitudes. The adaptive significance of particular morphological traits is discussed. 相似文献
62.
谷蠹生态学特性的研究 总被引:3,自引:0,他引:3
谷蠹Rhyzoperthadominica(F.)生态学特性的试验结果表明,温度、湿度、粮食含水量对其各发育历期影响非常显著;食物影响不显著。 相似文献
63.
Uncultivated plants growing on disturbed sites may be useful for assessing the bioavailability of some metals in soils, and thus the potential for metal mobilization up the terrestrial food chain, an important element in ecological risk assessment. A planted chicory cultivar (Cichorium intybus L. var. foliosum Hegi.) and the uncultivated plants horseweed (Canada fleabane) (Erigeron canadensis L.) and dogfennel (Eupatorium capillifolium (Lam.) Small) were evaluated for their ability to act as index plant species for soil Cd, Cr, Ni, and V at two field sites where these metals had been applied five yr previously to two highly weathered sandy Ultisols. Soil Cd was available to all analyzed plant tissues of all three plant species at both sites, particularly on the sandier Blanton soil. Chicory was an effective index plant for Cd on the finer textured Orangeburg soil but functioned as an indicator plant (toxicity symptoms were observed) on the sandier Blanton soil. Horseweed and dogfennel were effective index plants for Cd in both contaminated soils. Soil Cr, Ni, and V were less bioavailable than soil Cd and plant metal uptake was more sensitive to residual soil Cr, Ni, and V than was soil extraction with double acid. Horseweed and chicory may have potential as index plants for soil Cr. Chicory may have potential as a Ni index plant. Chicory and dogfennel may have potential as V index plants. 相似文献
64.
ITE Merlewood Land Classification of Great Britain 总被引:7,自引:0,他引:7
R. G. H. Bunce C. J. Barr R. T. Clarke D. C. Howard A. M. J. Lane 《Journal of Biogeography》1996,23(5):625-634
Abstract. The Institute of Terrestrial Ecology (ITE) has classified the 1 km squares in Great Britain (GB) into thirty-two environmental strata, termed land classes, as a basis for ecological survey. The classes have been used in biogeographical studies of the distribution of individual species and species assemblages. The concept behind the technique is that there is an association between the environmental character of land and ecological parameters. The initial classification was based on a sample of squares drawn from a regular grid. The data for the 12121 km squares classified were drawn from published maps; the number of squares was limited by the available computing power. Subsequently the availability of more powerful computers and the need to improve geographical definition have led to the allocation of every 1 km square to its appropriate class. This paper has been written to summarise the principles involved in the development of the system and indicate the range of projects for which it has been used. The extension of the classification from a sample to the complete coverage of GB revealed the importance of the structure and style of data used to produce the classification. The significance of these conclusions for future work is discussed, with particular reference to automated methods of data capture. 相似文献
65.
66.
67.
68.
69.
不同生境下蓝桉的木材解剖研究 总被引:9,自引:0,他引:9
用光镜及扫描电镜对不同生境下的蓝桉(Eucalyptusglobulus Labill.)木材结构进行了解剖学观察,描述了木材结构特征。以蓝桉木材的11 个主要特征——导管频率、单孔率、导管直径、导管分子长度、管胞长度和直径、纤维管胞长度和直径、纤维长度、射线频率和高度为指标,对生长在不同生境下的蓝桉进行比较发现:随着纬度的增加,生长轮变化不明显;木材中大部分组成分子的数量特征,除导管频率有降低趋势外均与纬度成正相关。评述了年降雨量对蓝桉木材结构的影响 相似文献
70.
合理用药生态控制茶小绿叶蝉主要措施与评价 总被引:10,自引:0,他引:10
合理用药生态控制茶小绿叶蝉主要措施与评价张觉晚,王沅江(湖南省茶叶研究所,长沙410145)PrincipalTechniquesofRationalUseofPesticideforEcologicalControloverTeaGresnLeaf... 相似文献