全文获取类型
收费全文 | 13113篇 |
免费 | 90篇 |
国内免费 | 244篇 |
专业分类
安全科学 | 159篇 |
废物处理 | 659篇 |
环保管理 | 1636篇 |
综合类 | 1840篇 |
基础理论 | 3889篇 |
污染及防治 | 2578篇 |
评价与监测 | 991篇 |
社会与环境 | 1647篇 |
灾害及防治 | 48篇 |
出版年
2023年 | 58篇 |
2022年 | 130篇 |
2021年 | 104篇 |
2020年 | 67篇 |
2019年 | 99篇 |
2018年 | 676篇 |
2017年 | 659篇 |
2016年 | 645篇 |
2015年 | 206篇 |
2014年 | 262篇 |
2013年 | 697篇 |
2012年 | 482篇 |
2011年 | 1086篇 |
2010年 | 710篇 |
2009年 | 778篇 |
2008年 | 976篇 |
2007年 | 1207篇 |
2006年 | 308篇 |
2005年 | 357篇 |
2004年 | 317篇 |
2003年 | 376篇 |
2002年 | 400篇 |
2001年 | 353篇 |
2000年 | 227篇 |
1999年 | 120篇 |
1998年 | 107篇 |
1997年 | 84篇 |
1996年 | 107篇 |
1995年 | 126篇 |
1994年 | 96篇 |
1993年 | 85篇 |
1992年 | 90篇 |
1991年 | 92篇 |
1990年 | 79篇 |
1989年 | 71篇 |
1988年 | 69篇 |
1987年 | 75篇 |
1986年 | 52篇 |
1985年 | 63篇 |
1984年 | 72篇 |
1983年 | 62篇 |
1982年 | 58篇 |
1981年 | 64篇 |
1980年 | 47篇 |
1979年 | 46篇 |
1977年 | 38篇 |
1976年 | 36篇 |
1974年 | 34篇 |
1973年 | 43篇 |
1972年 | 51篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
122.
123.
124.
125.
126.
127.
In angiosperms, archesporial cells in the anther primordium undergo meiosis to form haploid pollen, the sole occupants of anther sacs. Anther sacs are held together by a matrix of parenchyma cells, the connective tissue. Cells of the connective tissue are not known to differentiate. We report the differentiation of parenchyma cells in the connective tissue of two Gordonia species into pollen-like structures (described as pseudopollen), which migrate into the anther sacs before dehiscence. Pollen and pseudopollen were distinguishable by morphology and staining. Pollen were tricolpate to spherical while pseudopollen were less rigid and transparent with a ribbed surface. Both types were different in size, shape, staining and surface architecture. The ratio of the number of pseudopollen to pollen was 1:3. During ontogeny in the connective tissue, neither cell division nor tetrad formation was observed and hence pseudopollen were presumed to be diploid. Only normal pollen germinated on a germination medium. Fixed preparations in time seemed to indicate that pseudopollen migrate from the connective tissue into the anther sac. 相似文献
128.
129.
There are numerous reports of invertebrates that are visual mimics of ants, but no formal reports of mimicry of an ant, by
an ant. Two endemic Australian ants, Myrmecia fulvipes and Camponotus bendigensis are remarkably similar in colour and size; both are generally black but have red legs and golden gasters. The density and
hue of the pubescence of each ant's gaster are relatively uncommon in ants, but are very rare when combined with the black
forebody and red legs. The ants are similarly sized but are smaller than other species closely related to M. fulvipes. The range of C. bendigensis lies entirely within that of M. fulvipes, and both species excavate ground nests in open woodland. Finally, workers of both species are crepuscular and forage solitarily.
These data suggest that the relatively benign formicine C. bendigensis is a Batesian mimic of the formidable myrmeciine M. fulvipes.
Received: 9 August 1999 / Accepted in revised form: 22 December 1999 相似文献
130.