全文获取类型
收费全文 | 16304篇 |
免费 | 84篇 |
国内免费 | 168篇 |
专业分类
安全科学 | 196篇 |
废物处理 | 1075篇 |
环保管理 | 1768篇 |
综合类 | 2127篇 |
基础理论 | 4643篇 |
环境理论 | 8篇 |
污染及防治 | 3934篇 |
评价与监测 | 1496篇 |
社会与环境 | 1258篇 |
灾害及防治 | 51篇 |
出版年
2023年 | 80篇 |
2022年 | 202篇 |
2021年 | 217篇 |
2020年 | 110篇 |
2019年 | 129篇 |
2018年 | 1627篇 |
2017年 | 1562篇 |
2016年 | 1478篇 |
2015年 | 380篇 |
2014年 | 426篇 |
2013年 | 597篇 |
2012年 | 870篇 |
2011年 | 1742篇 |
2010年 | 990篇 |
2009年 | 865篇 |
2008年 | 1246篇 |
2007年 | 1565篇 |
2006年 | 359篇 |
2005年 | 270篇 |
2004年 | 280篇 |
2003年 | 261篇 |
2002年 | 276篇 |
2001年 | 128篇 |
2000年 | 77篇 |
1999年 | 70篇 |
1998年 | 42篇 |
1997年 | 38篇 |
1996年 | 38篇 |
1995年 | 45篇 |
1994年 | 41篇 |
1993年 | 20篇 |
1992年 | 27篇 |
1991年 | 28篇 |
1990年 | 23篇 |
1989年 | 21篇 |
1988年 | 20篇 |
1987年 | 13篇 |
1986年 | 29篇 |
1985年 | 16篇 |
1984年 | 25篇 |
1983年 | 21篇 |
1982年 | 21篇 |
1980年 | 13篇 |
1976年 | 11篇 |
1972年 | 10篇 |
1966年 | 19篇 |
1965年 | 12篇 |
1964年 | 12篇 |
1956年 | 11篇 |
1954年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
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
82.
83.
84.
85.
86.
Jan Gustafsson Göran Annerén Ulla-Britt Ericsson Lars Svanberg Sten A. Ivarsson 《黑龙江环境通报》1995,15(5):451-454
The observation that thyroid disease is frequent in mothers of children with Down syndrome (DS) has suggested that maternal thyroid antibodies could be a factor predisposing to trisomy 21 in their offspring. In this study, the incidences of thyroglobulin (Tg) and thyroid peroxidase (TPO) antibodies were analysed with a sensitive solid-phase immunosorbent radioassay in sera from 29 mothers giving birth to children with trisomy 21 and 87 control mothers. The serum samples were collected at delivery. There was no statistical difference regarding the proportion of thyroid antibodies (against Tg and/or TPO) in the two groups. Thyroid antibodies were detected in 6/29 (20.7 per cent) of the DS mothers and in 23/87 (26.4 per cent) of the control mothers. Among the women with thyroid antibodies, 4/6 (66.7 per cent) of the DS mothers and 12/23 (52 per cent) of the control mothers had antibodies against both Tg and TPO. There was no increase in the relative risk of having a child with DS if the titre of either Tg or TPO antibodies or both were positive, i.e. ≥ 1/5. The results indicate that the presence of thyroid antibodies in the serum of a pregnant woman has no prognostic value for the birth of an infant with DS. 相似文献
87.
88.
Maguelone G. Forest Hervé Bétuel Philippe Couillin André Boué Michel David Daniel Floret René Francois Pierre Guibaud Henri Plauchu Raphaël Rappaport 《黑龙江环境通报》1981,1(3):197-207
Amniotic fluid (AF) levels of 17-hydroxyprogesterone (17OHP) and testosterone (T) were determined at 16–17 weeks in 17 pregnancies at risk for CAH and results compared to 75 normal controls. The fetus was predicted to be unaffected in 12 cases on the findings of normal AF levels of both 17OHP and T and the latter allowed a correct prediction of fetal sex in all instances. HLA typing confirmed normality in 12 cases revealing 5 carriers, 5 homozygous normal and 2 indeterminate. Steroid levels of the 2 groups were similar. Three fetuses were predicted to be CAH affected on unambiguously high levels of 17OHP and T (in female only). HLA typing was in agreement, and the diagnosis was confirmed in 2 abortuses and a female newborn by physical and hormonal studies. In the last 2 cases AF levels of OHP and T were normal but HLA (A/B/C) genotypes were identical to the CAH affected siblings. Normal physical and hormonal findings in the 2 aborted fetuses would exclude the possibility of an in utero virilizing form of CAH. The discrepancy could be explained on the basis that the fetuses had an allelic form of 21-hydroxylase deficiency or on the basis of recombination (not fully tested). It is concluded that a fully informative prenatal diagnosis of CAH should not rely entirely on HLA typing but on hormonal studies. 相似文献
89.
Masera Omar R. Cerón Alma Delia Ordóñez Antonio 《Mitigation and Adaptation Strategies for Global Change》2001,6(3-4):291-312
We examine carbon (C) reference and mitigation scenarios for the Mexicanforest sector between the year 2000 and 2030. Estimates are presentedseparately for the period 2008–2012.Future C emissions and capture are estimated using a simulation modelthat: a) allocates the country land use/land cover classes among differentfuture uses and categories using demand-based scenarios for forestryproducts; b) estimates the total C densities associated to each land usecategory, and c) determines the net carbon implications of the process ofland use/cover change according to the different scenarios.The options analyzed include both afforestation/reforestation, such ascommercial, bionenergy and restoration plantations, and agroforestrysystems, and forest conservation, through the sustainable management ofnative forests and forest protection.The total mitigation potential, estimated as the difference between the totallong-term carbon stock in the reference and the mitigation scenario reaches300 × 106 Mg C in the year 2012 and increases to 1,382 × 106 Mg C in 2030. The average net sequestration in the 30 year period is 46 × 106 Mg C yr-1, or 12.5 × 106 Mg C yr-1 within the period 2008 to 2012. The costs of selected mitigation options range from 0.7–3.5 Mg C-1 to 35 Mg C-1. Some options are cost effective. 相似文献
90.
Pregnancies with fetal trisomy 21 have been associated with low amniotic fluid alpha-fetoprotein levels (AFAFP). This observation led to the suggestion that low AFAFP levels be used as a criterion for completion of a chromosomal analysis in patients who are not otherwise at increased risk for a fetal chromosome abnormality and in whom karyotyping might not have been completed for economic reasons. In order to assess the usefulness of such criteria, we reviewed the AFAFP levels of 90 cases of fetal trisomy 21, 23 cases of trisomy 18, and 10 cases of trisomy 13. These were compared with 2400 control samples with normal chromosome constitution. AFAFP levels were generally lower in pregnancies with trisomy 21, showing a median value of 0·72 MoM. However, 40 per cent of the trisomy 21 samples had AFAFP values greater than 0·8 MoM and 20 per cent were over 1·0 MoM. These data imply that over 50 per cent of Down syndrome cases might have been missed using a cut-off level of 0·70 MoM for completion of chromosome analysis. Using a higher cut-off level will leave only a small percentage of samples unkaryotyped. The distribution of AFP levels in trisomy 13 and 18 is no different from controls; we therefore believe that fetal karyotyping should be completed in every amniotic fluid sample obtained. 相似文献