全文获取类型
收费全文 | 1568篇 |
免费 | 61篇 |
国内免费 | 255篇 |
专业分类
林业 | 243篇 |
农学 | 107篇 |
基础科学 | 533篇 |
280篇 | |
综合类 | 486篇 |
农作物 | 65篇 |
水产渔业 | 37篇 |
畜牧兽医 | 71篇 |
园艺 | 21篇 |
植物保护 | 41篇 |
出版年
2024年 | 18篇 |
2023年 | 24篇 |
2022年 | 58篇 |
2021年 | 73篇 |
2020年 | 76篇 |
2019年 | 78篇 |
2018年 | 60篇 |
2017年 | 64篇 |
2016年 | 100篇 |
2015年 | 99篇 |
2014年 | 97篇 |
2013年 | 107篇 |
2012年 | 129篇 |
2011年 | 106篇 |
2010年 | 72篇 |
2009年 | 84篇 |
2008年 | 90篇 |
2007年 | 68篇 |
2006年 | 86篇 |
2005年 | 59篇 |
2004年 | 42篇 |
2003年 | 29篇 |
2002年 | 34篇 |
2001年 | 43篇 |
2000年 | 13篇 |
1999年 | 14篇 |
1998年 | 16篇 |
1997年 | 10篇 |
1996年 | 16篇 |
1995年 | 22篇 |
1994年 | 17篇 |
1993年 | 10篇 |
1992年 | 12篇 |
1991年 | 14篇 |
1990年 | 9篇 |
1989年 | 17篇 |
1988年 | 12篇 |
1987年 | 2篇 |
1981年 | 1篇 |
1978年 | 2篇 |
1976年 | 1篇 |
排序方式: 共有1884条查询结果,搜索用时 187 毫秒
111.
陈侠群 《中国农业大学学报》1991,(3):5-10
试验采用农大139等5~6个冬小麦品种观察表明:春一、二叶伸出4~5 cm 左右时,分别与单棱、二棱两个穗分化始期相对应;春三、四、五、六叶伸出9~10 cm 左右时,分别与小花、雌雄蕊、药隔形成和覆盖器官伸长四个穗分化始期相对应。一般趋势为当每片春生叶片伸出2/5~3/4全长时,分别可与相应的穗分化始期相对应。四分体的分化都在旗叶展开以后;当花药黄绿色、长度为1.5(±0.1)mm 时,发生四分体的机率可在80%以上。 相似文献
112.
113.
大型烟囱一般是工业厂区的高耸建筑,经过几十年的使用,需要重新鉴定其结构可靠性,烟囱的垂直度,是一个很重要的安全指标。由于烟囱为圆形截面,不能直接测量,而且工业厂区内建筑较多,通视条件较差,给测量带来很多困难。通过测量烟囱上下两个标高处的外轮廓水平角度,换算出上下标高处的相对水平偏移角,再用角度交汇方法求出最终偏移值,经过精度分析,完全满足工程精度要求。本文以上海某大型烟囱为例,提出了对已有烟囱垂直度偏差的检测方法,以用于大型服役烟囱的监测。 相似文献
114.
The effect of water state of soil on eco-boundary over leaf surface was disscussed by takingPopulus davidiana as experimental material. The results show that to change soil water state by some methods was to adjust and control the
forming quality of eco-boundary in fact. The plant in good water condition must be accompanied by thinner eco-boundary. This
eco-boundary is beneficial to exchange and flow of the substances between plant and environment.
Suifenhe Animal and Plant Quarantine Bureau
The study was supported by The Chinese Natural Science Foundation, No. 39470136
(Responsible Editor: Chai Ruihai) 相似文献
115.
116.
J M PEÑA-BARRAGÁN F LÓPEZ-GRANADOS M JURADO-EXPÓSITO & L GARCÍA-TORRES 《Weed Research》2007,47(2):164-172
Ridolfia segetum is a frequent umbelliferous weed in sunflower crops in the Mediterranean basin. Field and remote sensing research was conducted in 2003 and 2004 over two naturally infested fields to determine the potential of multispectral imagery for discrimination and mapping of R. segetum patches in sunflower crops. The efficiency of the four wavebands blue (B), green (G), red (R) and near‐infrared (NIR), selected vegetation indices and the spectral angle mapper (SAM) classification method were studied using aerial photographs taken in the late vegetative (mid‐May), flowering (mid‐June) and senescence (mid‐July) crop growth stages. Discrimination efficiency of R. segetum patches in sunflower crops is consistently affected by their phenological stages, in this order: flowering > senescence > vegetative. In both fields, R. segetum patches were efficiently discriminated in mid‐June, corresponding to the flowering phase, by using the waveband G, the ratio R/B or SAM with overall accuracies ranging from 85% to 98%. The application of the median‐filtering algorithm to any of the classified images improved the accuracy. Our results suggest that mapping R. segetum weed patches in sunflower to implement site‐specific weed management techniques is feasible with aerial photography when images are taken from 8 to 10 weeks before harvesting. 相似文献
117.
旋转编码器在回转体回转角度测量中的应用 总被引:4,自引:0,他引:4
针对回转体回转角度测量的实际情况,研制了新型回转体回转角度测量仪。该测量仪是以光、机、电为一体的旋转编码器作为传感器,测量回转体的回转角度。这种测量输出的信号为数字信号,克服了零漂、测量准确度低等缺点,提高了测量的准确度和可靠性。 相似文献
118.
119.
120.
Fibril angle variability in earlywood of Norway spruce using soft rot cavities and polarization confocal microscopy 总被引:4,自引:0,他引:4
Anna Bergander Jonas Br?ndstr?m Geoffrey Daniel Lennart Sahnen 《Journal of Wood Science》2002,48(4):255-263
The main purpose of this study was to investigate the variability of the fibril angle of tracheids in earlywood of Norway spruce (Picea abies L. Karst.). Polarization confocal microscopy was chosen and compared with the method utilizing the orientation of soft rot cavities. There was a significant correlation between the soft rot and polarization confocal microscopy methods, which showed the same trend of high fibril angles in the first part of the earlywood followed by a decrease toward the end of earlywood. This declining trend was less pronounced in annual rings containing compression wood. Moreover, large variations in fibril angle occurred between neighboring tracheids. The investigation also emphasized the differences between X-ray diffraction and microscopic methods, as the large variation seen by the latter methods is not seen by the X-ray diffraction approach because of its large area of measurement. No correlation was found between fiber morphology (i.e., average length, width, density) and the average fibril angle in the investigated annual rings. 相似文献