首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   492篇
  免费   0篇
林业   135篇
农学   12篇
  104篇
综合类   36篇
农作物   16篇
水产渔业   38篇
畜牧兽医   123篇
园艺   7篇
植物保护   21篇
  2020年   7篇
  2019年   6篇
  2018年   8篇
  2017年   6篇
  2016年   10篇
  2015年   8篇
  2014年   12篇
  2013年   11篇
  2012年   18篇
  2011年   30篇
  2010年   10篇
  2009年   15篇
  2008年   28篇
  2007年   20篇
  2006年   22篇
  2005年   17篇
  2004年   19篇
  2003年   22篇
  2002年   12篇
  2001年   7篇
  2000年   7篇
  1997年   4篇
  1992年   4篇
  1989年   4篇
  1986年   3篇
  1984年   5篇
  1983年   4篇
  1982年   4篇
  1977年   4篇
  1975年   3篇
  1974年   5篇
  1972年   6篇
  1969年   3篇
  1968年   3篇
  1967年   5篇
  1966年   3篇
  1965年   4篇
  1962年   3篇
  1956年   4篇
  1948年   7篇
  1941年   3篇
  1934年   3篇
  1933年   3篇
  1932年   12篇
  1927年   7篇
  1919年   6篇
  1918年   5篇
  1915年   4篇
  1904年   5篇
  1902年   3篇
排序方式: 共有492条查询结果,搜索用时 19 毫秒
41.
42.
Invasion of non-native species is among the top threats for the biodiversity and functioning of native and agricultural ecosystems worldwide. We investigated whether the herbivory of the slug Arion vulgaris (formerly Arion lusitanicus; Gastropoda), that is listed among the 100 worst alien species in Europe, is affected by soil organisms commonly present in terrestrial ecosystems (i.e. earthworms—Annelida: Lumbricidae and arbuscular mycorrhizal fungi—AMF, Glomerales). We hypothesized that slug herbivory would be affected by soil organisms via altered plant nutrient availability and plant quality. In a greenhouse experiment, we created a simple plant community consisting of a grass, a forb, and a legume species and inoculated these systems with either two earthworm species and/or four AMF taxa. Slugs were introduced after plants were established. Earthworms significantly reduced total slug herbivory in AMF-inoculated plant communities (P?=?0.013). Across plant species, earthworms increased leaf total N and secondary metabolites, AMF decreased leaf thickness. Mycorrhizae induced a shift in slug feeding preference from non-legumes to legumes; the grass was generally avoided by slugs. AMF effects on legume herbivory can partly be explained by the AMF-induced increase in total N and decrease in C/N ratio; earthworm effects are less clear as no worm-induced alterations of legume plant chemistry were observed. The presence of earthworms increased average AMF colonization of plant roots by 140 % (P?<?0.001). Total shoot mass was significantly increased by AMF (P?<?0.001). These data suggest that the feeding behavior of this invasive slug is altered by a belowground control of plant chemical quality and community structure.  相似文献   
43.
Beside the clinical examination of udder/teat imaging methods e.g. sonography, radiography and endoscopy allow diagnosis. This publication will give a summary of currently used and potentially usable methods of diagnostic imaging applied to the teat. Advantages and disadvantages of the several methods were compared--as quoted in literature and completed by own experiences.  相似文献   
44.
45.
    
Ohne Zusammenfassung  相似文献   
46.
The present study investigated the influence of ovarian fluid (OV) on the gamete physiology in the brown trout (Salmo trutta f. fario). In comparison to water OV prolongs the duration of sperm motility for >'5 min and the fertilizability of eggs for >'10 min independent from the quality of the egg batch from which the OV derived. These stabilizing effects are related to the inorganic composition of OV as similar results are obtained in buffered artificial saline solution (AFS). OV compensates also for sub-optimal environmental conditions as demonstrated in fertilization experiments with mixtures of OV and buffer solutions of different pH. At a dilution ratio of OV: water of 1:1 the stabilizing effect on gametes is significantly decreased and at a dilution ratio of 1:8 it is completely lost. Due to the described effects significantly higher fertilization rates are obtained in OV than in water. In OV the fertilization rates are also higher than in AFS but only at low sperm to egg ratios or when semen with low motility rates and swimming velocities is used. This is considered to be due to a semen dependent egg activation which is partly suppressed by a OV protein with a molecular weight between 20 and 62 kdaltons. The phenomen is discussed in detail.  相似文献   
47.
48.
49.
Summary One of the essential aims of land planning is to satisfy the people's need for recreation. For this purpose it seems to be efficient to differentiate between “local” recreation and “long-distance” recreation areas and to consider them separatly in regional planning. To determine the location of recreational forests we need appropriate aids for decision making. Therefore activities that bring about deterioration of natural conditions, e.g. water pollution, soil erosion etc., as well as injuries to the optical value of the landscape, are not allowed in natural recreational areas. On the other hand it is possible to delimit forests after an analysis of stand composition and also by interpretation of maps giving the physiological effects of the climate. Above all the habits of the people are of importance, especially when they are based on irrational motives. These factores are of great importance in addition to the sociological conditions of an area and therefore do not allow the organization of long-distance recreation areas from simply a regional point of view. In the case of areas designed for local recreation, however, it might be sufficient to appraise the demands of the people through a regional analysis.

Herrn Professor Dr. Dr. h. c.Julius Speer zum 65. Geburtstag gewidmet.  相似文献   
50.
  总被引:2,自引:0,他引:2  
In rainbow trout (Oncorhynchus mykiss) freshly ovulated eggs and over-ripened eggs which had been retained in the coelomic cavity for 7, 14 and 21 days were investigated in aspects of morphology, physiology and biochemistry. Egg viability was significantly reduced from 85.9±16.4% in freshly ovulated eggs to 25.1±21.9% in over-ripened eggs which had been retained in the coelomic cavity for 21 days. Further during over-ripening in the ovarian fluid the pH significantly decreased, while the levels of proteins, of esterified and non esterified fatty acids and the activities of aspartate aminotransferase and acid phosphatase significantly increased. Also egg parameters changed: the wet weight of the unhardened eggs increased, the weight increase during hardening and the levels of esterified and of non esterified fatty acids significantly decreased. In freshly ovulated eggs the yolk consisted of a homogenous mass and the perivitelline space was small, but in over-ripened eggs the yolk was non homogenous with numerous vesicular inclusions and the perivitelline space was enlarged. When freshly ovulated eggs were incubated in water the cortical reaction was detectable within 5 min, in over-ripened eggs hardly no extrusion of cortical vesicles was visible and the width of the perivitelline space was very irregular.For the investigated freshly ovulated and over-ripened samples the egg viability significantly correlated with ovarian fluid parameters (pH, protein, non esterified fatty acids, esterified fatty acids, aspartate aminotransferase, acid phosphatase) and egg parameters (weight increase during hardening, weight of the hardened eggs).  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号