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   2641篇
  免费   251篇
  国内免费   137篇
  3029篇
  2024年   9篇
  2023年   32篇
  2022年   49篇
  2021年   74篇
  2020年   75篇
  2019年   105篇
  2018年   71篇
  2017年   79篇
  2016年   109篇
  2015年   119篇
  2014年   110篇
  2013年   125篇
  2012年   103篇
  2011年   104篇
  2010年   94篇
  2009年   111篇
  2008年   117篇
  2007年   123篇
  2006年   100篇
  2005年   97篇
  2004年   94篇
  2003年   87篇
  2002年   70篇
  2001年   76篇
  2000年   61篇
  1999年   67篇
  1998年   62篇
  1997年   57篇
  1996年   58篇
  1995年   49篇
  1994年   60篇
  1993年   46篇
  1992年   44篇
  1991年   47篇
  1990年   40篇
  1989年   38篇
  1988年   47篇
  1987年   25篇
  1986年   23篇
  1985年   36篇
  1984年   22篇
  1983年   14篇
  1982年   19篇
  1981年   25篇
  1980年   16篇
  1979年   13篇
  1977年   12篇
  1975年   6篇
  1974年   5篇
  1972年   1篇
排序方式: 共有3029条查询结果,搜索用时 9 毫秒
11.
Using weight and skinfold thickness to calculate relative body fat, the fat content of 21 captive female rhesus macaques was estimated. Although age and. pregnancy had no effect on fat, rank was significant, especially during winter, with high-rank females having the highest fat scores. Rank therefore has a significant effect on the health of captive animals and may also affect the health of individuals in feral groups.  相似文献   
12.
Nuñas (Thaseolus vulgaris, Fabaceae), commonly called popping beans, are traditionally grown in the Andean highlands of South America, and are consumed as a snack food after a quick toasting process. Proximate analysis of their nutritive value revealed that nunas have a higher content of starch, amylose, and copper than four dry bean varieties and a lower mean content of protein, phosphorous, iron, and boron. The unique texture and taste of nuñas may be related to their high starch content. Antinutritional factors such as lectins were higher in raw and boiled nuña samples than in toasted nuñas, while tannin levels did not change from raw to toasted treatments. Overall in-vitro digestibility was slightly lower for toasted nunas than boiled dry bean.  相似文献   
13.
14.
This article summarises the way in which mycorrhizal infection of roots affects the mineral nutrition of plants and how the symbiosis may interact with the evaluation of efficiency of nutrient uptake and use by plants. A brief account of the processes of infection and the way they are affected by host genotype and environmental conditions is given and the relationships between this and mineral nutrition (especially phosphate nutrition) are outlined.The interactions between mycorrhizal infection and P efficiency are considered at two levels. Mycorrhizas may act as general modifiers of efficiency regardless of the extent to which the plants are infected and in some mycorrhiza-dependent plants infection may change the ranking of genotypes. The extent of infection is also under genetic control and shows considerable variability between genotypes in some species. This variation could be used in programs to select varieties in which infection is rapid and nutrient uptake from nutrient deficient or low input systems is, in consequence, increased.  相似文献   
15.
Summary Silicon during the early vegetative stage did not affect the oven dry weight of any of the various tissues of the soybean plant. Silicon did, however, decrease the Mn concentration in the youngest fully mature leaf at intermediate levels of Mn. This effect did not occur at the lowest or highest Mn levels. Deficiency and toxicity symptoms were moderated to a slight degree by Si except at the highest level of Mn.  相似文献   
16.
During the life cycle of the mermithid nematode Gastromermis boophthorae the main components of the body wall, namely the cuticle, hypodermis and somatic musculature, undergo radical changes in structure and relative proportions. In the non-feeding, free-living stages the cuticle is a well-defined, multilayered structure which lies upon a generally thin hypodermis; the somatic musculature has the extensive and highly organised appearance typical of motile nematodes. In parasitic juveniles the body wall has a completely different structure which is adapted for the suggested purpose of nutrient uptake from the host insect's haemocoelomic fluid; the cuticle is characteristically thin and rather diffuse, the underlying hypodermis becomes very extensive and metabolically active, and there is a corresponding reduction in the proportion of somatic musculature. The sub-cuticular surface of the enlarged hypodermis has a microvillous configuration and has associated alkaline phosphatase activity.  相似文献   
17.
18.
Parasitoid wasps of the genus Trichogramma are used worldwide as biological control agents against lepidopteran pests. Trichogramma wasps develop inside eggs of a wide range of host species, most of them moths. They are generally considered as diurnal insects. Here, we investigated whether Trichogramma wasps can also successfully parasitise host eggs at night under controlled laboratory conditions. Eggs of the moth Ephestia kuehniella were offered under dark conditions (scotophase) to females of Trichogramma brassicae and Trichogramma evanescens either from 9:00 PM to 9:00 AM or from 11:00 AM to 5:00 PM at four different temperatures (5°C, 10°C, 15°C and 20°C). Both species are known to parasitise E. kuehniella eggs in the photophase during daytime. The results show that T. brassicae did not parasitise eggs in the scotophase at night and only very few in the artificially induced scotophase during daytime from 10°C to 20°C. In contrast, T. evanescens parasitised more eggs in the dark both at night and artificially induced scotophase during daytime. Parasitism in the scotophase already started at 5°C, with more eggs being parasitised and more offspring being produced at higher temperatures. T. evanescens displayed higher parasitism activity in the induced scotophase during daytime than in the scotophase at night. The present study suggests that Trichogramma are capable of successfully parasitising host eggs at night, even at low temperatures, but that nocturnal activity with respect to parasitism varies between wasp species.  相似文献   
19.
While enteral nutrition is the basis for the critically ill, parenteral nutrition is often used when a sufficient enteral nutrition is not or not fully achievable. Lipids are a mainstay of caloric supply in both cases as they combine the provision of building blocks for the membranes and are precursors for function molecules including lipid mediators bearing the ability to influence immunity. Pro-inflammatory lipid mediators as prostaglandins and leukotrienes are generated from arachidonic acid (AA), a key member of the n-6 polyunsaturated fatty acids (PUFA). In contrast, lipid mediators derived from the n-3 fatty acids eicosapentaenoic acid (EPA) or docosahexaenoic acid (DHA) may exhibit less inflammatory properties compared to their AA-derived counterparts. Furthermore, intercellular mediators as resolvins and protectins are generated from n-3 fatty acids. They induce the resolution of inflammation, hence the name resolution phase interaction product—resolvin. Modulating the amount of PUFA and the n-6/n-3 ratio were investigated as means to change the inflammatory response and improve the outcome of patients. Experimental data showed that n-3 fatty acids may improve acute lung injury and sepsis in animal models. Studies in patients undergoing major surgery with application of n-3 fatty acids demonstrated beneficial effects in terms of reduction of length of stay and infectious complications. Clinical data hints that this concept may also improve outcome in critically ill patients. Additionally, experimental and clinical data suggest that a reduction in n-6 PUFA may change the immune response. In conclusion, modulating the amount of PUFA, the n-6/n-3 ratio and the composition of lipid emulsions may prove to be a useful means to improve the outcome of critically ill patients.  相似文献   
20.
The ectomycorrhizal fungus Cenococcum geophilum was grown in low-iron medium and the excreted siderophores were extracted, purified and analyzed by HPLC. The principal hydroxamate siderophore produced, was identified as ferricrocin as confirmed by analytical HPLC, FAB-mass spectrometry and 1H- and 13C-NMR spectra. Although the occurrence of ferricrocin has been shown earlier to occur in the ericoid mycorrhizal fungi, this is the first report of ferricrocin in a true ectomycorrhizal fungus which is taxonomically related to the ascomycetes.  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