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   681篇
  免费   46篇
林业   45篇
农学   23篇
基础科学   4篇
  193篇
综合类   46篇
农作物   31篇
水产渔业   52篇
畜牧兽医   243篇
园艺   24篇
植物保护   66篇
  2023年   6篇
  2022年   5篇
  2021年   23篇
  2020年   24篇
  2019年   25篇
  2018年   15篇
  2017年   22篇
  2016年   22篇
  2015年   7篇
  2014年   18篇
  2013年   45篇
  2012年   42篇
  2011年   61篇
  2010年   25篇
  2009年   27篇
  2008年   45篇
  2007年   45篇
  2006年   37篇
  2005年   38篇
  2004年   35篇
  2003年   26篇
  2002年   33篇
  2001年   12篇
  2000年   10篇
  1999年   8篇
  1997年   4篇
  1996年   4篇
  1995年   2篇
  1994年   5篇
  1993年   2篇
  1992年   4篇
  1991年   6篇
  1989年   3篇
  1988年   3篇
  1986年   3篇
  1985年   3篇
  1983年   4篇
  1981年   3篇
  1980年   4篇
  1979年   1篇
  1978年   1篇
  1976年   2篇
  1975年   1篇
  1974年   2篇
  1973年   2篇
  1972年   2篇
  1971年   2篇
  1969年   2篇
  1963年   2篇
  1961年   1篇
排序方式: 共有727条查询结果,搜索用时 15 毫秒
61.
Supercritical fluid extraction (SFE) from aniseed using carbon dioxide was performed at 30 degrees C and pressures of 80-180 bar. The chemical composition of the SFE extract was determined by GC-MS; the quantitative analysis was done by GC-FID and TLC. The total amount of extractable substances or global yield (mass of extract/mass of feed) for the SFE process varied from 3.13 to 10.67% (mass). The solubilities of the anise essential oil in CO(2) were 0.0110, 0.0277, 0.0143, and 0.0182 kg of solute/kg of CO(2) at 80, 100, 140, and 180 bar, respectively. The major compounds identified and quantified in the extracts were anethole ( approximately 90%), gamma-himachalene (2-4%), p-anisaldehyde (<1%), methylchavicol (0.9-1.5%), cis-pseudoisoeugenyl 2-methylbutyrate ( approximately 3%), and trans-pseudoisoeugenyl 2-methylbutyrate ( approximately 1.3%). The Sovová model described quite well the experimental overall extraction curves.  相似文献   
62.
The in-vivo toxicity data of 33 O,O-diethyl O-(1-alkyl-3-R″-1,2,4-triazol-5-yl) phosphorothioates, reported in Part I of this series, were subjected to multiregression analyses for the following organisms: Leptinotarsa decemlineata (larvae), Macrosiphum euphorbiae (aphids), Culex pipiens (adults), Spodoptera littoralis (larvae) and the rat. The activities were found to depend mainly on the steric parameters of R″, these being best expressed by the values L, B1 and B4 of the computer program STERIMOL. It was shown that there were different steric requirements for each species and these are discussed. The predictive values of the best equations were also estimated.  相似文献   
63.
A new class of compounds, derived from hydroquinones, has been synthesised and bioassayed for juvenile hormone activity on yellow mealworm (Tenebrio molitor), confused flour beetle (Tribolium confusum), and mediterranean flour moth (Anagasta kuehniella). Structure-biological activity relationships of the most significant products are tabulated and discussed. Two of the new compounds, 1, 4-bis(5-chloropent-4-ynyloxy) benzene (I) and 1-(5-chloropent-4-ynyloxy)-4-phenoxybenzene (II), showed outstanding activity in morphogenetic assay with T. molitor, and good field persistence. Product II is of particular interest for the control of various Homopteran species, mosquitoes, stored product pests and fire ants.  相似文献   
64.
In the search for new juvenile hormone analogues (JHAs), a general approach was chosen based on the substitution of the isoprene unit, that carries the epoxy function in the natural hormone JHIII, by 3,3,3-trichloropropyl, 2,2-dichlorocyclopropyl- methyl, 3,3-dichloroallyl, or 3-chloroprop-2-ynyl groups. Accordingly, several terpenoid aromatic JHAs were prepared with, or without, oxygen atoms in the aliphatic chain. Structure-biological activity relationships of the most active compounds toward the yellow mealworm (Tenebrio molitor) and the large cabbage white butterfly (Pieris brassicae) are given. With a view to preparing synthetically accessible analogues, some alkyl phenyl ethers were also made, which were not branched in the aliphatic chain and were related terminally to the most active compounds. They were only moderately active against insects when tested for morphogenetic activity, but exhibited a good field persistence. The occurrence of a moderate acaricidal activity among these analogues prompted the examination of several 1,4-disubstituted-phenyl derivatives. The ovicidal effectiveness of these products against a susceptible strain of the two-spotted spider mite, Tetranychus urticae, was ten or 100 times higher than that of the well known and highly potent JHAs.  相似文献   
65.
A total of 74 embryos and fetuses were used in a comparative analysis of the epithelium of the non-glandular stomach compartments of merino sheep during development. The mechanical protection showed by the tegumentary epithelium in the superficial layers of the rumen, reticulum and omasum is supported by a buffer system of neutral mucopolysaccharides secreted by the deeper strata. Neutral mucopolysaccharides first appeared in epithelial cells at 46 days of fetal life. Acid mucopolysaccharides, mucins, and mucoid compounds were not detected. Growth curves and formulas were constructed for the epithelial layers.  相似文献   
66.
A 27-year-old stand of flooded gum (Eucalyptus grandis Hill ex Maiden) in the North Coast Region of N.S.W. was assessed in relation to aboveground distribution and turnover of organic matter, nitrogen, phosphorus, calcium, magnesium and potassium. Of the 453 t ha?1 of aboveground organic matter present, 394 t was in the tree, 42 t in the understorey and 28 t in the forest floor. The total nitrogen, phosphorus, calcium, magnesium and potassium contents of the stand were 739, 44, 1254 and 658 kg ha?1, respectively, and the understorey contained 35%, 35%, 16%, 24% and 49% of the above-ground distribution of these nutrients respectively. Although the developing rainforest understorey comprised a relatively small portion (9.3%) of the total aboveground biomass, it played a disproportionate role in nutrient accumulation and uptake, and had an annual net accumulation of 14%, 55%, 59%, 30%, 44% and 69% of the aboveground organic matter, nitrogen, phosphorus, calcium, magnesium and potassium respectively. The net annual removal from the soil was 30, 1, 38, 5 and 31 kg ha?1 year?1 for nitrogen, phosphorus, calcium, magnesium and potassium, respectively. Flooded gum had very high accumulations of calcium in the bark and the effect of this in nutrient cycling is discussed. An idealised management system, to exploit and optimise the nutrient cycle of flooded gum, has been hypothesided.  相似文献   
67.
An electronic sphygmomanometer which functions on oscillometric principles, was used to measure arterial blood pressure in foals. Its accuracy was assessed by comparison with results obtained by direct measurement in anaesthetised pony foals, aged 34 to 64 days. Correlations between data obtained by the two methods were highly significant but the sphygmomanometric readings were consistently lower than those obtained by direct measurement. An equation was derived from the pooled values for mean, systolic and diastolic pressure measurements. This was used as a correction factor when the oscillometric method was employed in the field on Thoroughbred foals, during the first three days post partum, and on pony foals during the first day only. There were no significant differences between systolic, diastolic and mean values in Thoroughbreds on Days 1, 2 or 3 after birth. Measurements recorded on the first day after foaling were significantly lower (P less than 0.025) in ponies than in Thoroughbreds. Blood pressure measurements in Thoroughbreds and ponies were substantially higher than those reported previously using the ultrasonic Doppler technique.  相似文献   
68.
69.
When treated with formaldehyde, Tween 80, sodium oleate and Nonidet P-40, avian infectious bronchitis virus, porcine transmissible gastroenteritis virus, neonatal calf diarrhea coronavirus, porcine hemagglutinating encephalomyelitis virus as well as the human coronavirus show similar inner structures by negative staining. The first one is an inner membranous bag. This structure could be evaginated following treatments used and does not show the characteristic projections of coronaviruses. Subsequently, the inner fold could be separated from the outer membrane at the point of junction between these two membranes. Each virus does not react in the same way to the action of the different products. The transmissible gastroenteritis virus appears more sensitive to treatments than other viruses. On the other hand, the hemagglutinating encephalomyelitis virus is the most resistant. The variable sensitivities of these viruses are not related to the type of host-cells. Also, a second internal structure, which is more dense than the viral particle, encircles partially the aperture of the internal tongue-shaped structure and seems to emerge from the viral particle through the aperture of the inner bag.  相似文献   
70.
The proposed technique consists of placing a catheter in the olfactory recess of the lateral ventricle of the brain of a sheep. Twenty adult ewes were used in the study. The olfactory recess is in the frontal region 25 +/- 3 mm below the skin, just caudal to the line joining the supraorbital foramen and approximately 8-10 mm from the median plane. The approach is relatively easy under normal experimental conditions. The wall of the olfactory bulb, however, is fragile, and the animals must not have been injured by Oestrus ovis.  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