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   330633篇
  免费   17937篇
  国内免费   9915篇
林业   32385篇
农学   25875篇
基础科学   9974篇
  54598篇
综合类   52022篇
农作物   21641篇
水产渔业   19285篇
畜牧兽医   104760篇
园艺   9527篇
植物保护   28418篇
  2022年   3623篇
  2021年   5725篇
  2020年   5692篇
  2019年   6453篇
  2018年   5818篇
  2017年   7051篇
  2016年   7322篇
  2015年   8055篇
  2014年   9292篇
  2013年   20519篇
  2012年   11205篇
  2011年   13328篇
  2010年   12919篇
  2009年   13014篇
  2008年   11787篇
  2007年   11037篇
  2006年   11429篇
  2005年   10063篇
  2004年   7782篇
  2003年   7487篇
  2002年   6673篇
  2001年   7177篇
  2000年   6966篇
  1999年   6961篇
  1998年   5742篇
  1997年   5632篇
  1996年   5203篇
  1995年   5621篇
  1994年   4840篇
  1993年   4584篇
  1992年   4939篇
  1991年   4996篇
  1990年   4664篇
  1989年   4538篇
  1988年   3999篇
  1987年   3886篇
  1986年   3660篇
  1985年   3938篇
  1984年   3648篇
  1983年   3487篇
  1982年   2907篇
  1981年   2809篇
  1980年   2765篇
  1979年   3219篇
  1978年   2854篇
  1977年   2678篇
  1976年   2490篇
  1974年   2653篇
  1973年   2637篇
  1972年   2421篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
1.
In a feeding trial lasting for months Halothane-positive fattening pigs were put on a diet containing different concentrations of vitamin E and selenium. After a Myostress application vitamin E and selenium were additionally injected in order to examine stress resistance (CK-test) and regenerative capacity (histological examination of Musculus longissimus dorsi) of skeletal muscle. The study revealed that neither high dietary levels of vitamin E/selenium nor a vitamin E/selenium treatment applied after stress are capable of preventing stress related myopathy nor can they influence its healing up favourably.  相似文献   
2.
46 one day old chickens, divided into five groups, were fed an alternately composed vitamin D3 ration during 36 days. In the group depleted of vitamin D3 the typical rickets were found. When treated with a standard ration (2000 I.U. vit. D3/kg food) after 15 days depletion of vitamin D3 the chickens showed control-group parameters within 3 weeks. Using 60,000 I.U. vitamin D3 after a period of 15 days depletion evoked rather an abrupt beginning of healing tendency accompanied by signs of intoxication which were enhanced in those chickens treated with 120,000 I.U. vitamin D3: increase in number of osteoblasts, calcification of osteoblastic mitochondrial membranes and endothelial cell membranes, as well as degeneration of osteoblasts together with a strong reaction of eosinophils. There did not occur necrosis of osteocytes or of bone structure. Erythrocytes, being embedded in ground substance without endothelial surrounding, was a common finding in every group.  相似文献   
3.
Differences in gas exchange parameters i.e. carbon exchange rate (CER), transpiration (Tr), water vapor conductance (g) were investigated, under a controlled environment, in a semi-dwarf (SD) Triticum aestivum line, its tall (TL) near isoline and the Payne (PA) variety characterized by small leaves. The plants were maintained in: (a) optimal substrate moisture condition (CTR); (b) salinized by watering with a 0.12 m NaCl solution (SLT); (c) water stressed by withholding waterings during a period of six days (STR); (d) stressed and salinized during the same period and with the same saline solution (S + S). CER and Tr were negatively affected by the stresses; SLT and S + S treatments had a higher WUE compared to their respective controls CTR and STR.
SD line had a better performance in terms of CER and WUE, particularly when water and salt stresses interacted. Leaf anatomy and assimilation/internal CO2 concentration curves contributed to explaining the SD performance.  相似文献   
4.
The proximal straight tubules of the female mouse kidney exhibit heavy periodic acid Schiff (PAS) staining in their brush borders and numerous cytoplasmic granules. In the present study, the female DBA/2Cr mouse kidney was examined, using various fixatives (formalin, PFA, PLP, Zamboni's, Bouin, or Carnoy solution) and various staining methods (HE, PAS, alcian blue, periodic acid methenamine-silver (PAM), toluidine blue, azan, or Congo red). Under azan and PAM, the staining pattern of the brush border was similar to that of PAS, and few effects of the fixative were observed. Cytoplasmic granules were clearly detected with PAM as well as PAS. However, these granules were not detected with Carnoy solution. Furthermore, distribution of granules differed between PAS and PAM.  相似文献   
5.
6.
对5个无核葡萄品种的试验表明,不同培养方式对离体胚发育的影响是很明显的,其中4个品种胚发育率液体培养高于固体培养,‘京早晶’在液体培养中胚发育率高达73.7%,是目前国内外胚发育率最高的报道;液体培养比固体培养胚的发育程度高,发育整齐,无畸形胚,易成苗.  相似文献   
7.
A disease causing the decline of papaya (Carica papaya) plants was noticed in the Jordan Valley in 1982. The disease caused severe yellowing of the upper leaves and dieback of the apex. The disease was not transmissible mechanically and continued efforts to associate viruses and viroids have so far been unsuccessful. Epidemiological observations suggested that the disease is airborne and probably caused by a mollicute. In order to test this hypothesis, plots were covered by an insect-proof net and with white nets providing 15, 30 and 50% shade. The 30%, 50% and insect-proof nettings provided complete protection, and the 15% netting reduced disease incidence to <2%, compared with>37% in the uncovered control plot. Spraying plants at weekly intervals with a commercial whitewash solution was also found to be effective in reducing disease incidence. The advantages of using a range of netting field treatments as a simple means for obtaining information on the epidemiological nature of a new disease are described.  相似文献   
8.
Intravenous frusemide (1.0 mg/kg bwt) or phentolamine (0.33 mg/kg bwt) was given to 7 horses 1 h before exercise and their effects on pulmonary artery and aortic pressure changes during strenuous exercise were examined. Short-term near-maximal treadmill exercise (10 m/sec, 3 degrees incline) produced increases in heart rate, mean pulmonary artery pressure (PAP), mean aortic pressure (AP), and packed cell volume (PCV). Frusemide did not affect heart rate, PAP or PCV during exercise. Frusemide significantly decreased mean AP by 10 to 15 mmHg during exercise. Phentolamine produced an increase in heart rate relative to control only early in exercise but not during later, more strenuous, exercise. Phentolamine had no statistically significant effect on AP, PAP, or PCV, but a significant reduction was observed between 180 and 230 sec of exercise when PAP and AP were standardised against heart rate. Frusemide did not prevent horses from haemorrhaging during exercise in this study. Treatment with phentolamine did not sufficiently reduce the PAP and AP to test our hypothesis that a reduction in PAP and AP would eliminate EIPH.  相似文献   
9.
A systematic immunohistochemical study of the ultimobranchial tubule (UBT) has been carried out in 45 Wistar rats of different ages (0, 5, 10, 15, 20, 25, 30, 60 and 120 days). The existence of calcitonin immunoreactive cells in the UBT wall has been demonstrated in a 5-days old rat. In addition, immunohistochemical studies for thyroglobulin revealed positive staining in follicular cells connected to the UBT and, occasionally, in isolated cells lying within solid clusters from the UBT. These last results together with the continued and repeated existence of numerous mitosis and PAS (+) microfollicles, apparently rising from the UBT, support the hypothesis that the ultimobranchial body (UBB) may contribute partially to the formation of a part of the follicular component.  相似文献   
10.
Fifteen and eight mature beagles, without (normal group) and with experimental mitral regurgitation (MR group), respectively, were given 0.02 mg/kg/day digoxin powder for 10 days orally. The optimum time for sample collection after administration of digoxin was observed to be 8-18 hr and 10-22 hr in the normal and MR groups, respectively. In both groups, a stable concentration was reached after 3-5 days of treatment. No differences in plasma level were observed between sexes. The optimum concentration of digoxin was attained at an earlier stage than has been previously reported for both dogs and humans.  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