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   48850篇
  免费   3008篇
  国内免费   972篇
林业   2418篇
农学   1975篇
基础科学   737篇
  7040篇
综合类   9226篇
农作物   2154篇
水产渔业   2590篇
畜牧兽医   22244篇
园艺   1051篇
植物保护   3395篇
  2022年   594篇
  2021年   800篇
  2020年   848篇
  2019年   832篇
  2018年   1179篇
  2017年   1343篇
  2016年   1134篇
  2015年   1103篇
  2014年   1320篇
  2013年   2542篇
  2012年   2213篇
  2011年   2431篇
  2010年   1834篇
  2009年   1726篇
  2008年   2228篇
  2007年   2016篇
  2006年   1773篇
  2005年   1547篇
  2004年   1388篇
  2003年   1283篇
  2002年   1165篇
  2001年   1483篇
  2000年   1423篇
  1999年   1022篇
  1998年   421篇
  1997年   434篇
  1995年   413篇
  1994年   354篇
  1992年   672篇
  1991年   792篇
  1990年   694篇
  1989年   731篇
  1988年   665篇
  1987年   632篇
  1986年   669篇
  1985年   597篇
  1984年   471篇
  1983年   418篇
  1979年   555篇
  1978年   434篇
  1977年   362篇
  1976年   366篇
  1975年   399篇
  1974年   459篇
  1973年   478篇
  1972年   448篇
  1970年   367篇
  1969年   401篇
  1968年   357篇
  1967年   361篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
以杉木或湿地松为主要树种,用南酸枣、檫树、樟树、乐昌含笑、福建柏、鹅掌楸、蓝果树等作伴生树种,开展了13个类型的混交林试验研究,在定性分析筛选的基础上,初选出9个比较优良的混交类型,再依据Ii=P1h/H+P2d/D+P3v/V计算出混交类型的优选指标,用I标准=I单均+aδn计算出类型的优化标准为6.12.据此对参试的混交类型进行优选,选出的最佳混交类型为杉木+南酸枣、杉木+檫树、杉木+樟树和湿地松+鹅掌楸4个混交类型,供林业生产上推广应用.  相似文献   
62.
A field experiment was conducted to find out the critical physiological stages of irrigation schedules inducing better growth, physiological efficiency and seed yield potential of berseem ( Trifolium alexandrinum L., Var. S-99-1). For this purpose eight irrigation treatments were made comprised of four treatments of three irrigation (W1, W2, W3 and W4), three treatments of four irrigation (W5, W7 and W8) and one treatment of five irrigation (W6) at various physiological stages i. e. regeneration, flower initiation, full bloom, seed initiation and advance seed development stage.
Thus based on the experimental results the physiological role of watering in berseem seed production could be discussed as:
With-holding of irrigation either at regeneration or at full bloom stage developed potential water stress in plants as indicated by high proline content of irrigation treatments — W4, W1 and W7; and further brought out disturbance on the formation of carotene, synthesis of water soluble sugar and translocation of sugar towards reproductive organs during grain development stage. These stresses adversely affected the plant growth and flowering behaviour. The irrigation at seed initiation stage increased the seed yield. Continuous irrigation did not appear to be useful. Thus it can be concluded that irrigation at three critical physiological stages i. e. regeneration, full bloom and seed initiation was found to be essential for obtaining potential seed yield of berseem.  相似文献   
63.
通过黄粉虫黄色品系(Y)和黑色品系(B)间杂交试验,对其杂种1代产卵量、个体大小、头宽及蛋白质含量等方面进行了研究和探讨。结果表明:杂交组合B♀×Y♂在产卵量、头宽、蛹重等方面较两个自交组合达到了显著或极显著水平。其中产卵量较自交组合B♀×B♂相比提高116.83%;而杂交组合Y♀×B♂的卵长、卵重、产卵量、幼虫头宽、幼虫体重、蛹重、蛋白质含量、耐高温能力与Y♀×Y♂、B♀×B♂相比均达到显著或极显著水平,其中蛋白质含量较自交组合B♀×B♂相比提高21.05%。  相似文献   
64.
Fibrotic degeneration was examined in the kidneys of ICR-derived glomerulonephritis (ICGN) mice, a novel inbred mouse line with a hereditary nephrotic syndrome of unknown etiology considered to be a good model of human idiopathic nephrotic syndrome. In the present study, we histochemically revealed changes in accumulation of extracellular matrix (ECM) components and in localization of integrins, cellular receptors for ECM, in the kidneys of ICGN mice with the progression of renal failure. Excessive accumulation of basement membrane (laminin and collagen IV) and interstitial (type III collagen) ECM components were demonstrated in the glomeruli and tubulointerstitum of ICGN mice. Marked deposition of type I collagen and tenascin was seen only in the glomeruli of ICGN mice but not in those of ICR mice as normal controls. Increased expression of integrin alpha1-, alpha2-, alpha5- and beta1-subunits in glomeruli with fibrotic degeneration and abnormal distribution of alpha6-subunit were noted in the kidneys of ICGN mice. Excessive laminin, a ligand of alpha6beta1-integrin, was demonstrated on the tubular basement membrane, but alpha6-subunit diffusely disappeared on the basal side of the tubular epithelial cells. We presumed that abnormal integrin expression in renal tubules causes epithelial cell detachment, and consequently tubular nephropathy, and results in disorder of ECM metabolism causing excessive accumulation of ECM components in the kidneys of ICGN mice.  相似文献   
65.
OBJECTIVE: To compare the efficacy of a Salmonella bacterin and a modified live Salmonella ser. Choleraesuis vaccine on a commercial dairy. ANIMALS: 450 cows in late gestation and 80 calves. PROCEDURE: Group-1 cows (n = 150) were vaccinated once with a modified live S. Choleraesuis (serogroup C1) strain 54 (SC54) vaccine, group-2 cows (150) were vaccinated on enrollment and 30 days later with a Salmonella ser. Montevideo (serogroup C1) bacterin, and group-3 cows (150) served as unvaccinated controls. One gallon of colostrum harvested from the first 80 cows to calve was fed to each calf. Outcome assessments included fecal shedding of Salmonella spp for the first 10 days after parturition (cows) or birth (calves), milk production, involuntary culling rate, mastitis incidence, antimicrobial use, and mortality rate. RESULTS: Salmonellae were isolated from 306 of 309 (99%) cows and 64 of 74 (86.5%) calves. Shedding frequency was less in SC54-vaccinated cows and calves that received colostrum from those cows, compared with the other groups, and vaccination was specifically associated with less shedding of serogroup C1 salmonellae. Production data were similar among groups. CONCLUSIONS AND CLINICAL RELEVANCE: Vaccination of pregnant cows with an autogenous Salmonella bacterin had no effect on fecal shedding of salmonellae, whereas vaccination with a modified live S. Choleraesuis vaccine reduced the frequency of fecal shedding of serogroup C1 salmonellae during the peripartum period. A commercial S. Choleraesuis vaccine licensed for use in swine may be more efficacious than autogenous Salmonella bacterins on dairies infected with serogroup C1 salmonellae.  相似文献   
66.
对浙江等地鸭大肠杆菌病流行病学进行调查,并研究其发病特点,分离鉴定出致病性大肠杆菌26株;已对其中20株菌株进行血清型鉴定,主要为078,占70%(14/20),其次是O1,占15%(3/20);人工感染试验表明,80%(8/10)菌株毒力强,皮下、肌肉、足蹼3种途径均易复制该病;病理组织学检查肝、肺、脾等,表现急性炎症,出血等病变;药敏试验表明,不同菌株对药物敏感性差异较大,除对鸭菌消、卡那霉素等高敏外,对多数常用抗菌药已耐药,最后就该病的防治提出了对策。  相似文献   
67.
A 13-year-old neutered female Yorkshire terrier presented with a history of progressive episodic weakness and disorientation of 4 months duration. Physical and neurologic examinations were normal at presentation. Abdominal ultrasound revealed a mass involving the right adrenal gland. Standard planar scintigraphy was performed at 4, 18, and 24 hours after intravenous injection 185 MBq (5mCi) of 123I-labeled metaiodobenzylguanidine (123I-MIBG). An area of focal intense uptake was identified in the area of the right adrenal gland. A pheochromocytoma was confirmed histologically after surgical excision.  相似文献   
68.
The tone of respiratory smooth muscle is largely determined by the input from autonomic nerves. The distribution of adrenergic and selected nonadrenergic, non-cholinergic (NANC) nerves in the normal equine trachealis muscle was investigated using immunohistochemistry. The smooth muscle of the trachealis was found to contain numerous nerves immunoreactive for an enzymatic marker of adrenergic nerves, as well as many nerves immunoreactive for a putative NANC neurotransmitter, peptide histidine isoleucine, a potent bronchodilator. The tissues surrounding the respiratory smooth muscle contained numerous nerves immunoreactive for the neuropeptides substance P and calcitonin gene-related peptide, which can cause marked vasodilation and bronchoconstriction. The complex innervation of the equine trachea should be kept in mind when interpreting the results of physiological experiments.  相似文献   
69.
70.
The temperature dependence of chemical reaction rates and microbial metabolism mean that temperature is a key factor regulating soil trace gas emissions and hydrochemistry. Here we evaluated a novel approach for studying the thermal response of soils, by examining the effects of temperature on gas emissions and hydrochemistry in (a) peat and (b) soil from a Sitka spruce plantation. A thermal gradient was applied along an aluminium bar, allowing soil to be incubated contemporaneously from 2 to 18 °C. The approach demonstrated clear differences in the biogeochemical responses of the two soil types to warming. The peat showed no significant emission of CH4 at temperatures below 6 °C, while above 6 °C, a marked increase in the rate of release was apparent up to 15 °C (Q10 = 2.5) with emissions being similar between 15 and 18 °C. Conversely, CH4 emissions from the forest soil did not respond to warming. Nitrate availability in the peat decreased by 90% between 2 and 18 °C (P < 0.01), whereas concentrations in the forest soil did not respond. Sulphate availability in the peat decreased significantly with warming (60%, P < 0.01), while the forest soil showed the opposite response (a 30% increase, P < 0.01). Conventionally, thermal responses are studied by incubating individual soil samples at different temperatures, involving lengthy preparation and facilities to incubate samples at different temperatures simultaneously. Data collected on a given thermal response is usually limited and thus interpolated or extrapolated. The thermal gradient method overcomes these problems, is simple and flexible, and can be adapted for a wide range of sample types (not confined to soil). Such apparatus may prove useful in the optimization of management practices to mitigate the effects of climate change, as thermal responses will differ depending on land use and soil type.  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