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   92436篇
  免费   22229篇
  国内免费   1294篇
林业   12643篇
农学   6805篇
基础科学   5628篇
  12364篇
综合类   51237篇
农作物   4593篇
水产渔业   4608篇
畜牧兽医   10078篇
园艺   3060篇
植物保护   4943篇
  2025年   935篇
  2024年   2080篇
  2023年   2107篇
  2022年   2435篇
  2021年   2665篇
  2020年   2777篇
  2019年   3282篇
  2018年   1781篇
  2017年   3031篇
  2016年   3742篇
  2015年   3376篇
  2014年   4851篇
  2013年   4935篇
  2012年   7394篇
  2011年   7662篇
  2010年   6251篇
  2009年   6199篇
  2008年   5757篇
  2007年   6866篇
  2006年   5938篇
  2005年   4996篇
  2004年   3780篇
  2003年   3308篇
  2002年   2588篇
  2001年   2352篇
  2000年   2114篇
  1999年   1679篇
  1998年   1445篇
  1997年   1288篇
  1996年   1184篇
  1995年   1139篇
  1994年   1089篇
  1993年   914篇
  1992年   886篇
  1991年   716篇
  1990年   654篇
  1989年   575篇
  1988年   447篇
  1987年   321篇
  1986年   151篇
  1985年   55篇
  1984年   35篇
  1983年   24篇
  1982年   19篇
  1981年   22篇
  1980年   23篇
  1979年   22篇
  1978年   16篇
  1976年   13篇
  1956年   10篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
本文述了“广东山区开发理论与实践研讨会”所达成共识的观点:广东山区开发具有重要的战略意义,广东山区开发必须走可持续肆展,农业产业化经营、工业化道路,同时必须搞好基础设施建设,组织好商品流通工作,优化资源配置。  相似文献   
2.
Bovine cumulus-oocyte complexes (COCs) were aspirated from antral follicles (3–6 mm in diameter). COCs with a compact multilayered cumulus investment were cultured for 9, 13 and 24 hours, respectively, and processed for scanning or transmission electron microscopy after different periods of culture including a 0 hour control group. In all 0 hour control COCs initial formation of cumulus cell projections, blebs and microvilli were observed. The start of cumulus-oocyte disconnection in cattle was observed after 9 hours of in vitro culture. These connections were almost completely discrupted after 13 hours, when a continuous production of extracellular matrix was observed. The full expansion of corona radiata cells was not observed even after 24 hours. Some cumulus oophorus cells were bound together with junctions of the zonula adherens type after 24 hours when extracellular matrix was found only in deeper layers. The full expansion of corona radiata cells was not the prerequisite for disconnection of cumulus-oocyte complex. Inhalt: Kumulus oophorus Expansion von in vitro kultivierten Rinderoozyten: Eine SEM und TEM Untersuchung Kumulus-Oozyten-Komplexe (COC) von Rindern wurden aus antralen Follikeln aspiriert (Durchmesser 3–6 mm). COC's mit kompakten mehrschichtigen Kumulus wurden für 9, 13 und 24 Stunden kultiviert und für die Scanner- oder Transmissionselektronenmikroskopie vorbereitet. Verschiedene Kulturzeiten incl. einer 0-Stundengruppe als Kontrolle wurden untersucht. In allen Kontroll COC's wurden beginnende Formationen yon Kumuluszellprojektionen, Bläschen und Mikrovilli beobachtet. Nach 9-stündiger in vitro Kultur begann die Lösung zwischen Kumulus und Oozyte. Die Verbindungen waren nach 13 Stunden vollständig unterbrochen, wobei eine kontinuierliche Produktion extrazellulärer Matrix beobachtet wurde. Eine vollständige Expansion der Corona radiata Zellen wurde auch nicht nach 24 Stunden beobachtet. Nach 24 Stunden waren einige Cumulus Oophorus Zellen über Verbindungen des Zonula adherens Typs verknüpft, wobei extrazelluläre Matrix nur in den tieferen Zellschichten gefunden wurde. Eine vollständige Kumulusexpansion ist keine Voraussetzung für die Auflösung des Kumulus-Oozyten-Komplexes.  相似文献   
3.
Dynamics of the cumulation of phosphorus, potassium, calcium and magnesium in developing seed of four soybean genotypes under the defined climatic conditions in the course of three years was observed. Samples were being taken at weekly intervals from the point at which the grain reached a size suitable to be analyzed up to the full maturity of the seed. By analyzing the results it was found out that the intense cumulation of the minerals tested took place during the stages III and IV of the seed development, that is, in the course of intense cumulation of dry matter and the most expressive metabolic activity in the seed. Investigation of the impact of climatic conditions provides evidence that cumulation of the tested macroelements has been favourably influenced by increased precipitation along with moderate air temperatures within the seed developing period. The highest content of the minerals found with the cultivar Maksimirka independently of the year investigated reveals that mineral cumulation in soybean seed is affected by the cultivar and genetic particularity.  相似文献   
4.
5.
6.
Germinating seeds and young plants of winter rape var. Górczañski were vernalized for 56–63 days under conditions of 9-hour day, at the temperature 2 and 5 °C and in continuous darkness at the temperature 2 °C. After vernalization the plants grew under conditions enabling to complete vernalization: in a glass-house at the temperature day/night 15/10 °C and in semi natural conditions of open vegetation hall in the period from June till August. After sub-optimal vernalization further growth of the plants at lowered temperature increased its effectiveness (completion of vernalization). Depending on the degree of the vernalization of the plants the completion of their vernalization was both obligatory, i.e. conditioning the acquisition of the ability of generative development, and facultative i.e. accelerating this development. It has been demonstrated that the population of plants of the examined variety is strongly differentiated not only with respect of vernalization requirements in the particular plants, but also what regards the effectiveness of vernalization completion. New observations have been made indicating that the mechanisms controlling the successive phases of generative development, i.e. phase of forming flower buds and the flowering phase are not identical which may be interpreted as indicating that the "flowering factor" is polymorphous.  相似文献   
7.
创新是一个民族进步的灵魂,是国家兴旺发达的不竭动力源泉。因此,对创新人才的培养,应该从基础教育抓起。为了在语文教学中培养学生的“创新精神”,教师首先应转变观念,使自己成为有创新精神的人。其次为学生创设和谐、民主的学习氛围,让学生最大限度地发展创造性思维能力。应让学生丰富知识、夯实基础、拓展视野,为创新思维的发展打下坚实的基础。  相似文献   
8.
综合英语课是研究生英语学习的基础课。笔者从教学实践论证了把合作学习理论引入综合英语教学的必要性和可行性。研究结果表明,通过合作学习有利于转变研究生英语学习理念,将语言学习与发展语言能力有机结合起来,有助于研究生综合语言能力的提高,为更高层次的英语选修课学习打好基础。  相似文献   
9.
Mycotoxic nephropathy was induced in 18 young pigs by diets contaminated with strains of Aspergillus ochraceus containing ochratoxin A (OTA) and penicillic acid (PA) at levels corresponding to those naturally encountered in animal feeds in Bulgaria. Haematological and biochemical parameters, as well as the morphological and ultrastructural changes in various internal organs, and especially in the kidneys, were examined at different stages of development of the disease. A mottled surface of the kidneys was only seen in pigs exposed to a mouldy diet containing 180 ppb OTA for 3 months, but microscopic lesions, as well as changes in various haematological and biochemical parameters, were observed in all groups exposed to the same mouldy diet containing only 90 or 180 ppb OTA. Histological examination showed two types of change: degenerative changes affecting the epithelial cells of the proximal tubules, which predominated at the initial stage, and proliferative changes in the interstitium, which predominated at the later stage of the disease. Telangiectasis and lymph stasis were also seen, as well as degenerative changes in the capillary endothelium. The characteristic renal lesions were similar to those observed in spontaneous cases of mycotoxic porcine nephropathy in Bulgaria, but they were a little different from the classic Danish porcine nephropathy. The enhanced toxicity of OTA in our study may be due to a synergistic effect between OTA and PA or to some other unknown metabolites produced by the same ochratoxinogenic strains of A. ochraceus.  相似文献   
10.
在人类居住的“地球村”里,生存着多种多样的生物,他们之间存在着千丝万缕的奇妙关系,生物共生就是这些关系之一。分析生物共生理念的产生根源,回顾生物共生理论的发展轨迹,对目前人类认识的生物共生现象进行了归类阐述,提出生物共生理论资源对构建和谐社会具有科学的指导意义。  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