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   1664篇
  免费   78篇
  国内免费   3篇
林业   131篇
农学   31篇
基础科学   21篇
  367篇
综合类   189篇
农作物   153篇
水产渔业   71篇
畜牧兽医   575篇
园艺   50篇
植物保护   157篇
  2022年   15篇
  2021年   16篇
  2020年   32篇
  2019年   35篇
  2018年   40篇
  2017年   35篇
  2016年   50篇
  2015年   26篇
  2014年   39篇
  2013年   81篇
  2012年   78篇
  2011年   89篇
  2010年   50篇
  2009年   59篇
  2008年   76篇
  2007年   78篇
  2006年   93篇
  2005年   98篇
  2004年   70篇
  2003年   86篇
  2002年   64篇
  2001年   22篇
  2000年   15篇
  1999年   21篇
  1998年   9篇
  1997年   21篇
  1996年   18篇
  1995年   16篇
  1993年   9篇
  1992年   20篇
  1991年   12篇
  1990年   19篇
  1989年   25篇
  1988年   12篇
  1987年   12篇
  1986年   12篇
  1985年   17篇
  1984年   14篇
  1982年   10篇
  1981年   14篇
  1980年   9篇
  1979年   11篇
  1977年   9篇
  1974年   19篇
  1973年   9篇
  1972年   9篇
  1971年   8篇
  1952年   8篇
  1926年   11篇
  1925年   10篇
排序方式: 共有1745条查询结果,搜索用时 109 毫秒
61.
Proteinuria (urine protein/creatinine ratio, 13.6) resolved after control of primary erythrocytosis in a dog. Hydroxyurea and doxorubicin administration and phlebotomy were used initially to manage erythrocytosis. Remission was maintained for approximately 2 years. Glomerulonephropathy, characterized by absence of routine histologic or immunofluorescent changes and ultrastructural evidence of basement membrane deterioration and podocyte fusion, was documented. These lesions may have been a result of hypoxia and/or hyperviscosity secondary to erythrocytosis.  相似文献   
62.
Roots from healthy and diseased mature ponderosa pine, Pinus ponderosa Laws., trees were excavated from a site near Burns, Oregon. The diseased trees were infected with black-stain root disease, Leptographium wageneri Kendrick, or annosus root disease, Heterobasidion annosum (Fr.) Bref., or both. Axial hydraulic conductivity of the roots was measured under a positive head pressure of 5 kPa, and the conducting area was stained with safranin dye to determine specific conductivity (k(s)). In diseased roots, only 8-12% of the cross-sectional xylem area conducted water. Resin-soaked xylem completely restricted water transport and accounted for 13-16% of the loss in conducting area. In roots with black-stain root disease, 17% of the loss in conducting area was associated with unstained xylem, possibly resulting from occlusions or embolisms. Based on the entire cross-sectional area of infected roots, the k(s) of roots infected with black-stain root disease was 4.6% of that for healthy roots, whereas the k(s) of roots infected with annosus root disease was 2.6% of that for healthy roots. Although these low values were partly the result of the presence of a large number of diseased roots (72%) with no conducting xylem, the k(s) of functional xylem of diseased roots was only 33% of that for healthy roots. The low k(s) values of functional xylem in diseased roots may be caused by fungus induced occlusions preceding cavitation and embolism of tracheids. The k(s) of disease-free roots from diseased trees was only 70% of that for healthy roots from healthy trees. The disease-free roots had the same mean tracheid diameter and tissue density as the healthy roots, suggesting that the lower k(s) in disease-free roots of diseased trees may also have been caused by partial xylary occlusions.  相似文献   
63.
64.
65.
Minh PT  Ngoc PH  Taylor WC  Cuong NM 《Fitoterapia》2004,75(6):552-556
A new ent-kaurane diterpenoid (1) was isolated from the leaves of Croton tonkinensis. The structure of 1 was determined as ent-7beta-hydroxy-15-oxokaur-16-en-18-ol from spectroscopic methods.  相似文献   
66.
A self-help watershed management project in the densely populated areas of Central Java, Indonesia, is discussed in terms of its achievements to combine activities to improve the physical, the social and the economic environment. Without simultaneous, well-planned action on these three essential components the project's improvements cannot be sustained by the farmers. It was found that activities such as tree planting and maintenance, construction of soil erosion control structures and training in agroforestry-type intensification techniques for land cultivation serve as a focus to induce farmers to participate. Participation can be sustained in spite of initial decreasing net income if increases in land productivity based on agroforestry practices together with appropriate training programmes organize and motivate the farmers. In future watershed management programmes more emphasis may be placed on the rural development planning aspects of e.g. agroforestry techniques, and on rural community organization.This paper is based on a M.Sc. research study conducted by the first author under the supervision of the second author at the Rural Development Planning Programme of the Division of Human Settlements Development, A.I.T., Bangkok. The first author is indebted to FAO and the Canadian International Development Agency for scholarship and research grants and to his employer, the Solo Watershed Development Centre of the Indonesian Government. The second author acknowledges his secondment by the Belgian Development Co-operation Administration, Brussels. Comments of the research committee members, Prof. Dr. H. Dias and J. Piper on the research project and of anonymous referees on the paper are gratefully acknowledged. However, the responsibility for the views presented here lies solely with the authors.  相似文献   
67.
Zusammenfassung Es wird von Fraßbeschädigungen an derPinus radiata Don. durchRattus rattus L. aus der nördlichen Forstregion Chiles berichtet. Stark beschädigte Kiefern mit mehreren Fraßstellen oder einer fast vollständigen Ringelung starben durch Vertrocknen ab. Bei weitgehender Ringelung kam es nicht selten infolge eines Staues der Assimilate oberhalb der Beschädigungen zu einem abnormen Dickenwachstum. Diese verformten Bäume wurden häufig vom Wind gebrochen oder geworfen. Der starke Befall führte in einem Waldgebiet zum frühzeitigen Abtrieb eines Bestandes von zirka 150 ha und zu einer starken Durchforstung angrenzender Bestände in erosionsgefährdeter Hanglage. Nach dreijähriger Dauer ließ der Fraß plötzlich wieder nach. Die Massenvermehrung der Ratten stand in keinem direkten Zusammenhang zurPinus radiata. Durch die besondere Form der Bewirtschaftung und die daraus resultierende mangelnde Pflege derPinus radiata wurde der Angriff der Ratten begünstigt.
Summary Damages are described which were caused byRattus rattus L. inPinus radiata Don. forests in the northern forest districts of Chile. Heavily damaged ones, some of them completely ringbarked, died. Partly ringbarked trees frequently showed an abnormal increase in size above the lesion (assimilation). Such crippled trees were frequently broken or roote by the wind. In one instant an area of 150 ha had to be cleared, and the adjacent areas were thinned. Thus, the slopes were exposed to erosion. The chewing by the rodents suddenly diminished after three years from onset. The tremendous reproduction rate of the rats was not related to thePinus radiata habitat. Lack of proficient management favored the rats.
  相似文献   
68.
竹材结构的防腐   总被引:2,自引:0,他引:2  
由于在露天环境中遭遇生物腐蚀,竹秆的持久性差,所以需要采取化学手段增强其耐力.但是它的解剖构造使化学物质难以像木材那样容易进入竹材.竹秆的外部由其表皮保护防水,不像木材那样有径向渗透的途径.而其内部腔隙也有保护性纤维.化学物质的主要渗透途径位于竹秆根部微管束的后生木质部.这些渗透途径分布于横剖面,很不均匀,且少,只占总面积的8~10%,而且在通过竹节时改变方向.由于在采伐时砍伤竹秆,通向导管的细胞腔被堵塞,渗透途径也受影响.周围的薄壁细胞是竹秆组织的主要部分,它们通过微小的纹孔互相连接,只能通过弥散进入.它们所含的淀粉是昆虫和某些真菌的食品.对纤维的保护也有赖于弥散.用化学物质进行保护,对于新鲜的含水量高的竹秆效果最好.就像简单的根部处理,或技术性较强的细胞液改善.新鲜竹秆的薄壁组织和纤维也能通过垂直弥散的方法得到保护.如进行滴渍和浸渍工作,最好在竹材含水分时劈开,因为薄壁组织容易接受弥散.技术措施如竹秆的水分储存和熏蒸的过程,与竹秆的自然结构,尤其是薄壁组织有关.  相似文献   
69.
Zusammenfassung Der Energiesto? eines guten Samenjahres der Fichte im Nationalpark Bayerischer Wald bewirkte bei M?usen im Frühjahr 1972 zun?chst eine starke Zunahme der Gelbhalsmaus. Nach dem Zusammenbruch dieser Population unter Stresserscheinungen, hervorgerufen durch Nahrungsverknappung und überbev?lkerung, trat im Herbst die R?telmaus st?rker in Erscheinung, die insbesondere an Laubh?lzern im Winter 1972/73 deutliche Spuren hinterlie?. Im Frühjahr 1973 dominierte nach einem schneereichen Winter bei sehr geringer Dichte die Erdmaus. Untersuchungen über die Nahrungswahl, Energie- reserven, Reproduktionsrate und Stresserscheinungen zeigten, da? die Populationsentwicklung der Gelbhalsmaus vorwiegend durch das Nahrungsangebot gesteuert wurde. Die Bev?lkerungsentwicklung der R?telmaus erschien dagegen von der Gelbhalsmaus negativ beeinflu?t oder durch nicht aufgedeckte endogene Faktoren geregelt. Es ergaben sich hier keine Hinweise für das Vorhandensein von Stressoren in der entscheidenden Vermehrungspause bis zum August 1972. Eine starke Zunahme der Adrenal- gewichte wurde erst im November bei h?chster Dichte und dem Einsetzen des Winters beobachtet.
Summary Studies about a turnover of a Field-Mouse-population in the Bavarian National Park. In 1972 and 73 research work was carried out about population dynamics of the three most commonly species of rodents in the Bavarian National Park near the czech border: The Yellownecked Field Mouse (Apodemus flavicollis L.), the Bank Vole (Clethrionomys glareolus) and the Common Vole (Microtus agrestis L.). A great amount of spruce- seeds seemed to be most important for increased numbers of the Field Mouse trapped in the spring 1972 on all five localities (A, B, C, D and E) which were observed (Abb. 1). After the crash of this population in August 1972 by good weather conditions (Abb. 2) numbers of the Bank Vole were increased and this species was now most numerous in all localities (Abb. 1). In June 1973 the lowest density of all species was observed and the Common Vole dominated. This latter fact may be related to the high snow-cover until May 1973. Studies about ingested food-stuffs, body-fat content (Tab. 1) reproduction-rate (Abb. 3) and signs of stress (Abb. 4) showed, that the crash of the Field Mouse was caused primarly by detioration of food coupled with crowding effects. The population dynamics of the Bank Vole seemed interfered by the Field Mouse. No signs of stress were however detected during the recess of reproduction until August 1972 when the Field Mouse was dominating.
  相似文献   
70.
Pinguinain is the name given to a proteolytic enzyme preparation obtained from Bromelia pinguin fruits that has been scarcely studied. The present paper deals on the reexamination of the proteases present in fruits of B. pinguin grown in Cienfuegos, Cuba. The preparation (partially purified pinguinain, PPP) showed the main characteristics of the cysteine proteases, i.e., optimum pH within alkaline range (pH 7.2-8.8), inhibition of proteolytic activity by thiol blocking reagents, which is usually reverted by addition of cysteine, a remarkable thermal stability and notable stability at high ionic strength values. Isoelectric focusing and zymogram of PPP revealed the presence of several proteolytic components between pI 4.6 and 8.1. Preliminary peptidase purification by cationic exchange chromatography showed the presence of two main proteolytic fractions with molecular masses of approximately 20.0 kDa, according to SDS-PAGE.  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