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   6755篇
  免费   288篇
  国内免费   13篇
林业   230篇
农学   344篇
基础科学   82篇
  832篇
综合类   379篇
农作物   247篇
水产渔业   197篇
畜牧兽医   3768篇
园艺   109篇
植物保护   868篇
  2023年   31篇
  2022年   36篇
  2021年   55篇
  2020年   81篇
  2019年   90篇
  2018年   102篇
  2017年   102篇
  2016年   120篇
  2015年   98篇
  2014年   133篇
  2013年   500篇
  2012年   299篇
  2011年   354篇
  2010年   182篇
  2009年   176篇
  2008年   342篇
  2007年   326篇
  2006年   277篇
  2005年   276篇
  2004年   285篇
  2003年   258篇
  2002年   249篇
  2001年   237篇
  2000年   215篇
  1999年   195篇
  1998年   55篇
  1997年   54篇
  1996年   59篇
  1995年   64篇
  1994年   43篇
  1993年   50篇
  1992年   106篇
  1991年   119篇
  1990年   136篇
  1989年   111篇
  1988年   102篇
  1987年   84篇
  1986年   67篇
  1985年   71篇
  1984年   66篇
  1983年   44篇
  1982年   30篇
  1981年   41篇
  1980年   44篇
  1979年   45篇
  1975年   25篇
  1974年   25篇
  1973年   26篇
  1972年   35篇
  1971年   32篇
排序方式: 共有7056条查询结果,搜索用时 15 毫秒
181.
Depending upon the variety, fruits of plum trees infected with sharka virus may show grooves and pits, red bands and thin red rings and lines. The latter two types of symptom were only found on fruits that become orange, red or purple during ripening. On fruits of trees free from sharka virus these discolorations were never observed and therefore these symptoms are diagnostic for sharka virus. In several varieties the grooves and pits, previously thought to be the main symptom produced by sharka virus on plum fruits, were observed more or less frequently on fruits of trees free from sharka virus. Therefore, this symptom was unreliable for diagnosis of sharka virus under Dutch conditions.Inclusions were present in parenchyma cells of fruits of all varieties, when infected with sharka virus. They may be helpful for diagnosis when external symptoms are not conclusive.Samenvatting De diagnose van de sharkaziekte van de pruim is met behulp van de bladsymptomen goed mogelijk van begin juni tot ongeveer half juli. Daarna zijn de bladsymptomen bij de meeste rassen moeilijk of in het geheel niet meer te vinden. Daarom werd nagegaan in hoeverre vruchtsymptomen bruikbaar zijn voor een betrouwbare diagnose in de zomer.Op vruchten van met het sharkavirus geïnfecteerde bomen werden drie typen symptomen waargenomen (Tabel 1): 1. brede, rood- of paarsachtige bandvormige schilverkleuring, meestal aan een zijde scherp en aan de andere zijde diffuus begrensd (Plaat 2b en 4); 2. dunne, scherp begrensde rood- of paarsachtige lijntjes en kringetjes, vaak op de onderzijde van de vrucht (Plaat 5); 3. onregelmatige lijn- en putvormige inzinkingen, waaronder bruin necrotisch vruchtvles (Plaat 1b, 2b en 3b). Per vrucht kon meer dan een symptoomtype voorkomen. De symptomen verschenen pas 2–4 weken voor de rijping van de vruchten.Zowel de bandvormige verkleuringen als de dunne lijntjes en kringetjes werden waargenomen op vruchten van met het sharkavirus geïnfecteerde bomen van rassen met oranje, rode en paarse vruchten, doch niet op vruchten van rassen met gele of groene vruchten. Deze verkleuringen van de vruchtschil werden niet waargenomen op vruchten van niet mét het sharkavirus geïnfecteerde bomen (Tabel 1 en 2).De lijn- en putvormige inzikingen werden waargenomen op vruchten van verscheidene pruimerassen, zowel met als zonder het sharkavirus (Tabel 1 en 2; Plaat la en b, 2a en b en 3a en b). Bij een aantal rassen bleek dit pseudo-pox (inzinkingen op vruchten van bomen zónder het sharkavirus) vrij algemeen, zoals bij Warwickshire Drooper (Plaat la), Zoete Kwets (Plaat 3a) en incidenteel ook bij Victoria (Plaat 2a) en Early Prolific. De inzinkingen waren veelal alleen zichtbaar op de eerst rijpende vruchten, onafhankelijk van het voorkomen van het sharkavirus. Vruchten met dit symptoom vielen vaak vóór de rijping van de overige vruchten van de boom. De symptoomexpressie was op de vruchten van met het sharkavirus geïnfecteerde bomen vaak heviger dan op vruchten van niet met dit virus geïnfecteerde bomen, terwijl ook een groter aantal vruchten het sympttom toonden. Het is onmogelijk gebleken verschillen aan te geven tussen inzinkingen op vruchten van bomen die al dan niet met het sharkavirus waren geïnfecteerd. Alleen op vruchten van het ras Czar werden nooit inzinkingen gevonden.In de parenchymcellen van rijpende vruchten van sharkazieke bomen van alle onderzochte rassen werden voor het virus karakteristieke insluitsels gevonden. Deze insluitsels ontbraken in vruchten van niet met het sharkavirus geïnfecteerde bomen.Voor de diagnose van sharka kan dus bij rassen met oranje, rood of paars kleurende vruchten gebruik worden gemaakt van de beide typen schilverkleuring. Indien vruchten worden gevonden met inzinkingen, dan kan de aanwezigheid van het sharkavirus alleen worden vastgesteld op grond van andere symptomen op de vruchten en de bladeren, door insluitselonderzoek of door toetsing op indicatorplanten.  相似文献   
182.
Calculations from data of egg quality obtained at four Regional Experimental Farms for Poultry Husbandry and at the Central Institute for Poultry Research “Het Spelderholt” at Beekbergen, the Netherlands, support the conclusions advanced in recent American investigations that the uniform regression of Haugh (1937), which is used for the correction of the height of the thick albumen in relation to the egg weight (0.05 mm. per g. difference in weight), has no general validity. Only for fresh eggs at the beginning of the laying period was an average regression found which did not deviate significantly from the Haugh regression. Significant differences between farms and strain crosses were not established as long as exceptionally deviating circumstances were not considered. It was also found that as the laying period proceeded the Haugh correction factor for the height of the thick albumen in relation to the egg weight could have an adverse effect. The same conclusion is valid for stored eggs produced at the end, and probably at the beginning, of the laying period.  相似文献   
183.
184.
Forty-seven suspensions of type A virus grown in surviving bovine tongue epithelium (Frenkel) cultures were compared in a quantitative complement fixation test (CFT) and an antibody combining test (ACT) to evaluate the antigenic mass of the 140 S component, and with three assay methods for infectivity titration.The cultures were then converted into vaccines and the potency of these was measured with a guinea-pig PD50 test and again with the ACT. The relationship between the results obtained with different methods used for titration of infectivity, i.e. baby mice and plaque counting in BHK cells in monolayer (BHK-M) or suspended in agar (BHK-S) was poor and, with the exception of the BHK-S technique, showed little correlation with the results of the serological tests.The correlation coefficient between values obtained by the CFT and the ACT on virus cultures was 0.82 and that between CFT and ACT on vaccines 0.88. The guinea-pig PD50's of 19 vaccines were compared with the ACT and with the CFT and ACT of the corresponding cultures yielding r values of 0.74, 0.62 and 0.62 respectively. Regression lines are presented for the different relations.An accurate cattle PD50 was determined for two vaccines of the series, using groups of 20 animals per vaccine dilution. The relationship between guinea-pig PD50 (X) and cattle PD50 (Y), expressed by the regression formula Y = 1.05 X + 0.84, showed that differences in cattle PD50's are of the same magnitude as those observed in the guinea-pig test, provided that sufficient numbers of animals are used for both tests.The results of the quantitative CFT and ACT were in good agreement with the guinea-pig test and as such these tests can provide valuable information on the quality of Frenkel cultures.  相似文献   
185.
Experiments on the effect of various compounds with plant growth regulating activity led to the hypothesis that conditions inhibitory to indoleacetic acid (IAA) action or leading to a decrease in the IAA level in cucumber seedlings would be unfavourable for the development of cucumber scab, caused byCladosporium cucumerinum. Susceptibility decreased as the result of treatment with growth retardants, which cause an increase in IAA-oxidase activity in the plants, whereas application of indoleacetic acid or compounds expected to decrease the rate of oxidative breakdown of IAA in the seedlings resulted in an increase of susceptibility. Growing the plants under various periods of illumination also influenced both the susceptibility of the plants and the IAA-oxidase activity in the hypocotyl tissue.To obtain further information about the relation between the increase in resistance and the increase in the rate of IAA oxidation, the effect of one compound, viz. L-threo-β-phenylserine (abbr. phenylserine) was studied in more detail. The rate of oxidative breakdown of IAA was increased in extracts of hypocotyls and cotyledons of plants treated with phenylserine as compared with extracts of tissue of untreated plants. Extracts of phenylserine-treated plants were found to contain a higher cofactor and/or a lower inhibitor content than those of control plants. The substances involved were not identified, but this result may indicate a shift in the concentrations of various phenols in the tissue.The question must remain open whether a shift in the concentration of phenols some-how effected by phenylserine treatment determines both degree of susceptibility and rate of IAA oxidation independently, or whether the rate of IAA breakdown and the resulting change in IAA content in the plants is directly related to the degree of susceptibility.  相似文献   
186.
187.
188.
189.
A method whereby resistance data can be analysed by means of frequency distributions is described. This method established that Rhipicephalus appendiculatus and R. evertsi evertsi show either little or only developing resistance to the pesticides chlorfenvinphos and dioxathion. Boophilus spp., however, show more instances and higher levels of resistance to these 2 pesticides.  相似文献   
190.
In adultFasciola hepatica glucose is broken down to volatile fatty acids, carbon dioxide and lactate. In the cytosol glucose is converted into malate and lactate and malate is metabolized further in the mitochondrial compartment via a dismutation. The dismutation of malate can proceed under anaerobic conditions. To a limited extent oxygen can function as terminal electron acceptor.It is shown that the reaction catalysed by fumarate reductase is a reversible reaction. The direction of the reaction depends on the relative concentrations of fumarate and succinate.Mechanisms of ATP production and electron transport under aerobic and anaerobic conditions are discussed. It is concluded that the energy metabolism of the adult liver fluke is adapted to the life in an environment with a low oxygen content.
Kurzfassung Beim adultenFasciola hepatica wird Glucose in flüchtige Fettsäuren, Kohlendioxid und Laktate gespalten. Im Zytosol wird die Glucose in Malate und Laktate gespalten und Malate werden im Mitochondrienbereich durch Redox-Dismutation weiter verstoffwechselt. Die Dismutation der Malate kann unter anaeroben Bedingungen ablaufen. In gewissem Ausmass kann Sauerstoff terminal Elektronen binden.Es wird gezeigt, dass die Reaktion, die durch Fumaratreduktase katalysiert wird, reversibel ist. Der Ablauf der Reaktion hängt von den relativen Konzentrationen der Fumarate und Succinate ab.Ferner werden die Mechanismen der ATP-Produktion und des Elektronentransports unter aeroben und anaeroben Bedingungen diskutiert. Zusammenfassend wird festgestellt, dass der Energiestoffwechsel des adulten grossen Leberegels an ein Leben in einer sauerstoffarmen Umwelt adaptiert ist.

Resume Chez les douves adultes le glucose est dégradé en acides gras volatil, gaz carbonique et lactate. Dans le cytosol le glucose est convertit en malate et en lactate et le malate est métabolisé ensuite dans la mitochondrie selon un phénomène de transformation biochimique. La transformation biochimique du malate peut se réaliser dans des conditions anaérobies. Dans une certaine mesure l'oxygène peut fonctionner comme un accepteur terminal d'électrons.Il est démontré que la réaction catalysée par la fumarate réductase est réversible. Le sens de la réaction dépend de la concentration relative en fumarate et succinate.Le mécanisme de production de l'ATP, du transport des électrons dans des conditions aérobies et anaérobies est discuté. Il est retenu que le métabolisme énergétique de la douve adulte est adapté à la vie dans un environnement pauvre en oxygène.

Riassunto Negli adulti diFasciola hepatica il glucosio si scinde in acidi grassi volatili, biossido di carbonio e lattato. Nel citosol il glucosio è convertito in malato e lattato, ed il malato è ulteriormente metabolizzato nel comparto mitocondriale attraverso un processo di dismutazione. La dismutazione del malato può proseguire in condizioni di anaerobiosi. In misura limitata l'ossigeno può agire come accettore terminale di elettroni.E'stato dimostrato che la reazione catalizzata dalla fumarato-reduttasi è una reazione reversibile. La direzione della reazione dipende dalle concentrazioni relative di fumarato e di succinato.Vengono discussi i meccanismi di produzione di ATP e del trasporto di elettroni in condizione di aerobiosi e di anaerobiosi. Si conclude che il metabolismo energetico dell'adulto diFasciola hepatica è adattato alla vita in un ambiente con bassi livelli di ossigenazione.
  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