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   100717篇
  免费   5123篇
  国内免费   41篇
林业   5126篇
农学   3625篇
基础科学   692篇
  13492篇
综合类   15592篇
农作物   4233篇
水产渔业   4964篇
畜牧兽医   49524篇
园艺   1668篇
植物保护   6965篇
  2018年   2294篇
  2017年   2437篇
  2016年   1787篇
  2015年   1144篇
  2014年   1392篇
  2013年   3516篇
  2012年   2872篇
  2011年   3949篇
  2010年   3087篇
  2009年   2912篇
  2008年   3727篇
  2007年   3797篇
  2006年   2662篇
  2005年   2619篇
  2004年   2400篇
  2003年   2614篇
  2002年   2327篇
  2001年   2879篇
  2000年   2832篇
  1999年   2269篇
  1998年   939篇
  1997年   1004篇
  1995年   1069篇
  1994年   922篇
  1993年   953篇
  1992年   1866篇
  1991年   2059篇
  1990年   1960篇
  1989年   1964篇
  1988年   1836篇
  1987年   1773篇
  1986年   1821篇
  1985年   1765篇
  1984年   1538篇
  1983年   1299篇
  1982年   944篇
  1979年   1471篇
  1978年   1130篇
  1977年   1074篇
  1976年   1030篇
  1975年   1084篇
  1974年   1311篇
  1973年   1325篇
  1972年   1251篇
  1971年   1165篇
  1970年   1121篇
  1969年   1133篇
  1968年   999篇
  1967年   1117篇
  1966年   926篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
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   
2.
Abstract.— The freshwater mussel Elliptio complanata was provided water containing green algae and cyanobacteria delivered from the Partitioned Aquaculture System (PAS) at eight flow rates to determine algal filtration rates as mg of particulate organic carbon (POC)/kg wet tissue weight per h. The dominant taxon in cyanobacterial waters was Microcystis while the dominant taxa in green algal waters were Scenedesmus and Ankistrodesmus . The cell counts of Scenedesmus and Ankistrodesmus were the only algal taxa that were significantly different between the incoming water and water filtered by mussels. Filtration rates of POC obtained from green algal water were significantly greater than from cyanobacteria-dominated waters at all flow rates. A significant increase in mean filtration rate was observed as flow rates increased. The filtration rate of green algae increased as POC concentration increased, peaking at 28 mg C/L. A maximum filtration rate was not observed with cyanobacterial waters.  相似文献   
3.
Grain yield and yield components (grains per ear, grain weight, 1000-grain weight, ear weight, ear seed ratio and dry matter partitioning between ear and seed) were examined in a wheat genotype (PBW-343) with well-filled grains and a Triticale genotype (DT-46) with poorly filled grains (showing grain shrivelling) grown in pots. Six days after anthesis (DAA), benzyladenine (BA) @2 µg ear−1 and abscisic acid (ABA) @4 µg ear−1 were injected at the base of the mother shoot ear in both species. It was observed that, in both wheat and Triticale , BA increased the grain weight, grain number and partitioning of dry matter between ear and seed, whereas ABA decreased the grain weight, grain number and dry matter partitioning between ear and seed. However, these decreases were slower in Triticale than in wheat. BA treatment increased the grain dry matter accumulation, which in turn resulted in better filling of grains and increased the grain weight in both wheat and Triticale . The average grain weight of Triticale was lower than that of wheat. Thus, it appears that variation in grain weight between wheat and Triticale might be due to different availabilities of growth-promoting phytohormones such as cytokinins and assimilates.  相似文献   
4.
Twelve (54.5%) of 22 free-roaming dogs in Ishigaki Island had tick infestation identified as Rhipicephalus sanguineus. There were 121 ticks recovered and consisted of 28 females, 58 males, 22 nymphs and 3 larvae. Infection of dogs possibly with canine ehrlichial pathogens was examined by both indirect immunofluorescence assay and polymerase chain reaction (PCR). Two dogs of the 13 examined were sero-positive for the human granulocytic ehrlichia agent, and one of two dogs was PCR positive for Ehrlichia platys. This dog had platelet numbers slightly lower than normal value, however, no morulae were found within platelet on peripheral blood smear stained with Giemsa.  相似文献   
5.
The isolation of (+)-totarol as active compound against Mycobacterium tuberculosis is reported from Chamaecyparis nootkatensis outerbark.  相似文献   
6.
Abstract – The reproductive biology of three species of rainbowfish (Melanotaeniidae) in northeastern Australian rainforest streams was investigated. Two species, Melanotaenia eachamensis and Cairnsichthys rhombosomoides are endemic to the area, whereas the third, M. splendida splendida, is more widespread. The species were all highly fecund, producing many hundreds of eggs between 1.10 and 1.24 mm in diameter. Melanotaenia eachamensis was the most fecund, produced the largest eggs of the three species, and consequently exhibited the greatest maternal investment (as measured by gonadosomatic index). The majority of reproductive effort occurred during the dry season, although reproductively active fish were present year-round for each of the species, but particularly so for M. s. splendida and C. rhombosomoides. No evidence for a role by temperature or photoperiod as environmental cues for reproduction was found, and it was suggested that gonad development was strongly tied to somatic growth. The concentration of reproduction to the dry season ensures that larvae are produced during a period of relatively stable and benign physical conditions. Comparison of temporal changes in gonadosomatic index values suggest that the spawning season of M. eachamensis , which occurs in high-elevation streams, is more restricted and commences about 1 month earlier than either other species. A similar phenology was observed for the M. s. splendida population found at high elevation and highlights the potential for spatial differences in stream productivity to influence life history. Note  相似文献   
7.
8.
9.
10.
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