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   421篇
  免费   42篇
  国内免费   201篇
  2024年   6篇
  2023年   5篇
  2022年   9篇
  2021年   22篇
  2020年   13篇
  2019年   20篇
  2018年   11篇
  2017年   14篇
  2016年   16篇
  2015年   19篇
  2014年   23篇
  2013年   26篇
  2012年   53篇
  2011年   40篇
  2010年   42篇
  2009年   39篇
  2008年   35篇
  2007年   34篇
  2006年   23篇
  2005年   23篇
  2004年   18篇
  2003年   13篇
  2002年   26篇
  2001年   20篇
  2000年   11篇
  1999年   10篇
  1998年   14篇
  1997年   3篇
  1996年   3篇
  1994年   8篇
  1993年   4篇
  1992年   6篇
  1991年   5篇
  1990年   2篇
  1989年   8篇
  1988年   2篇
  1987年   4篇
  1986年   2篇
  1985年   4篇
  1984年   4篇
  1983年   3篇
  1982年   3篇
  1979年   2篇
  1976年   2篇
  1975年   2篇
  1972年   1篇
  1967年   1篇
  1959年   1篇
  1957年   2篇
  1956年   3篇
排序方式: 共有664条查询结果,搜索用时 171 毫秒
141.
对武汉生物工程学院问天湖湖水的水质进行监测,其中氨氮0.03~0.19 mg/L,总磷0.08~1.01 mg/L,化学需氧量(COD)22.26~58.22 mg/L,溶解氧(DO)2.79~4.31 mg/L,pH 7.60~8.25。由监测结果可知,pH值在正常范围内,氨氮与DO达标,总磷与COD超标,问天湖水质污染原因主要是有机物污染及磷引起的富营养化现象,并对如何改善水质提出了相应的治理对策。  相似文献   
142.
143.
The present study aimed to investigate the relationship between growth rate, final mass, and larval development, as well as how this relationship influences reproductive trade‐offs, in the context of a gregarious life‐style and the need to keep an optimal group size. We use as a model two sympatric populations of the pine processionary moth Thaumetopoea pityocampa, which occur in different seasons and thus experience different climatic conditions. Thaumetopoea pityocampa is a strictly gregarious caterpillar throughout the larval period, which occurs during winter in countries all over the Mediterranean Basin. However, in 1997, a population in which larval development occurs during the summer was discovered in Portugal, namely the summer population (SP), as opposed to the normal winter population (WP), which coexists in the same forest feeding on the same host during the winter. Both populations were monitored over 3 years, with an assessment of the length of the larval period and its relationship with different climatic variables, final mass and adult size, egg size and number, colony size, and mortality at different life stages. The SP larval period was reduced as a result of development in the warmer part of the year, although it reached the same final mass and adult size as the WP. Despite an equal size at maturity, a trade‐off between egg size and number was found between the two populations: SP produced less but bigger eggs than WP. This contrasts with the findings obtained in other Lepidoptera species, where development in colder environments leads to larger eggs at the expense of fecundity, but corroborates the trend found at a macro‐geographical scale for T. pityocampa, with females from northern latitudes and a colder environment producing more (and smaller) eggs. The results demonstrate the importance of the number of eggs in cold environments as a result of an advantage of large colonies when gregarious caterpillars develop in such environments, and these findings are discussed in accordance with the major theories regarding size in animals. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 105 , 340–349.  相似文献   
144.
该研究旨在建立联体小鼠模型并评估小鼠应对联体状态时的生理变化。选取13对同基因联体小鼠,在两只小鼠肘关节远端4~5mm处,沿着肱骨和肋骨侧面直到腰围线末端,连接皮肤和皮下组织,建立联体模型,术后观察联体小鼠运动变化、体重、粪便皮质酮活性及血液交换率等。结果发现:术后联体小鼠出现兴奋、焦虑等特征性运动失常,随后数周小鼠活动逐渐适应联体状态,120天内无一死亡;术后三天体重下降,术后一月恢复至正常;术后粪便中皮质酮代谢物水平快速升高,至第75天降至正常;检测术后其中三对联体小鼠之间的血液交换率,结果基本正常,第12周血液完全交换率分别为63、46和107min。该结果提示小鼠能够很好地适应联体状态,该模型具有广泛的应用前景。  相似文献   
145.
目的:获得清晰可靠、重复性较好的ISSR-PCR扩增反应体系,应用于芋种质资源进行遗传多样性的研究中.方法:以芋的幼叶提取基因组DNA为材料,采用正交试验设计L16(45),从模板DNA浓度、引物浓度、Mg2+浓度、dNTPs浓度及TaqDNA聚合酶的用量5因素4水平出发,构建芋最佳反应体系.结果:芋ISSR-PCR的最佳反应体系为:在25μl的反应体系中,40ng DNA模板、0.4μmol/L引物浓度、2.5 mmol/L Mg2+、0.2 mmol/L dNTPs、1 U Taq DNA聚合酶.结论:利用芋种质资源对最佳反应体系的验证,结果显示该反应体系具有扩增稳定性.  相似文献   
146.
We describe in detail the floral ontogeny of Nymphaea tetragona from a wild population to provide evidence regarding the phylogenetic position of Nymphaea and to reveal evolutionary trends of flowers in Nymphaeaceae by comparison with that of the other genera. Four sepals are initiated unidirectionally. The basal petals are initiated unidirectionally and alternate with the sepals. The dome‐shaped floral apex continues to expand and produces more petal and stamen primordia. The remaining petals and all stamens are initiated in spirals or whorls. Later, the periphery of the floral apex grows more quickly than the centre and results in a depression in the centre of the apex after all stamens have been initiated. Carpels are simultaneously initiated in a cycle at the periphery of the depression. They are ascidiate. After all organs have been initiated, the centre of the depression on the floral apex grows and develops into a globular structure. The connected inferior ovary, stigma caps and the globular floral apex together form an extragynoecial compitum. Within Nymphaeaceae, the floral ontogeny of Nymphaea is most similar to that of Euryale and Victoria. It differs more from Ondinea and Barclaya, and differs most from Nuphar. © 2009 The Linnean Society of London, Botanical Journal of the Linnean Society, 2009, 159 , 211–221.  相似文献   
147.
应用3天种群增长实验方法,在(25±1)℃、无光照、以3.0×106 cells/mL的斜生栅藻为轮虫的食物等条件下,研究了亚致死浓度(0.01、0.1、1.0、10.0、100.0、1000.0和10000.0 µg/L)的甲胺磷和乙酰甲胺磷对萼花臂尾轮虫实验种群动态的影响。结果显示,甲胺磷和乙酰甲胺磷显著地影响萼花臂尾轮虫的种群增长率、混交雌体数与非混交雌体数的比值和混交率。甲胺磷显著地影响萼花臂尾轮虫种群中的带卵雌体数与不带卵雌体数的比值,但乙酰甲胺磷对其无显著的影响。和对照组相比,浓度为100.0 µg/L 的甲胺磷和浓度为1.0-10,000.0 µg/L 的乙酰甲胺磷均使轮虫种群增长率显著增大,而浓度为1000.0 µg/L和10000.0 µg/L的甲胺磷却使之显著减小;1000.0 µg/L 的甲胺磷使轮虫种群中的带卵雌体数与不带卵雌体数的比值显著上升,0.1 µg/L的甲胺磷和10.0-10000.0 µg/L的乙酰甲胺磷均使轮虫种群中的混交雌体数与非混交雌体数的比值显著上升,0.1µg/L 的甲胺磷和10 µg/L的乙酰甲胺磷均使轮虫的混交率显著增大,10.0-0000.0 µg/L 的乙酰甲胺磷使轮虫休眠卵产量显著提高。上述结果表明,亚致死浓度的甲胺磷和乙酰甲胺磷对萼花臂尾轮虫实验种群动态具有显著的影响。  相似文献   
148.
选择福建省梁野山、龙栖山、闽侯旗山、南靖书洋乡和南平大坪村南方红豆杉(Taxus chinensis var. marei)天然群体分布相对集中区,采集胸径相同的南方红豆杉的当年生针叶和树皮并取树下土壤样品。利用超高效液相色谱分析测定南方红豆杉当年生针叶及树皮中紫杉醇及10-去乙酰基巴卡亭Ⅲ(10-deacetylbaccatin Ⅲ, 10-DAB)含量,另测定土壤pH值、速效磷、速效氮、铁、钙、镁、锌含量。结果表明,不论是紫杉醇还是10-DAB含量,不同地理种源南方红豆杉间差异均较大,说明地理位置对于红豆杉中紫杉醇和10-DAB含量具有明显的影响;树皮中紫杉醇含量与土壤中铁含量显著相关(P<0.01);南方红豆杉当年生针叶中及树皮中紫杉醇含量与各种源地气象因子的相关性不显著(P>0.05)。  相似文献   
149.
DIANHUA KE  & XIN LU 《Ibis》2009,151(2):321-331
The Tibetan Ground Tit Pseudopodoces humilis is a high-altitude passerine endemic to the Tibetan Plateau. A 4-year study in alpine meadows in Northern Tibet at 4300 m asl demonstrated that rather than using Pika Ochotona spp. holes as previously reported, the birds excavated one nest burrow themselves in spring and another after breeding, which they used for roosting in winter. Both the nesting and winter-roosting burrows comprised a straight tunnel leading to an ellipsoid chamber. There were no significant differences in placement or structural characteristics between the two types of burrows, except that winter-roosting burrows had a significantly smaller entrance diameter. Most burrows were 100–160 cm long and their chambers 20–40 cm deep. Tibetan Ground Tits tended to maximize the thermal benefits of their burrows by adjusting their spatial and structural characteristics in response to local solar radiation and wind regimes. Burrows tended to be oriented towards the sun and away from prevailing winds, presumably to maintain burrow temperature. Longer tunnels could function to save heat from solar radiation or reduce wind-disturbance, while shorter tunnels allow chambers to be warmed sooner in situations where wind potentially reduced soil temperatures. The thermal benefits to the birds of burrow architecture are likely to play a crucial role throughout the year in these extreme alpine environments.  相似文献   
150.
大鼠胰腺发育的过程分为以细胞增生、分化为主的早中期及以结构形成、功能完善为主的后期这两个阶段,后期涉及胚胎后期胰岛结构形成和生后胰岛结构重塑.而β细胞的功能完善需要多种细胞迁移和细胞间支持黏附因子的介导,其中表达于β细胞的问皮素(Mesothelin)对细胞间的信号识别以及相互黏附有重要的作用.Mesothelin也是胰腺癌等多种恶性肿瘤的细胞表面标记物.有关Mesothelin在大鼠胰岛结构形成和结构重塑过程中的作用及其与胰腺癌形成关系的作用正处于研究之中.  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