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   6250篇
  免费   589篇
  国内免费   5篇
  2023年   29篇
  2022年   53篇
  2021年   110篇
  2020年   64篇
  2019年   102篇
  2018年   93篇
  2017年   95篇
  2016年   159篇
  2015年   308篇
  2014年   323篇
  2013年   341篇
  2012年   472篇
  2011年   436篇
  2010年   270篇
  2009年   270篇
  2008年   374篇
  2007年   394篇
  2006年   366篇
  2005年   381篇
  2004年   341篇
  2003年   310篇
  2002年   299篇
  2001年   78篇
  2000年   44篇
  1999年   73篇
  1998年   74篇
  1997年   58篇
  1996年   45篇
  1995年   58篇
  1994年   48篇
  1993年   46篇
  1992年   38篇
  1991年   34篇
  1990年   40篇
  1989年   29篇
  1988年   25篇
  1987年   43篇
  1986年   21篇
  1985年   28篇
  1984年   38篇
  1983年   28篇
  1982年   36篇
  1981年   34篇
  1980年   26篇
  1979年   24篇
  1978年   27篇
  1976年   23篇
  1974年   20篇
  1973年   27篇
  1972年   20篇
排序方式: 共有6844条查询结果,搜索用时 15 毫秒
941.
Tourism accounts for 9% of global GDP and comprises 1.1 billion tourist arrivals per annum. Visits to wildlife tourist attractions (WTAs) may account for 20–40% of global tourism, but no studies have audited the diversity of WTAs and their impacts on the conservation status and welfare of subject animals. We scored these impacts for 24 types of WTA, visited by 3.6–6 million tourists per year, and compared our scores to tourists’ feedback on TripAdvisor. Six WTA types (impacting 1,500–13,000 individual animals) had net positive conservation/welfare impacts, but 14 (120,000–340,000 individuals) had negative conservation impacts and 18 (230,000–550,000 individuals) had negative welfare impacts. Despite these figures only 7.8% of all tourist feedback on these WTAs was negative due to conservation/welfare concerns. We demonstrate that WTAs have substantial negative effects that are unrecognised by the majority of tourists, suggesting an urgent need for tourist education and regulation of WTAs worldwide.  相似文献   
942.
The cytochrome composition of 19 strains of Actinoplanaceae has been investigated. The principal cytochrome found was cytochrome b, and occasional strains showed the presence of cytochrome a.  相似文献   
943.
We report on the first phytochemical investigation of a member of the African genus Resnova (Hyacinthoideae: Hyacinthaceae). From the dichloromethane extract of the bulbs of both Resnova humifusa and Eucomis montana (Hyacinthoideae: Hyacinthaceae) a novel 3-benzyl-4-chromanone homoisoflavonoid, 5,6-dimethoxy-7-hydroxy-3-(4′-hydroxybenzyl)-4-chromanone, was isolated. A further 11 known homoisoflavonoids were also identified, the 12 in total presenting a clear biosynthetic sequence. Eight of the 12 compounds found were common to both species.  相似文献   
944.
945.
Although defeminization of the rat brain appears to depend significantly on the conversion of testosterone (T) to estradiol (E2), the antiandrogenic steroid cyproterone acetate (CA) is able to attenuate defeminization. In order to study the mechanism of action of CA on brain sexual differentiation, newborn male rats were given subcutaneous injections of this steroid on postnatal Days 2–6. When castrated on Day 70 and given estrogen and progesterone, these CA-treated males displayed elevated lordosis quotients (LQ) compared to controls. CA-treated neonatal males were also examined at the end of the drug treatment to ascertain the mechanism of drug action: (1) Serum T levels were normal; (2) Brain cell nuclear estrogen receptor occupation, estimated by an exchange assay, was reduced by ≈ 30% in the brains of the CA-treated males, although the ability of exogenous E2 to occupy these brain estrogen receptors was not reduced. Other work has demonstrated a weak competitive effect of cyproterone on aromatization, and thus cyproterone acetate may have interfered with the conversion of T to E2 CA also has progestogenic activity, and 5-mm capsules of a potent synthetic progestin, R5020, given to newborn male rats on Days 2–6, are shown to elevate the LQ after postnatal Day 70 to the same extent as CA. However, R5020 did not reduce estrogen receptor occupation in the neonatal male rat brain and was without effect on serum T levels in the neonatal male. Because of the implied role of T-derived estrogens in defeminization, an experiment was conducted showing that the defeminizing action of estradiol benzoate given to 3-day-old female rat pups is attenuated by the antiestrogen, CI628, and not by the potent inhibitor of aromatization, 1,4,6-androstatriene-3,17-dione (ATD). This result complements previous experiments showing that both ATD and CI628 attenuate the defeminization produced by T. Taken together, the results lend further support to a pivotal role for aromatization and for estrogen-receptor interactions in the defeminizing effects of T. The actions of progestins such as CA and R5020 in attenuating defeminization are discussed in relation to the recent demonstration of progestin receptors in the neonatal rat brain. It is concluded that CA may act by a combination of actions, both by inhibiting aromatization and by acting as a progestin.  相似文献   
946.
Journal of Plant Growth Regulation - Trichoderma is an important genus of symbiotic fungi, commonly used around the world as biocontrol agents and as biofertilizer. Although their beneficial...  相似文献   
947.
948.
949.
Michael Bradbury  Neil R. Baker 《BBA》1981,635(3):542-551
An analysis of the photo-induced decline in the in vivo chlorophyll a fluorescence emission (Kautsky phenomenon) from the bean leaf is presented. The redox state of PS II electron acceptors and the fluorescence emission from PS I and PS II were monitored during quenching of fluorescence from the maximum level at P to the steady state level at T. Simultaneous measurement of the kinetics of fluorescence emission associated with PS I and PS II indicated that the ratio of PS I/PS II emission changed in an antiparallel fashion to PS II emission throughout the induction curve. Estimation of the redox state of PS II electron acceptors at given points during P to T quenching was made by exposing the leaf to additional excitation irradiation and determining the amount of variable PS II fluorescence generated. An inverse relationship was found between the proportion of PS II electron acceptors in the oxidised state and PS II fluorescence emission. The interrelationships between the redox state of PS II electron acceptors and fluorescence emission from PS I and PS II remained similar when the shape of the induction curve from P to T was modified by increasing the excitation photon flux density. The contributions of photochemical and non-photochemical quenching to the in vivo fluorescence decline from P to T are discussed.  相似文献   
950.
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