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   570篇
  免费   31篇
  国内免费   12篇
  2014年   5篇
  2013年   16篇
  2012年   12篇
  2011年   22篇
  2010年   30篇
  2009年   39篇
  2008年   18篇
  2007年   26篇
  2006年   14篇
  2005年   15篇
  2004年   10篇
  2000年   4篇
  1999年   4篇
  1998年   9篇
  1997年   17篇
  1996年   16篇
  1995年   13篇
  1994年   9篇
  1993年   6篇
  1992年   7篇
  1991年   13篇
  1990年   15篇
  1989年   8篇
  1988年   8篇
  1987年   12篇
  1986年   6篇
  1985年   4篇
  1983年   6篇
  1981年   4篇
  1979年   4篇
  1978年   4篇
  1977年   7篇
  1976年   5篇
  1974年   4篇
  1973年   11篇
  1972年   9篇
  1971年   9篇
  1968年   4篇
  1959年   15篇
  1958年   25篇
  1957年   12篇
  1956年   17篇
  1955年   12篇
  1954年   7篇
  1953年   5篇
  1952年   8篇
  1951年   10篇
  1950年   14篇
  1949年   10篇
  1948年   9篇
排序方式: 共有613条查询结果,搜索用时 15 毫秒
131.
132.
For the triangle, duo‐trio, same‐different and 2‐AFC methods, using a model system, mean d′ values for the same subjects, discriminating between the same taste stimuli, were not significantly different. This confirmed the postulated cognitive strategies used for these methods in their respective Thurstonian/signal detection models. Introduction of perceptual variance as a result of the effects of sequences of tasting within a test, forgetting stimulus perceptions and τcriterion variation resulted in the 2‐AFC eliciting a significantly higher d′ than the other three methods. Yet, after a warm‐up procedure, which not only significantly increased values of d′ for all methods but also aligned subjects' τcriteria, the same‐different test had a d′ comparable to that of the 2‐AFC, while both d′ values were significantly higher than those of the triangle and duo‐trio. This suggested that effects of memory were more important those of sequence of tasting.  相似文献   
133.
Tropical forests have a high diversity of plant species; are they associated with a correspondingly rich microbial flora? We addressed this question by examining the symbiotic rhizobium bacteria that nodulate a diverse pool of forest legume species in Brazil. The 44 strains studied had been isolated from 29 legume tree species representing 13 tribes including all three subfamilies of the Leguminosae, and were chosen to represent major groups from a larger sample that had previously been characterized by SDS–PAGE of total proteins. Partial 16S rRNA gene sequence was determined, corresponding to positions 44–303 in the Escherichia coli sequence. Fifteen sequences were found, including six novel ones. However, all but one of them could be assigned to a genus because they grouped closely with sequences from previously described rhizobial species. Fast-growing strains had sequences similar to Rhizobium spp., Sinorhizobium spp. or Mesorhizobium spp., while the slow-growing strains had sequences similar to Bradyrhizobium spp. One strain with an intermediate growth rate had a unique sequence which indicated that the strain might belong to the genus Azorhizobium. Although the strains showed a variety of sequences, it was surprising that these strains isolated from taxonomically very diverse host plants in previously unexplored environments were mostly very similar to strains described previously, largely from agricultural systems.  相似文献   
134.
1. We examined whether long-term exposure to chemical cues of predatory longear sunfish (Lepomis megalottis) affected growth and rates of leaf processing by the isopod, Lirceus fontinalis, an important facultative shredder in low-order streams in the eastern United States, and whether isopods habituated to sunfish chemicals. 2. Long-term (24 days) exposure to fish chemicals did not affect growth or the extent of leaf processing by isopods in the laboratory. Thus, chemical cues alone may not be important in triggering long-term predator avoidance behaviours that could potentially reduce fitness or affect community processes like Litter breakdown. 3. Isopods exposed to fish chemicals for 3, 9 and 15 days were significantly more active than individuals unexposed to fish chemicals, when transferred to another environment with fish chemicals. This result, coupled with results of the growth experiment, suggests that isopods habituate to fish chemical stimuli and that time to habituation is c. 3 days or less. Because fish chemicals can mislead prey about predator presence, and hence be a non-threatening stimulus, they are probably important in eliciting only short-term antipredatory behaviours by isopods.  相似文献   
135.
The dissipation of excess excitation energy in British plant species   总被引:33,自引:1,他引:32  
The reversible dissipation of excitation energy in higher plants is believed to protect against light-induced damage to the photosynthetic apparatus. This dissipation is measured as the non-photochemical quenching of chlorophyll fluorescence. A method is described whereby the saturated capacity for rapidly reversible non-photochemical quenching can be compared between plant species. This method was applied to 22 common British plant species whose habitat was quantified using an index that describes shade tolerance. An association was found between occurrence in open habitats and a high capacity for non-photochemical quenching. It was found that, whilst this capacity was species dependent, it did not depend upon the conditions under which the plant was grown. The possible role of zeaxanthin as a determinant of quenching capacity was examined by measuring the contents of xanthophyll cycle carotenoids for each species. Comparing species, no correlation was seen between the saturated level of non-photochemical quenching and zeaxanthin content expressed relative to either total carotenoid or to chlorophyll. When zeaxanthin was expressed relative to the amount of xanthophyll cycle intermediates (zeaxanthin, antheraxanthin and violaxanthin), a weak correlation was seen.  相似文献   
136.
ABSTRACT. The application of an immunocytochemical method to identify precystic stages and to analyse the encystment kinetics, by using a polyclonal antiserum against isolated cyst walls from the ciliate Colpoda inflata , is reported for the first time. Three different precystic phases were chosen on the basis of morphological changes and degree of cyst wall formation. By using this procedure a better identification of mature resting cysts with regard to precystic cells or young cysts is provided. An average consensus encystment kinetics of C. inflata , by using an accumulated class frequency analysis, is reported.  相似文献   
137.
杨平澜 《昆虫学报》1979,(1):108-114
从1974年起连续两年参加了全国松干蚧防治研究协作会议,看到了各地不少标本,使我们对于我国松干蚧的种类和分布有了大体上的了解。但是,在我国还有一些空白地区,在有的种类的生物学知识还很缺乏。为此,我们写了《中国的松干蚧》报告(杨平澜等1976),希望通过各地进一步调查研究,使这些方面的知识日臻完善。 昆虫学报21卷2期登载了《关于松干蚧的讨论》,其中提出的主要问题是关于日本松干蚧的鉴定问题。对于这个问题在我们过去的报告里已有足够的讨论。本文将着重论述日本松干蚧是鉴定,以澄清对于日本松干蚧还存在的一些错误认识。  相似文献   
138.
Garlic yellow streak virus, a potyvirus infecting garlic in New Zealand   总被引:1,自引:0,他引:1  
In New Zealand, all garlic (Allium sativum) plants tested were infected by a virus with flexuous filamentous particles 700–800 nm long. This virus, called garlic yellow streak virus (GYSV), infected only two of 12 species tested and was transmitted to garlic by the aphid Myzus persicae in a non-persistent manner. In garlic sap, GYSV was infective at a dilution of 10-4 but not 10-3, after heating for 10 min at 60°C but not 65°C, and after 2 days but not 3 days at 25°C. The yield of virus, purified from naturally infected garlic, was 3–4 mg/kg fresh leaf. Preparations had A260/A280= 1.28 and Aman/Amin= 1.08. The virus particles had a sedimentation coefficient of 149S and a buoyant density in CsCl of 1.334 g/cm3. Mol. wt estimates for the virus nucleic acid were 2.95 × 106 by electrophoresis in polyacrylamide gels and 3.46 × 106 from the sedimentation coefficient (41.4S) in linear-log sucrose density gradients. Two polypeptides were detected in virus preparations; one (mol. wt 30 500) was possibly a breakdown product of the other (mol. wt 33 000). GYSV was serologically distantly related to onion yellow dwarf and leek yellow stripe viruses but was considered to be a separate virus because it differed from them in host range.  相似文献   
139.
中国的松干蚧   总被引:4,自引:1,他引:3  
松干蚧是在松树枝干上为害的珠蚧科(Margarodidae)、松蚧属(Matsucoccus)的蚧虫。关于我国松干蚧的种类和经济意义,在解放前一无所知。解放后,首先在山东崂山地区发现松干蚧为害严重,才开始进行防治研究。文化大革命以后,群众性的科学实验运动蓬勃发展,对于松干蚧的防治研究通过各地协作,进展很快。我们得到全国松干蚧防治研究协作组的帮助,研究了各地的标本,使我们对于我国松干蚧的种类有了一些了解。现将  相似文献   
140.
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