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   391篇
  免费   7篇
  2015年   3篇
  2014年   6篇
  2013年   10篇
  2012年   10篇
  2011年   19篇
  2010年   21篇
  2009年   26篇
  2008年   20篇
  2007年   20篇
  2006年   17篇
  2005年   13篇
  2004年   10篇
  2003年   7篇
  2002年   3篇
  2001年   5篇
  2000年   5篇
  1999年   4篇
  1998年   7篇
  1997年   12篇
  1996年   8篇
  1995年   6篇
  1994年   9篇
  1993年   10篇
  1992年   10篇
  1991年   6篇
  1989年   6篇
  1988年   5篇
  1987年   7篇
  1986年   4篇
  1985年   7篇
  1984年   8篇
  1983年   7篇
  1982年   4篇
  1981年   2篇
  1980年   8篇
  1979年   5篇
  1978年   2篇
  1977年   6篇
  1976年   7篇
  1975年   3篇
  1974年   2篇
  1972年   10篇
  1971年   3篇
  1970年   4篇
  1969年   3篇
  1968年   3篇
  1959年   4篇
  1956年   3篇
  1953年   2篇
  1951年   3篇
排序方式: 共有398条查询结果,搜索用时 15 毫秒
41.
42.
The nature of plant behaviour is discussed, and it is concluded that it is best described as what plants do. The possibility that plant behaviour is simply signal‐induced phenotypic plasticity is outlined, and some limitations of this assumption are considered. Natural environments present many challenges to growing plants, and the consequent signalling that plants perceive is becoming extremely complex. Plant behaviour is active, purposeful and intentional, and examples are discussed. Much plant behaviour, concerned with stress and herbivory, is also based on an assessment of the future likelihood of further damaging episodes and is therefore predictive. Plant behaviour involves the acquisition and processing of information. Informational terminology provides a suitable way of incorporating the concepts of learning, memory and intelligence into plant behaviour, capabilities that plants are rarely credited with. Finally, trade‐offs, cost–benefit assessments and decision making are common plant behavioural attributes. It is suggested that intelligent assessments that involve the whole plant are essential to optimize these adaptive capabilities.  相似文献   
43.
44.
45.
Abstract The developmental process leading to the formation of the abscisic acid (ABA) induced turion of Spirodela polyrrhiza was accompanied by a repression of nucleic acid and protein synthesis. DNA synthesis in the developing lurion (induced by 10−4mol m−3 ABA) was inhibited within 3h of ABA addition, followed by a repression of protein synthesis after 24 h, while RNA synthesis was not inhibited until 3 d. The inhibitory effect of ABA on protein synthesis was found to be selective and the synthesis of several novel proteins appeared to be induced. These effects were specific to ABA-sensitive tissue. The relationship between the changes in the protein and mRNA profiles during the development of the turion was investigated. The rapid general inhibition of protein synthesis at early stages of lurion formation could not be accounted for by the level of translatable mRNA, indicating an effect of ABA at the translational level. The specific alteration to the pattern of in vivo labelled proteins could have resulted, however, from control of the level of specific mRNAs for those particular proteins. Only after 3 d in ABA, when the developing primordium is committed to the turion developmental pathway, is there a total inhibition in the production of mRNA leading to the shutdown of all primary processes and the onset of the irreversible events leading to the dormant state.  相似文献   
46.
SYNOPSIS. Thymidylate synthetase (E.C.2.1.1.45) has been demonstrated in unsporulated oocysts of Eimeria tenella. The properties of this enzyme have also been investigated in Tetrahymena pyriformis, as a protozoan model, and 7-day-old chick embryo, as a host model. The enzymes from E. tenella and chick embryo were inhibited by all concentrations of MnCl2 and MgCl2 tested. Tetrahymena pyriformis thymidylate synthetase was stimulated by low concentrations of both these cations but was inhibited by high concentrations. Subsequent data refer to chick embryo, E. tenella and T. pyriformis respectively: the apparent Km was 5.89 μM, 5.94 μM, and 0.53 M for the substrate dUMP: and 5.13 μM, 1.10 μM and 4.65 μM, respectively for the cofactor N5N10-methylenetetrahydrofolate. The pH optimum for the enzyme from both chick embryo and T. pyriformis was 8.0, with Tris-HCl buffer; activity of E. tenella thymidylate synthetase was still increasing at pH 8.2. The E. tenella enzyme was found to have a molecular weight of 4.6–4.9 × 105 daltons. The effects of nucleotides, inhibitors, and the omission of assay components on each enzyme are presented. Thymidylate synthetase from E. tenella is not greatly different from that of chick embryo, but does not resemble the enzyme from T. pyriformis. A case for using thymidylate synthetase as a chemotherapeutic target in the treatment of Eimeria infections remains. Indeed Eimeria may be considered as a model for infections caused by other protozoan parasites, such as Toxoplasma and Plasmodium, provided that suitable inhibitors can be found that are not toxic to the host.  相似文献   
47.
Abstract To prioritize weed management activities it is necessary to predict the extent of future infestations of individual weed species. Management effort should focus on populations which are likely to spread and produce new satellite populations. Deciding which aspects of an invading population to manage, new emerging distant satellites or the existing core from which most seed originates has been the subject of much debate. We use extensive survey data to define the core and satellite populations and dispersal function of Parkinsonia aculeata in the semi‐arid rangelands of the Northern Territory of Australia. We described populations in terms of three size classes, classified on the basis of plant height (small (<0.4 m), medium (>0.4 and <2.5 m) and large (>2.5 m)). Data were recorded as a spatial point pattern and a dispersal function was derived from the distribution of the distance of each medium plant to the nearest P. aculeata neighbour that was greater than 2.5 m. All large and medium P. aculeata were classified as members of core or satellite populations. From these classifications, we tested critical assumptions of Moody and Mack's model, namely (i) the satellites form multiple disjunct foci that do not overlap; (ii) the system is homogenous and there are no abiotic or biotic restrictions to the weeds expansion enabling it to expand radially in any direction; (iii) the densities of all foci are uniform; (iv) there is no extinction through stochastic processes; and (v) there are no threshold effects. When the medium size class was considered, the population density and area occupied by the weed increased. Two satellite populations were comprised solely of medium plants, suggesting the number of satellites increased marginally from eight to 10 when data from both size classes were considered. Most individuals were part of the core population. The core population increased in density and we argue this core population is most likely to give rise to new satellites and should be managed in preference to emerging satellite populations. We suggest the invasion is behaving like a source‐sink system, where the core population is the source of most new medium plants.  相似文献   
48.
Aphidophagous and coccidophagous ladybirds, similar to their prey, show marked differences in their pace of life (Dixon, 2000), in particular in their rate of development, with all stages of aphidophagous species developing much faster than those of coccidophagous species. Two hypotheses are proposed to account for the large difference in the pace of life of these two groups. These are that differences in the rate of development are a result of differences in lower temperature thresholds for development or the quality of their respective prey as food (Dixon et al., 2011). Analysis of published results on the rates of development of the eggs of ladybirds indicates that the inverse relationships between the number of day‐degrees required for development (K) and the lower temperature threshold for development (tdmin) of these two groups are significantly different. In particular, the respective tdmin overlap and K of the aphidophagous and coccidophagous species with a similar tdmin are, on average, 38 and 117 day‐degrees (Do). The relationship between the rate of development (R) and temperature (T) for aphids reared on poor‐ or high‐quality foods indicates that, although the value of tdmin of a species depends on food quality, K does not, showing that it is unlikely that K is governed by food quality. Thus, there is little support for differences in either the tdmin or food quality governing the difference in the pace of life of these two groups of ladybirds. The results indicate that the physiological mechanism that may govern the difference in the pace of life between these two groups is the number of day‐degrees (K) needed to complete their development. The possible evolutionary reason for this is discussed.  相似文献   
49.
Bactrocera dorsalis (Hendel) and B. papayae Drew & Hancock represent a closely related sibling species pair for which the biological species limits are unclear; i.e. it is uncertain if they are truely two biological species, or one biological species which has been incorrectly split taxonomically. The geographical ranges of the two taxa are thought to abut or overlap on or around the Isthmus of Kra, a recognised biogeographic barrier located on the narrowest portion of the Thai Peninsula. We collected fresh material of B. dorsalis s.l. (i.e. B. dorsalis s.s.+ B. papayae) in a north–south transect down the Thai Peninsula, from areas regarded as being exclusively B. dorsalis s.s., across the Kra Isthmus, and into regions regarded as exclusively B. papayae. We carried out microsatellite analyses and took measurements of male genitalia and wing shape, both used previously to separate the taxa. No significant population structuring was found in the microsatellite analysis, consistent with one, predominantly panmictic population. Both morphological datasets showed consistent, clinal variation along the transect, without disjunction. No evidence supported historical vicariance driven by the Isthmus of Kra, and no dataset supported the current taxonomy of two species. Rather, within and across the area of range overlap or abutment between the two species, only continuous morphological and genetic variation was recorded. Recognition that morphological traits previously used to separate these taxa are continuous, and that there is no genetic evidence for population segregation in the region of suspected species overlap, is consistent with a growing body of literature that reports no evidence of biological differentiation between these taxa.  相似文献   
50.
Comparison of the behaviour of slugs before and during trailfollowing shows that there is no change in their rates of locomotionbut that there are decreases in measures of tentacle activity(distance moved and the frequency of substrate contacts) duringtrail following. Bilateral amputation of tentacles has the generaleffect of slowing the speed of locomotion whether trail followingor not, and preventing the decrease in tentacle activity duringtrail following. Amputation of the anterior tentacles results in a decrease inthe accuracy with which trails are followed. Amputation of theposterior tentacles on the other hand, results in a decreasein the frequency with which slugs turned onto trails. Removalof all the tentacles prevents trail following. It is concludedthat the posterior tentacles are concerned with the detectionand identification of trails which are subsequently followedand that the anterior tentacles normally control the behaviourof the slug on the trail. (Received 22 May 1984;  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