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   979篇
  免费   41篇
  国内免费   87篇
  2024年   2篇
  2023年   10篇
  2022年   14篇
  2021年   9篇
  2020年   14篇
  2019年   23篇
  2018年   17篇
  2017年   21篇
  2016年   30篇
  2015年   32篇
  2014年   39篇
  2013年   42篇
  2012年   26篇
  2011年   38篇
  2010年   37篇
  2009年   45篇
  2008年   72篇
  2007年   60篇
  2006年   44篇
  2005年   59篇
  2004年   49篇
  2003年   45篇
  2002年   26篇
  2001年   22篇
  2000年   26篇
  1999年   16篇
  1998年   27篇
  1997年   20篇
  1996年   26篇
  1995年   25篇
  1994年   23篇
  1993年   18篇
  1992年   14篇
  1991年   10篇
  1990年   11篇
  1989年   13篇
  1988年   9篇
  1987年   6篇
  1986年   16篇
  1985年   18篇
  1984年   7篇
  1983年   4篇
  1982年   9篇
  1981年   7篇
  1980年   7篇
  1978年   8篇
  1977年   3篇
  1975年   2篇
  1965年   1篇
  1963年   1篇
排序方式: 共有1107条查询结果,搜索用时 218 毫秒
41.
Summary Flower and fruit characters were measured in ten female, five male and five fruiting male selections of A. deliciosa var deliciosa (A. Chev) Liang and Ferguson. Flowers from female vines had functional pistils, which contained many ovules. Stamens appeared to be fully developed but produced only empty pollen grains. Flowers from male vines had functional stamens that produced high percentages of pollen grains with stainable cytoplasmic contents. Pistils did not contain ovules and were generally small with vestigial styles. Fruiting male vines had both staminate and bisexual flowers. Staminate flowers were similar to those found on strictly male vines. Bisexual flowers produced ovules and stainable pollen. Pistils were smaller than in pistillate flowers. Although the three flower sexes differed in style length, ovary dimensions and ovules per carpel, staminate and bisexual flowers were similar in number of flowers per inflorescence, stamen filament length, pollen stainability, inflorescence rachis length and carpel number, and differed from pistillate flowers in these characters. The three flower sexes had similar sepal and petal numbers. The fruit of fruiting males were considerably smaller than those of females. Low ovule number appears to be the major factor limiting fruit size in the fruiting males studied. Prospects for developing hermaphroditic kiwifruit cultivars through breeding are discussed.  相似文献   
42.
Blondel  J.  Isenmann  P.  Maistre  M.  Perret  P. 《Plant Ecology》1992,(1):129-136
By comparison with deciduous oaks, the lower yearly production of new leaves in sclerophyllous oaks is hypothesized to have several consequences on animal communities. In particular, the production of arthropod communities that feed upon the leaves should be lower in sclerophyllous than in deciduous oaks, this causing changes in breeding patterns and the demographic balance in insectivorous birds. Studies in both deciduous and sclerophyllous habitats in southern France have shown that: 1) the spring development of new leaves occurs later and more slowly in sclerophyllous than in deciduous oaks, 2) the biomass of caterpillars is much lower in sclerophyllous oak forests, and 3) there is a large variation in life history traits of the Blue Tit depending in which type of habitat they breed. Laying date occurs later and clutch size is lower in sclerophyllous habitats than in deciduous habitats. The evolution of life history traits is discussed according to whether poor sclerophyllous habitats are isolated (e.g. on Corsica) or are parts of landscapes including both habitat types.  相似文献   
43.
The postmetamorphic growth and survival of the salamander Hynobius nebulosus tokyoentisTago were surveyed in the study site located in Habu village of Hinodemachi, a suburb of Tokyo City, during 1975–1981. A laboratory experiment on the growth rate of juveniles was conducted in parallel with the field survey. The result indicated that this salamander grew at the rate of 8,mm in s.v.l. per year during the juvenile stage, but its growth rate decreased markedly as low as 1.8 mm for males and 1.1 mm for females, once it had attained sexual maturity. According to the “capture-recapture” procedure the annual survival rate after metamorphosis was found to be quite high; that is, approximately 0.7. By using the growth rate of juveniles and the difference between the sizes at metamorphosis and sexual maturity, the age at first reproduction was estimated to be 4 year for males and 5 year for females. From the data obtained in this study, the intrinsic rates of increase (r) were calculated for various values of age at first reproduction under different survival schedules, and the relationship between the age at first reproduction and fitness as measured by r was examined. The result indicated that an optimal age maximizing fitness always existed under respective survival schedules, and the observed age at first reproduction of this salamandei was found to coincide well with the predicted optimal age.  相似文献   
44.
  • Enantiostyly is a floral polymorphism in which two floral forms in the same species differ in deflection of the stigma to right or left position. In monomorphic enantiostylous plants, flowers of the two morphs occur within the same individual, usually in the same proportion. In self‐compatible species the function of monomorphic enantiostyly is proposed to increase outcrossing rates and offer a reproductive advantage under pollination limitation. Enantiostylous species are usually self‐compatible and show heteranthery, with poricide anthers and pollen as pollinator reward; however, there are families, such as Vochysiaceae, that have different characteristics.
  • We analysed the reproductive system and pollination biology of Qualea parviflora and Q. multiflora, two enantiostylous species from the Brazilian Cerrado that have specific morphological and physiological traits. For this, we characterized flower traits, performed hand pollinations and studied floral visitors.
  • We found no differences between morphs in the proportion of flowers, nectar produced or its concentration, pollen quantity and fruit set. Both species were self‐incompatible and quite generalist regarding floral visitors.
  • Enantiostyly in self‐incompatible plants seems to confer a reproductive advantage by reducing self‐interference resulting from stigma clogging. This novel result helps to expand our knowledge on this complex floral polymorphism and opens new avenues for future research on this topic.
  相似文献   
45.
46.
The reproductive biology of six native Tillandsia species that co-occurred in woody areas of Santa Fe Province (Argentina) was studied for floral biology, breeding systems and reproductive success. Flower longevity, floral phases, hand self- and cross-pollinations or natural pollinations and fluorescence microscopy observations of post-pollination events were analysed. Also pollen/ovule (P/O) ratio and pre-emergent reproductive success (PERS) were compared between species. In T. aëranthos, T. ixioides and T. meridionalis, self-pollinated flowers mostly failed to form fruits, and fluorescence microscopy studies showed that in selfed pistils pollen tube growth was arrested in the style. It is likely that these species have homomorphic, gametophytic self-incompatibility. In contrast, T. capillaris, T. tricholepis and T. recurvata regularly formed fruits following autonomous self-pollination. Floral traits, the P/O ratios and PERS indices correlate with the breeding systems found in these two groups of species: the three selfing species have inconspicuous closed flowers, and they have low P/O ratios and high PERS values. In contrast, the three self-incompatible (SI) species have conspicuous flowers with relatively high P/O and low PERS values. We described here three new cases of SI in Tillandsia species subgenera Anoplophytum and confirm the autogamous and cleistogamous status in three species of Tillandsia subgenera Diaphoranthema.  相似文献   
47.
以灰黄青霉菌(Penicillium griseofulvum HL)为出发菌株,试验得到灰黄青霉原生质体制备的优化条件为:菌体培养48h,用0.7mol/L的NaC1溶液作为渗透压稳定剂,用0.5%的蜗牛酶+0.5%的纤维素酶,在pH为6,30℃条件下酶解3h,所得原生质体数最多,达到3.14×107/ml.原生质体再生的最佳条件为采用双层平板培养法,在用0.7mol/L的蔗糖溶液配制的改进察氏培养基上其再生率最高,达到24.93%.灰黄青霉原生质体经过紫外线诱变,DES诱变,紫外线-DES诱变复合诱变,紫外线-氯化锂复合诱变选育异抗坏血酸高产菌株,通过对再生平板上长出的诱变菌株进行初筛和摇瓶复筛,最终获得一株异抗坏血酸产量较高的菌株ZD4,其产量为5.28mg/ml,提高到出发菌株产量(1.08 mg/ml)的488.9%,且连续传代6代遗传稳定.  相似文献   
48.
海南鳽(Gorsachius magnificus)隶属于鹈形目(Pelecaniformes)鹭科(Ardeidae),为我国Ⅱ级重点保护野生动物。本文报道了2019年5月至9月海南鳽 在贵州雷公山自然保护区干脑村南柳河(26?16? N,108?06? E,海拔771 m)的繁殖情况。海南鳽巢址选择在常绿阔叶林中,营巢于高大、枝叶繁茂、枝桠较多并有很强隐蔽性的阔叶树上。结合发现海南鳽幼鸟在贵州雷公山自然保护区分布区的增加,我们认为在贵州雷公山地区应该至少有1或2个繁殖种群。本次记录是海南鳽在贵州省的首次繁殖记录。  相似文献   
49.
Road ecology, the study of the impacts of roads and their traffic on wildlife, including birds, is a rapidly growing field, with research showing effects on local avian population densities up to several kilometres from a road. However, in most studies, the effects of roads on the detectability of birds by surveyors are not accounted for. This could be a significant source of error in estimates of the impacts of roads on birds and could also affect other studies of bird populations. Using road density, traffic volume and bird count data from across Great Britain, we assess the relationships between roads and detectability of a range of bird species. Of 51 species analysed, the detectability of 36 was significantly associated with road exposure, in most cases inversely. Across the range of road exposure recorded for each species, the mean positive change in detectability was 52% and the mean negative change was 36%, with the strongest negative associations found in smaller-bodied species and those for which aural cues are more important in detection. These associations between road exposure and detectability could be caused by a reduction in surveyors’ abilities to hear birds or by changes in birds’ behaviour, making them harder or easier to detect. We suggest that future studies of the impacts of roads on populations of birds or other taxa, and other studies using survey data from road-exposed areas, should account for the potential impacts of roads on detectability.  相似文献   
50.
The techniques and biological functions of avian flight are briefly presented. The reasons for rendering zoo birds flightless are explained taking into account the tasks and obligations of modern zoos, and the various deflighting procedures are described. The legal situation regarding deflighting as it currently exists in Germany and other countries is clarified. It is discussed in detail under which circumstances it would be justifiable to render a bird flightless, to what extent keeping the birds in aviaries would be an alternative and whether reversible or irreversible methods should be preferred. A legal opinion has been sought which came to the conclusion that even under the restrictive Animal Welfare Law of Germany interventions to render a bird flightless were admissible if based on veterinary indication on a case-by-case basis. Such indication may be justified by the anticipation that a bird may be injured or die from an accident in future if not deflighted. Contrary to the views of some legal experts commenting on the German Animal Welfare Law the authors consider feather clipping not to be an intervention prohibited under the law. In the interest of the good functioning of european and international breeding programmes the authors suggest that the German legislation should be modified with a view of containing a general derogation for rendering flightless at least certain species of zoo birds.  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