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   968篇
  免费   49篇
  国内免费   155篇
  2023年   17篇
  2022年   23篇
  2021年   14篇
  2020年   23篇
  2019年   18篇
  2018年   26篇
  2017年   21篇
  2016年   29篇
  2015年   36篇
  2014年   46篇
  2013年   64篇
  2012年   41篇
  2011年   50篇
  2010年   44篇
  2009年   61篇
  2008年   48篇
  2007年   55篇
  2006年   47篇
  2005年   44篇
  2004年   41篇
  2003年   36篇
  2002年   24篇
  2001年   25篇
  2000年   25篇
  1999年   23篇
  1998年   25篇
  1997年   22篇
  1996年   28篇
  1995年   20篇
  1994年   19篇
  1993年   24篇
  1992年   22篇
  1991年   15篇
  1990年   22篇
  1989年   12篇
  1988年   10篇
  1987年   15篇
  1986年   8篇
  1985年   16篇
  1984年   5篇
  1983年   2篇
  1982年   4篇
  1981年   4篇
  1980年   4篇
  1979年   3篇
  1977年   3篇
  1976年   2篇
  1975年   1篇
  1974年   3篇
  1953年   1篇
排序方式: 共有1172条查询结果,搜索用时 919 毫秒
41.
作者在内蒙科尔沁草原实验观察了绵羊体内62日龄及三年龄的土耳其斯坦东毕吸虫雌雄虫。三年龄虫体变小,大多数雄虫的睾丸内质呈不同程度空泡伏,部分雌虫的卵巢和卵黄腺萎缩或消失,体形变异。实验羊粪便的虫卵孵化率逐年下降,粪便散入水中后连续4—5天有毛蚴孵出,毛蚴数最多的是在次日上午,随后每天上午有一递减的小高峰,下午极少、夜间消失。  相似文献   
42.
The 13C NMR data of 51 iridoid glucosides or glucoside acetates are tabulated. The collection includes 20 pairs of C-6, C-7 or C-8 epimers. Three parameters in using the data for the configurational assignment of 6-O-substituents are given. The chemical shift for C-9 in a range of substituted compounds is shown to be numerically related to the stereochemistry at C-8. This allows the determination of the configuration at this centre for most types of substitution patterns by calculation of the C-9 shift using increments for each substituent. Such increments are given for 25 substituents in three different solvents. A method for simulation of spectra of unknown iridoid glucosides is presented. By this method, the structures of five novel iridoid glucosides have been elucidated, and that of tecomoside has been revised. The methods used to assign the configurations to C-6 and C-8 epimeric iridoid glucosides by 1H NMR spectroscopy are discussed and a table with selected data is presented. It is suggested that the structures in the literature for ajugol and myoporoside should be interchanged. Consequently, Horeau's method has failed in these instances. Finally, the differences in the 13C NMR spectra of pairs of C-6 and C-8 epimeric iridoid glucosides have been interpreted as originating from cis/trans-interactions.  相似文献   
43.
Nicolaia elatior is used as an example to demonstrate that the mucronate tepals ofZingiberaceae correspond to hypsophylls (bracts) consisting of a leaf sheath and a rudimentary Oberblatt (= leaf petiole + lamina) represented by the mucro. Evidence for this interpretation is furnished by all available criteria: leaf sequence (exhibiting a complete continuum of forms from foliage leaves over cata- and hypsophylls to the tepals), nervature, and ontogeny.The present conception is compared with the well-founded thesis ofLeinfellner that the perigone ofLiliaceae is derived from the androecium. The different morphological status of the perigone in both families is not regarded as the result of different phylogenetic origin, but as a manifestation of morphogenetic transgressions from one phyllome category to an adjacent one: In theLiliaceae the perigone is under a strong morphogenetic influence of the androecium, and therefore displays staminal characters, in theZingiberaceae it is under the dominating influence of the extrafloral region, and thus appears as a hypsophyllous structure. If this assumption of a morphologically oscillating perigone is correct, it will be fundamentally impossible to demonstrate unequivocally the phylogenetic origin of the monocotyledonous perigone.
Im wissenschaftlichen Werk Prof. Dr.Walter Leinfellners steht an erster Stelle die Morphologie der Blütenorgane. Als sein dankbarer Schüler möchte ich ihm aus Anlaß seines 70. Geburtstages die folgende Studie zu einem Thema zueignen, das ihn wie mich gleichermaßen angesprochen hat und schon Gegenstand der Forschungsarbeit des Jubilars war: die Homologie des Monokotylen-Perigons.  相似文献   
44.
Against the background of cercarial fine structure, ultrastructural changes were compared in schistosomules of Schistosoma mansoni 30 min and 1 hr after their production in vivo by skin penetration and in vitro by shear pressure. The same developmental pattern was observed in schistosomules of both derivations. In vitro schistosomules, however, developed more slowly, resembled cercariae more closely, and varied less among organisms than did in vivo schistosomules. The greatest morphological changes were observed in the 1-hr in vivo schistosomules. These were as follows: (1) in tegument, formation of transient microvilli, a hepatalaminate outer membrane and accented surface invaginations, loss of glycocalyx, movement outward of cyton vesicles via bridges, accumulation of multilaminate bodies around bridge openings; (2) in the anterior organ (oral sucker), movement of head gland vesicles via the ducts into tegument followed by collapse of the gland fundus, disappearance of the circumfundal cells and two large support cells, and the appearance in these areas of membranes and parenchymal cells; (3) secretion of the acetabular gland contents, collapse of the glands and replacement by membranes and parenchymal cells; (4) peristaltic activity of the digestive tract as shown by alternate areas of lumen constriction and dilation; (5) loss of bladder and contraction of the small aboral collecting tubules; and (6) conversion of heterochromatic parenchymal cell nuclei to euchromatic. In contrast, the 1-hr in vitro shear schistosomules resembled 30-min in vivo schistosomules, retaining many cercarial features.  相似文献   
45.
Three species of the genusPicris L. are native in Tropical Africa:P. abyssinica Sch. Bip. (Ethiopia),P. xylopoda Lack, spec. nova (Nigeria, Ethiopia) andP. humilis DC. (Senegal, Mali). There are indications that the two perennial species,P. abyssinica andP. xylopoda, are related to and have evolved from a primitive Central Asiatic stock in a manner parallel to many African species ofCrepis L.P. humilis, on the other hand, is a small annual plant with a high number of derived characters. The introduced species of European origin growing south of the Sahara are briefly mentioned.  相似文献   
46.
中国是全球生物多样性最丰富的国家之一。最近40年, 中国的植物多样性保护取得了巨大成就, 实施了多项政策和法律, 尤其是《野生植物保护条例》和《国家重点保护野生植物名录》先后颁布, 奠定了中国植物保护的法律和政策框架, 就地保护和迁地保护网络基本形成。但与生态文明建设的要求相比, 野生植物保护依然存在许多不足。本文系统回顾了中国野生植物保护管理的政策和法律制度, 从就地保护、迁地保护、开发利用活动管理三方面分析了其优缺点并提出建议; 重点对修订《野生植物保护条例》进行讨论并提出建议, 包括修订野生植物和人工培植的定义、优化对开发利用活动的管理程序、加强国际法和国内法的衔接、细化优化罚则等。  相似文献   
47.
以‘窄叶藤本月季花’(Rosa chinensis‘Zhaiye Tengben Yuejihua’)ב月月粉’(R.chinensis‘Old Blush’)杂交群体为材料,分析其花瓣数量的分离特点,对单瓣花与重瓣花的花芽分化过程进行观察,并对花瓣、雄蕊及瓣化雄蕊进行表皮细胞超微结构的观察。结果显示:杂交群体的花瓣数量分离明显,出现从5~54片的连续变异;花瓣数量、瓣化雄蕊数量、雌蕊数量的遗传模型为2MG-AD(2对加性-显性-上位性主基因控制),雄蕊数量的遗传模型为0MG(无主基因控制);月季重瓣花形成的原因为雄蕊瓣化,重瓣花形成的关键时期为雄蕊原基形成后期,可见到雄蕊瓣化为花瓣的现象;月季瓣化雄蕊的表皮细胞形态、褶皱程度介于单瓣花花瓣和重瓣花外轮花瓣之间。  相似文献   
48.
Tibial bone defect is a critical problem for revision knee arthroplasty. Instead of using metallic spacer or cement, biodegradable scaffolds could be an alternative solution. A numerical model of a revision knee arthroplasty was thus developed to estimate the mechanical resistance of the scaffold in this demanding situation. The tibia, scaffold, and prosthesis were represented by simplified parameterised geometries. The maximal gait cycle force was applied asymmetrically to simulate a critical loading. Several parameters were analysed: 1) inter-individual variability, 2) cortical bone stiffness, 3) cortical bone thickness, 4) prosthesis fixation quality, and 5) scaffold thickness. The calculated scaffold strain was compared to its experimental ultimate strain. Among the tested parameters, failure was only predicted with scaffold thickness below 5 mm. This study suggests that biodegradable bone scaffolds could be used to fill bone defects in revision knee arthroplasty, but scaffold size seems to be the limiting factor.  相似文献   
49.
Abstract

The research conducted here presents a syntaxonomical revision of the montane mesophilous meadows of the Polygono bistortae-Trisetion flavescentis alliance in central Europe – the Slovak part of the western and eastern Carpathians. These typical semi-natural grasslands occur mainly as small islands over the calcareous bedrocks. Associations of this alliance have tight relationships to the Arrhenatherion, Mesobromion and Nardo-Agrostion alliances. Formal definitions based on combination of the species groups were used regarding the diversity and geographical range of the Polygono bistortae-Trisetion flavescentis alliance. Following the formal definitions from the eight associations reported previously for Slovakia, only four of them can be placed within the Polygono bistortae-Trisetion flavescentis: Campanulo glomeratae-Geranietum sylvatici, Geranio sylvatici-Trisetetum, Crepido mollis-Agrostietum capillaries and Geranio-Alchemilletum crinitae. A comparison of traditional and formalized classification, and the advantages/disadvantages of the formalized classification are discussed.  相似文献   
50.
The genus Lasinus Sharp, 1874 of the Pselaphodes complex of genera (Pselaphitae: Tyrini: Tyrina) is revised. The three so far known species, Lasinus mandarinus Raffray, 1890, Lasinus monticola Sawada, 1961 and Lasinus spinosus Sharp, 1874 are redescribed. Eight new species, Lasinus sinicus sp. n. from China, Lasinus mikado sp. n., Lasinus yamamotoi sp. n., Lasinus inexpectatus sp. n., Lasinus yakushimanus sp. n., Lasinus amamianus sp. n., Lasinus saoriae sp. n., and Lasinus okinawanus sp. n. from Japan, are described. And all species are illustrated. Lectotypes are designated for Lasinus mandarinus and Lasinus spinosus. An identification key to species of the genus Lasinus is provided.  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