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   2238篇
  免费   207篇
  国内免费   56篇
  2024年   3篇
  2023年   44篇
  2022年   49篇
  2021年   69篇
  2020年   79篇
  2019年   67篇
  2018年   75篇
  2017年   64篇
  2016年   63篇
  2015年   73篇
  2014年   100篇
  2013年   123篇
  2012年   64篇
  2011年   91篇
  2010年   73篇
  2009年   101篇
  2008年   96篇
  2007年   92篇
  2006年   101篇
  2005年   76篇
  2004年   89篇
  2003年   88篇
  2002年   77篇
  2001年   72篇
  2000年   46篇
  1999年   50篇
  1998年   51篇
  1997年   43篇
  1996年   39篇
  1995年   49篇
  1994年   38篇
  1993年   26篇
  1992年   39篇
  1991年   26篇
  1990年   26篇
  1989年   17篇
  1988年   20篇
  1987年   22篇
  1986年   18篇
  1985年   16篇
  1984年   25篇
  1983年   7篇
  1982年   13篇
  1981年   22篇
  1980年   25篇
  1979年   19篇
  1978年   9篇
  1977年   16篇
  1976年   6篇
  1971年   4篇
排序方式: 共有2501条查询结果,搜索用时 281 毫秒
51.
Pericarp structure was investigated in 158 species of the familiesLamiaceae andVerbenaceae. Data from 221 out of 262 genera ofLamiaceae s.l. and a few ofVerbenaceae s.str. were collected in a table. A cladistic analysis was performed on the basis of pericarp characters only. The abandonment of subfam.Pogostemonoideae as a taxonomic unit is considered. Examples of groups given additional support by similarities in pericarp characters are: (1) the gynobasic-styled labiates (subfamiliesPogostemonoideae, Lamioideae, Nepetoideae); (2) aLamioideae-Pogostemonoideae-group; (3)Nepetoideae; (4) aWestringia-Hemigenia-Hemiandra-Microcorys group (in subfam.Chloranthoideae); (5) aLepechinia-Chaunostoma-group (inNepetoideae); (6) aPrunella-Cleonia-group (inNepetoideae).  相似文献   
52.
The root bark structure of Quercus robur L. was analysed at different stages of root development and compared to the structure of stem bark. Root bark thickness varied considerably between different roots. Sclereid quantity decreased with increasing distance from the stem, which means it increased with age. Visible growth increments diminished with increasing distance from the stem. In lateral roots crystal quantity decreased with increasing distance from the stem. In lateral roots secondary phloem fibre length, sieve tube member length, and sieve tube diameter showed no regular trend. There were only a few basic structural differences between root and stem bark. The zone of cell differentiation (cell expansion, lignification) was wider in root bark; sieve tube collapse was delayed. In lateral root bark fewer sclereids were formed. The first-formed periderm often originated from deeper cell layers. Thus, primary elements were lacking after periderm formation. In root bark the phellem cell walls were of equal thickness. Thus, phellem lacked visible growth increments. Root bark phellem cells were slightly larger. The root phelloderm was more distinct. The secondary phloem fibres were slightly shorter than those in stem bark. Sieve tube members of stem and root bark were of similar length and diameter. The qualitative bark anatomical characters of oak root bark are suitable for root identifications. Due to minor structural differences between root and stem bark the characters must be used with care.  相似文献   
53.
火棘属果实发育解剖学研究   总被引:2,自引:0,他引:2  
火棘属植物的托附杯呈深杯状,具有5个分离心皮,子房半不位。托附杯发育成肉质果壁,主要藉细胞体积增大,子房壁全部石细胞化,发育成骨质瘦果状小果的果皮。火棘属的果实不是典型和梨果,是蔷薇科果实在演化过程中的一个过渡类型。火棘属肉质果壁的厚角组织经过角隅减薄后再转变成厚壁组织。  相似文献   
54.
The effect of thyrotropin releasing hormone (TRH) alone and in combination with morphine on the gastrointestinal transit was investigated by using the charcoal meal test in mice. The intraperitoneal (IP) administration of TRH decreased the transit when given in a dose of 1.0 mg/kg 10 min prior to the meal. The intracerebroventricular (ICV) administration of TRH (10 μg/mouse) also inhibited the transit when given just prior to the charcoal meal. Subcutaneous (SC) administration of morphine (5, 10 and 20 mg/kg) inhibited gastrointestinal transit in a dose dependent manner. When TRH (1, 3 and 10 mg/kg, IP as well as 0.3 μg, ICV) which had no effect on the transit by itself was combined with morphine (10 mg/kg, SC), an enhancement in the inhibition of the transit was observed. TRH-induced inhibition of the transit was antagonized by naloxone (0.1 mg/kg, SC). It is concluded that TRH inhibits gastrointestinal transit in the mouse possibly via the opiate receptor system.  相似文献   
55.
Four collections of three species ofTrimenia and one collection ofPiptocalyx were studied; early-formed and later-formed wood was analyzed for oneTrimenia. Liquid-preserved material permitted analysis of mucilage and starch storage in wood ofT. neocaledonica andP. moorei. BecausePiptocalyx is scandent whereasTrimenia is arborescent, wood differences relative to evolution of a climbing habit could be examined.Piptocalyx contrasts withTrimenia in having wider vessels, more numerous per mm2, resulting in a conductive area five times greater per unit area than that of theTrimenia woods averaged.Piptocalyx has appreciably fewer bars per perforation plate and thus much greater conductive area per perforation plate than have the species ofTrimenia. Rays inPiptocalyx are much taller and wider than those ofTrimenia. Wood ofTrimeniaceae is highly primitive in its scalariform perforation plates, scalariform lateral wall pitting on vessels, relatively long vessels elements, and heterocellular rays. Imperforate tracheary elements are septate nucleate fibertracheids (or even libriform fibers) rather than tracheids, but loss of borders on pits (and thus lowered conductive function of the imperforate tracheary elements) can be explained by the development of these elements into starchstoring cells. Some fiber-tracheids inT. neocaledonica are enlarged mucilagecontaining cells. Details of vessel structure inTrimeniaceae are similar to those ofMonimiaceae (s. s.), but similarity to some other lauralean (annonalean) families may be found: in mucilage presence,Trimeniaceae resembleLauraceae rather thanMonimiaceae. Wood ofTrimeniaceae may be regarded as highly mesomorphic, corresponding to the moist habitats in which all of the species occur.  相似文献   
56.
Two species ofPenaeaceae (Penaea mucronata andSaltera sarcocolla), a unique South African family ofMyrtales, were investigated embryologically.Penaeaceae clearly agrees with otherMyrtales in its basic embryological characteristics, and further is characterized by its highly specialized features: ephemeral endothecium, 16-nucleatePenaea-type embryo sac, and unique ovular form. A wider range of affinities of families includingPenaeaceae, Oliniaceae, Rhynchocalycaceae, Alzateaceae, andCrypteroniaceae sensu stricto, as well as a possible common divergence from an ancestral line leading toLythraceae and/orMelastomataceae, are discussed on embryological and other grounds.  相似文献   
57.
Leaf architectural pattern has been studied in 27 genera and 35 species of the Acanthaceae. The major venation pattern conforms to pinnate camptodromous with eucamptodromous or festooned brochidodromous secondaries, pinnate craspedodromous inAcanthus ilicifolius and acrodromous inLepidagathis trinervis. Intersecondary veins are common. The marginal ultimate venation is looped. The areoles are variable in size. The vein endings are usually simple, linear or cuved or divide once or twice dichotomously. Isolated vein endings, tracheids, isolated free vein endings and extension cells are observed. Transfusion tracheids are seen inDicliptera verticillata. Miniature vessel elements with simple perforation plates are noticed lying free in the areole or at the end of tracheids inSeriocalyx scaber andThunbergia grandiflora.Elytraria acaulis, Nelsonia canescens andStaurogynae zeylanica exhibit venation pattern shown by majority of the taxa studied, of the Acanthaceae.  相似文献   
58.
The remarkable diversity of fruits inValerianella andFedia is revealed by a comparative morphological and anatomical analysis. Characters are evaluated in regard to a new systematic grouping. The knowledge on dispersal biology is summarized and supplemented. Finally, salient features of fruit differentiation are discussed.
3. Teil der Publikationsserie Beiträge zur Systematik und Evolution vonValerianella undFedia (Valerianaceae) (vgl.Ernet 1977a, b).  相似文献   
59.
Summary Pectic strands are shown to connect the lower stomatal ledges and to develop across the posterior chamber of a number of plant species. Similar strands are formed between guard cells and subsidiary cells, and between epidermal cells.  相似文献   
60.
Summary The distribution and origins of vasoactive intestinal peptide (VIP) in the gut and pancreas of the turkey were studied by radioimmunoassay of tissue extracts and by immunocytochemistry. Several antisera were used that vary in their specificity for different regions of porcine or chicken VIP. Radioimmunoassays using NH2-terminal specific antisera that react almost equally with porcine and chicken VIP's revealed significant amounts of immunoreactive VIP in extracts of pancreas, brain and all regions of the gastrointestinal tract from crop to colon. Highest concentrations (300pmol/g) were found in the colon muscle, and concentrations were generally low (< 20 pmol/g) in the mucosal layers of the small intestine. After ion exchange chromatography of extracts on CM-Sephadex three immunoreactive forms of VIP were separated corresponding to the three molecular forms previously found in mammalian gut extracts. In immunocytochemical studies nerve fibres were found throughout the gut, and in the pancreas. Immunoreactive nerve cell bodies were also identified in the submucous plexus throughout the gut, but were particularly prominent in the oesophagus and pancreas. It has previously been shown that VIP is a strong stimulant of the flow of pancreatic juice in birds whereas the structurally related hormone secretin, which is known to control the flow of pancreatic juice in mammals, is a weak stimulant. It is proposed that in birds VIP might regulate the pancreas, and other aspects of gut function, as a neurotransmitter or neurohormone.  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