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   914篇
  免费   60篇
  国内免费   146篇
  2024年   1篇
  2023年   10篇
  2022年   12篇
  2021年   18篇
  2020年   11篇
  2019年   9篇
  2018年   24篇
  2017年   34篇
  2016年   43篇
  2015年   26篇
  2014年   39篇
  2013年   53篇
  2012年   45篇
  2011年   40篇
  2010年   43篇
  2009年   45篇
  2008年   48篇
  2007年   41篇
  2006年   45篇
  2005年   57篇
  2004年   38篇
  2003年   47篇
  2002年   31篇
  2001年   29篇
  2000年   29篇
  1999年   39篇
  1998年   20篇
  1997年   19篇
  1996年   23篇
  1995年   22篇
  1994年   29篇
  1993年   14篇
  1992年   12篇
  1991年   18篇
  1990年   7篇
  1989年   10篇
  1988年   14篇
  1987年   8篇
  1986年   9篇
  1985年   15篇
  1984年   8篇
  1983年   3篇
  1982年   3篇
  1981年   5篇
  1980年   5篇
  1979年   7篇
  1978年   8篇
  1977年   2篇
  1974年   2篇
排序方式: 共有1120条查询结果,搜索用时 15 毫秒
1.
BATEMAN, R. M. & FARRINGTON, O. S., 1989. Morphometric comparison of populations of Orchis sitnia Lam. (Orchidaceae) from Oxfordshire and Kent. Univariate and multivariate morphometric analyses of Orchis simia Lam. from its two remaining naturally-founded populations in Britain (Goring, Oxfordshire and Faversham, Kent) reveal only minor morphological differences between the populations and suprisingly low levels of intra-population variation. This probably reflects recent founding from Continental seed at Faversham and penecontemporaneous, human-induced, catastrophic contraction of the gene pool at Goring. The apparently restricted gene pools of these populations may adversely effect their long-term survival.  相似文献   
2.
椭圆斜羽叶的解剖研究及斜羽叶的系统分类位置   总被引:2,自引:0,他引:2  
本文根据对椭圆斜羽叶(Plagiozamites oblongifolius)钙质石化化石的研究,认为其主要特征如下:羽状复叶,叶肉无海绵组织和栅栏组织分化。羽片叶脉维管束外韧式,木质部外始式,与现代苏铁属叶脉特征相近。羽轴维管束为“U”形,外始式木质部、梯状纹孔管胞,与现代苏铁属的羽轴维管束相似。根据上述特征,目前可将斜羽叶属归入原始苏铁类植物。但是,如果斜羽叶的生殖器官同 Noeggerathia 一样,为异孢型孢子囊穗,则斜羽叶可能为一种与苏铁类起源有关的原裸子植物。  相似文献   
3.
The -galactosidase (EC 3.2.1.32) of Corynebacterium murisepticum (inducible by lactose and galactose) was purified by successive column chromatography on Sephadex G-200, DEAE-Sephadex A-50 and DEAE-cellulose (DE52). The enzyme was found to be a dimer of identical subunits of molecular mass 100,000 daltons. The K m values of the enzyme for the substrates lactose and o-nitrophenyl--d-galactopyranoside (ONPG) are 16.7 mM and 4.4 mM, respectively, indicating, its low affinity for the substrates. The Ouchterlony immunodiffusion method exhibited immunological homogeneity of the enzyme preparation. The catalytic site of the enzyme does not take part in antigen-antibody reaction.  相似文献   
4.
Larvae of all three southern hemisphere anadromous parasitic lampreys were collected from rivers in Australia, New Zealand and South America. Body intervals were measured, trunk myomeres counted and the frequency of pigmentation in different body regions recorded. Morphometric data were subjected to multiple group principal components analysis (MGPCA) which took into account changes during growth. The components (together with myomere counts) and the pigmentation data were both subjected to discriminant analysis. Ordination and rank correlation tests revealed no evidence for either latitudinal clines or a continuum of circumpolar change amongst larval lamprey populations. Clustering of population centroids clearly distinguished between Mordacia lapicida (Gray) from Chile and M. mordax (Richardson) from south-eastern Australia. Populations of Geotria australis Gray divided into groups representing three geographical regions, namely Argentina, Chile and Australasia (Western Australia, Tasmania and New Zealand). Ammocoetes from Argentina were the most divergent, possessing a more posterior cloaca, taller dorsal fins, a greater gap between dorsal fins, and distinctive pigmentation on the head and caudal fin. Within the Australasian group, Western Australian and New Zealand populations clustered closer than either did with those from Tasmania. The cluster analyses for larval populations of G. australis suggest that, during their marine trophic phase, the adults of this species originating from Argentinian and Chilean rivers follow different migratory routes, whereas those from Western Australia, New Zealand and, to a lesser extent, Tasmania intermix.  相似文献   
5.
The dental and cranial morphologies of all species ofSaguinus, S. oedipus, S. geoffroyi, S. leucopus, S. nigricollis, S. fuscicollis, S. labiatus, S. mystax, S. imperator, S. bicolor, andS. midas are examined. The following hypotheses are developed by cladistic methodology, using only synapomorphic characters to assess the interspecific relationships ofSaguinus.Saguinus are divided into two main groups; one consists ofS. oedipus, S. geoffroyi, andS. leucopus, and the other includesS. inustus, S. nigricollis, S. fuscicollis, S. labiatus, S. mystax, S. imperator, S. bicolor, andS. midas. In the former group,S. oedipus is more closely related toS. geoffroyi than either is toS. leucopus. In the latter group,S. labiatus, S. mystax, andS. imperator are classified into one group, andS. bicolor andS. midas form one monophyletic group.  相似文献   
6.
On the systematic position of the family Gyrinidae (Coleoptera: Adephaga)   总被引:2,自引:0,他引:2  
Various characters of adult and larval members of Adephaga and Cupedidae were analyzed, and suggest that Gyrinidae are the sister-group of the remaining Adephaga, and are not closely related to the remaining aquatic Adephaga. The aquatic families Noteridae, Amphizoidae, Hygrobiidae and Dytiscidae seem to form a well founded monophyletic unit. The following characters are considered as synapomorphies of Adephaga excluding Gyrinidae: bifurcate condition of the muscle (= M.) tentoriopraementalis inferior, reduction of hypopharynx, strongly developed prosternal process, reduction in size and specialized modification of the ventral sclerite of the mesothorax, strongly developed mesofurcal arms, a high mesopleural ridge, globular mesocoxae restricted to rotatory movements, invaginated sternum VIII (coxostemum), the strongly curved base of the median lobe of the aedeagus, which articulates with the parameres, the rotated position of the aedeagus in repose, fusion of the larval clypeolabrum with the frons and reduction of the larval lacinia. Mesal shifting of M. episterno-coxalis prothoracis, and the fusion of the apical portions of the malpighian tubules of either side are considered as synapomorphies of Adephaga excluding Rhysodidae and Gyrinidae. Lateral reduction of the meta “sternal” transverse ridge and the presence of the subcubital setal binding patch of the hind wing are considered as synapomorphic characters of Trachypachidae, Noteridae, Amphizoidae, Hygrobiidae and Dytiscidae. We postulate that the metacoxal fusion occurred independently in gyrmids and the common ancestor of Trachypachidae, Noteridae, Amphizoidae, Hygrobiidae and Dytiscidae. Consequently we consider this character state as another synapomorphy of Trachypachidae and Hydradephaga excluding Haliplidae and Gyrinidae. The following characters are considered as synapomorphies of Noteridae, Amphizoidae, Hygrobiidae and Dytiscidae: Loss of tactile setae on the head capsule, metafurcal origin on the intercoxal wall, expansion of the intercoxal wall, elongation of the subcubital setal binding patch, loss of Mm. furca-coxale anterior and posterior, reduction of the larval abdominal segments IX and X, and the shifting of the uropmphi onto the ventral side of segment VIII. Presence of M. tentorio-mandibularis and M. stipitopalpalis intemus are certainly primitive features of adult gyrinids but the distribution of these character states among most members of Adephaga is yet unclear. Chemical defence gland constituents point towards a very isolated position of Gyrinidae. The old age of the group, documented by a larva found in upper Permian deposits, may support the hypothesis of a sister-group relation-ship between Gyrinidae and the remainder of Adephaga.  相似文献   
7.
8.
Besides Clostridium thermoaceticum and C. formicoaceticum other resting acetogenic clostridia such as C. aceticum and C. thermoautotrophicum and to a lesser extent non-clostridial acetogens such as Butyribacterium methylotrophicum and Eubacterium limosum were able to reduce propionate to propanol at the expense of carbon monoxide or formate. Methylviologen usually increased the reduction rate. Ten M molybdate in the growth medium decreased this capability for C. thermoaceticum but increased it or had no effect for the other organisms. Ten M tungstate in the growth medium increased the aldehyde oxidoreductase activity in all organisms. Crude extracts of C. thermoaceticum cells grown in the presence of 10 M or 1 mM molybdate showed by ELISA the same or even a 4 fold concentration of aldehyde oxidoreductase in the latter case. However, the enzymic activity was very low in both cases. Omission of dithionite in the growth medium diminished the antigen by a factor of about 8. The immunological distance between the enzyme from C. thermoaceticum and C. thermoautotrophicum was rather low but very large to C. formicoaceticum and undeterminably large to the other organisms.Abbreviations Ald-DH aldehyde dehydrogenase - AOR aldehyde oxidoreductase - CO-DH carbon-monoxide dehydrogenase - ELI-SA enzyme-linked immunosorbent assay - FDH formate dehydrogenase - MV methylviologen - V++ oxidised - V+. reduced viologen  相似文献   
9.
Cells of the purple non-sulphur bacterium Rhodobacter sphaeroides express a high-affinity K+ uptake system when grown in media with low K+ concentrations. Antibodies againts the catalytic KdpB protein or the whole KdpABC complex of Escherichia coli crossreact with a 70.0 kDa R. sphaeroides protein that was expressed only in cells grown in media with low K+ concentrations. In membranes derived from R. sphaeroides cells grown with low K+ concentrations (induced cells), a high ATPase activity could be detected when assayed in Tris-HCl pH 8.0 containing 1 mM MgSO4. This ATPase activity increased upon addition of 1 mM KCl from 166 to 289 mol ATP hydrolysed x min-1 x g protein-1 (1.7-fold stimulation). The K+-stimulated ATPase activity was inhibited approximately 93% by 0.5 mM vanadate but hardly by N,N-dicyclohexylcarbo-diimide (DCCD). These results indicate that the inducible K+-ATPase in R. sphaeroides resembles the Kdp K+-translocating ATPase of Escherichia coli. This Kdp-like transport system is also expressed in R. capsulatus and Rhodospirillum rubrum during growth in media with low K+ concentrations suggesting a wide distribution of this transport system among phototrophic bacteria.Abbreviations electrical potential difference across the cytoplasmic membrane - pH pH difference across the cytoplasmic membrane - BSA bovine serum albumine - PAGE polyacrylamide gel electrophoresis - HEPES 4-(2-hydroxyethyl)-1-piperazine-ethanesulfonic acid - PMSF phenyl-methyl-sulfonyl fluoride - DCCD N,N-dicyclohexylcarbodiimide - AIB 2--aminoisobutyric acid - TMG methyl--d-thiogalactopyranoside  相似文献   
10.
本文对鲴亚科Xenocyprinae共4属6种鱼和鲢亚科Hypophthal michthyinae的鲢、鳙Aristichihys nobilis进行了骨骼系统的研究,并对鲴亚科的6种鱼同工酶谱型,进行了分析比较。鲢亚科虽然具有鲴亚科的共同祖征和全部离征,但因其有较多的特化性状,仍将其列为和鲴亚科并列的一个亚科。应用分支系统学原理推导出鲴亚科的系统发育图。鲴亚科6种鱼的肝、肾、心、眼的晶状体、鳍、脑,肌肉等7种组织5种同工酶(LDH,MDH,ES,ADH,G6PDH)的电泳分析表明,生化分类和经典分类结果完全一致。  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