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   1301篇
  免费   82篇
  国内免费   220篇
  2023年   19篇
  2022年   18篇
  2021年   22篇
  2020年   23篇
  2019年   18篇
  2018年   37篇
  2017年   42篇
  2016年   47篇
  2015年   45篇
  2014年   58篇
  2013年   74篇
  2012年   53篇
  2011年   61篇
  2010年   52篇
  2009年   82篇
  2008年   69篇
  2007年   77篇
  2006年   78篇
  2005年   73篇
  2004年   61篇
  2003年   57篇
  2002年   39篇
  2001年   38篇
  2000年   36篇
  1999年   44篇
  1998年   38篇
  1997年   30篇
  1996年   37篇
  1995年   24篇
  1994年   35篇
  1993年   29篇
  1992年   27篇
  1991年   22篇
  1990年   20篇
  1989年   16篇
  1988年   14篇
  1987年   13篇
  1986年   13篇
  1985年   19篇
  1984年   8篇
  1983年   2篇
  1982年   3篇
  1981年   5篇
  1980年   5篇
  1979年   5篇
  1978年   5篇
  1977年   2篇
  1976年   2篇
  1974年   3篇
  1953年   1篇
排序方式: 共有1603条查询结果,搜索用时 15 毫秒
1.
A morphological mutation particularly detectable in the dikaryotic state was found in Lentinula edodes. The mutant dikaryon was readily distinguishable from the normal dikaryon by the irregularly branched short hyphae, very slow hyphal growth, and sparse aerial hyphae. Genetic analysis revealed that expression of this mutation was controlled by a single recessive gene, mor-13. Linkage analysis showed that the mor-13 was not linked to either the incompatibility factors (A and B) or the five kinds of mor genes that were segregated independently of each other in a previous study. Contribution no. 380 from the Tottori Mycological Institute  相似文献   
2.
The morphology of dissociated single cerebellar Purkinje cells obtained from wild-type P1 CD1 mice was assessed in the absence and in the presence of glia. A dedicated noninvasive technique based on optical microscopy was developed. Image processing algorithms were implemented to extract metrical features characterizing cell structure and dendritic arborization. The morphological features were analyzed in order to identify quantitative differences in Purkinje cell morphology due to interactions with astrocytes.  相似文献   
3.
Synopsis Experiments were carried out to study the feeding rates of the predator fish Therapon jarbua (Forsk) on mullet juveniles, before and after treatment with DDT. Mullet juveniles treated with a subacute concentration, were refused by the control predators, whereas predators treated with a subacute concentration consumed more mullet juveniles. In the present study crescent perch T. jarbua were exposed to subacute and acute concentrations of DDT, and their behaviour was compared with that of the control predators. There were changes in oriented behaviour and co-ordinated movements, and in feeding, aggression and comfort behaviour of the fish. Inflammation in the gills, and caudal fin serration, were noticed in treated fishes. The findings presented here throw light on fundamental pathways by which pollutants interact with the behaviour of fishes.  相似文献   
4.
The environmental conditions to which juvenile barnacle geese (Branta leucopsis) were exposed during growth were found to affect their body size at fledging as well as their final adult body size. Small juveniles showed compensatory growth from the time of fledging up to one year of age, but this did not fully compensate the differences in body size that were established before fledging. The variation in protein content in plants eaten during growth could probably explain the observed body size differences, sometimes of more than 10%, between different categories of adult geese. Our results imply that one cannot infer selection on morphological characters from differences between samples of adult birds from different localities or from different cohorts within a population, without first showing that environmental conditions during growth do not affect the development of the characters under study.  相似文献   
5.
Summary Immature embryos of 20 sorghum genotypes were cultured on MS 5 medium containing MS mineral salts supplemented with 2,4-D, zeatin, glycine, niacinamide, Ca-pantothenate, L-asparagine, and vitamins. For regeneration, calli were transferred onto the same medium with the exception that IAA was substituted for 2,4-D. In general, immature embryos obtained 9–12 days after pollination resulted in the best redifferentiation. Ability of calli to regenerate varied among genotypes; cultivars C401-1 and C625 had the highest redifferentiation frequencies. Ability to redifferentiate was heritable and acted as a dominant trait. At least two gene pairs were involved. Regenerated R0 plants were planted in a greenhouse and their selfed (R1 and R2) progenies were planted in the field and examined for morphological and cytological variations. The majority of the phenotypic variations noted in R0 were not transmitted to later generations. However, variants for plant height, degree of fertility, and midrib color persisted in R1 and R2 generations. A variation in tallness was attributable to one dominant mutant gene. Short stature and male sterility variants appeared to be consequences of recessive mutant genes controlling those traits. Minor variations in peroxidase banding patterns were found among R0 plants.This study was supported by a research grant from Kansas Sorghum Commission and by a Research Fellowship to the senior author from the Ministry of Agriculture, Animal Husbandry, and Fisheries, China. Contribution 86-456-J from the Kansas Agricultural Experiment Station  相似文献   
6.
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   
7.
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   
8.
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   
9.
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   
10.
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   
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