全文获取类型
收费全文 | 4127篇 |
免费 | 521篇 |
国内免费 | 1872篇 |
出版年
2024年 | 33篇 |
2023年 | 210篇 |
2022年 | 219篇 |
2021年 | 286篇 |
2020年 | 288篇 |
2019年 | 292篇 |
2018年 | 241篇 |
2017年 | 244篇 |
2016年 | 266篇 |
2015年 | 249篇 |
2014年 | 296篇 |
2013年 | 354篇 |
2012年 | 286篇 |
2011年 | 304篇 |
2010年 | 233篇 |
2009年 | 284篇 |
2008年 | 277篇 |
2007年 | 286篇 |
2006年 | 244篇 |
2005年 | 188篇 |
2004年 | 176篇 |
2003年 | 157篇 |
2002年 | 158篇 |
2001年 | 109篇 |
2000年 | 103篇 |
1999年 | 97篇 |
1998年 | 68篇 |
1997年 | 58篇 |
1996年 | 60篇 |
1995年 | 60篇 |
1994年 | 56篇 |
1993年 | 49篇 |
1992年 | 38篇 |
1991年 | 40篇 |
1990年 | 26篇 |
1989年 | 19篇 |
1988年 | 15篇 |
1987年 | 16篇 |
1986年 | 24篇 |
1985年 | 23篇 |
1984年 | 18篇 |
1983年 | 7篇 |
1982年 | 15篇 |
1981年 | 11篇 |
1980年 | 8篇 |
1979年 | 7篇 |
1978年 | 4篇 |
1977年 | 4篇 |
1976年 | 3篇 |
1975年 | 6篇 |
排序方式: 共有6520条查询结果,搜索用时 15 毫秒
41.
Anders Pape Mller 《Evolution; international journal of organic evolution》1994,48(3):658-670
The patterns of variation in fluctuating asymmetry were studied in four morphological characters of the barn swallow Hirundo rustica. The level of absolute and relative asymmetry was larger in the secondary sexual character “outer tail length” than in three nonsexual morphological traits (wing, central tail, and tarsus length). The extent of individual asymmetry in outer tail length was negatively correlated with tail-ornament size, whereas the relationship between asymmetry of all other morphological characters and their size was flat or U-shaped. Asymmetry in outer tail length was unrelated to asymmetry in other morphological characters, whereas asymmetries in the length of wing, central tail, and tarsus were positively correlated. Male bam swallows exhibited larger asymmetry in outer tail length than females. Asymmetry of most morphological traits exhibited intermediate repeatabilities between years, with the exception of male and female outer tail length, which were highly repeatable. Tail asymmetry of offspring weakly, though significantly, resembled that of their parents. Asymmetry in wing and outer tail length was also significantly related to several fitness components. Male barn swallows that acquired a mate were less asymmetric in wing and outer tail length than unmated males. Females with more asymmetrical tails laid eggs significantly later. Annual reproductive success was unrelated to fluctuating asymmetry. Male barn swallows that survived were less asymmetric in wing and outer tail length than nonsurvivors, whereas female survivors were less asymmetric in outer tail length than nonsurvivors. These results suggest that levels of fluctuating asymmetry in barn swallows are associated with differences in fitness. 相似文献
42.
The nest site characteristics of the freshwater three-spined stickleback, Gasterosteus aculeatus (form leiurus ), were quantitatively investigated in springs and the main stream of the Yamayoke and the Tsuya River system, central Japan. Most nests (93·4%) were on a muddy or sandy substratum, at depths of 10–40 cm (84·3%), in water velocities less than 15 cm s−1 (76·2%) and in the temperature range of 14 to 16° C (82·7%), Spring-fed localities provided more of these conditions than the main stream channel and hence contained more potential nesting areas. Thus, they were utilized by male sticklebacks both temporally (prolonged breeding season) and spatially (more nest sites). 相似文献
43.
A cladistic analysis was used to deduce the phylogenetic relationships within the colonial Volvocales. Forty-one pairs of characters related to gross morphology and ultrastructure of vegetative colonies as well as asexual and sexual reproduction were analyzed based on parsimony, using the PAUP 3.0 computer program, for 25 species belonging to nine volvocacean and goniacean genera of the colonial Volvocales. Chlamydomonas reinhardtii Dangeard was the outgroup. The strict consensus tree indicated the presence of two monophyletic groups, one composed of all the volvocacean species analyzed in this study and the other containing the goniacean species except for the four-celled species Gonium sociale (Dujardin) Warming. In addition, these two groups constitute a large monophyletic group, to which G. sociale is a sister group. A new combination Tetrabaena socialis (Dujardin) Nozaki et Itoh and a new family Tetrabaenaceae Nozaki et Itoh are thus proposed for G. sociale. In addition, the analysis suggests that the volvocacean genera Eudorina and Pleodorina are paraphyletic groups, respectively, and that the monotypic genus Yamagishiella has no autapomorphic characters and represents primitive features of the anisogamous and oogamous genera of the Volvocaceae. Phylogenetic relationships within the Volvocaceae and the Goniaceae, as well as the various modes of sexual reproduction exhibited by these organisms, are discussed on the basis of the analysis. 相似文献
44.
A simple classification of the complex parts of vascular plants 总被引:1,自引:0,他引:1
GERARD CUSSET F.L.S. 《Botanical journal of the Linnean Society. Linnean Society of London》1994,114(3):229-242
Four operational morphological units are defined to characterize 'typical' stems, leaves, roots and hairS. Any complex part can be classified among these OMUs by a graphical representation or by a quadruplet suitable for computer-aided analyses. Thirty-nine examples are given, suggesting a continuous view of plant morphology. 相似文献
45.
为了进一步研究Elymus sibiricus L.、E.nutans Griseb.和E.burchan-buddae(Neuski)Tzelev
[=Roegneria nutans(Keng)Keng]的外部形态差异及其系统学关系,本文对这三种植物的6个穗部形
态性状进行了观测和比较,并对这三个Elymus种进行了种间杂交及杂种F1的减数分裂染色体配对行
为的分析研究。结果表明:这三个Elymus种的穗长及颖长等性状均变异很大,而内稃的长、宽则变异
不大并具有明显的种间差异。E. nutans×E.barchan-buddae及E.nutans×E.sibiricus的杂种F1均完
全不育,减数分裂不规则。E.nutans×E.burchan-buddae杂种F1的减数分裂构型为:7.70I+13.40
Ⅱ+0.06Ⅲ+O.08 Ⅳ,而E.sibiricus×E.nutans杂种F1的构型为11.98 Ⅰ+9.61Ⅱ+O.64Ⅲ+0.39
Ⅳ+0.01V。由本实验的形态学和细胞学的研究结果得出以下结论:1.利用内稃形态性状结合穗部
其它性状的差异能对这三个物种进行较准确的鉴定;2.E.nutans与E.burchan-buddae的亲缘关系较
近,而E.nutans与E.sibiricus的亲缘关系则较远;3. E,burchan-buddae×E.nutans的杂种Fl中存在着染色体配对控制因子。 相似文献
46.
47.
Tansley Review No. 59 Leaf boundary layers 总被引:6,自引:0,他引:6
P. H. SCHUEPP 《The New phytologist》1993,125(3):477-507
Studies of heat and mass exchange between leaves and their local environment are central to our understanding of plant-atmosphere interactions. The transfer across aerodynamic leaf boundary layers is generally described by non-dimensional expressions which reflect largely empirical adaptations of engineering models derived for flat plates. This paper reviews studies on leaves, and leaf models with varying degrees of abstraction, in free and forced convection. It discusses implecations of finding for leaf morphology as it affects – and is affected by – the local microclimate. Predictions of transfer from many leaves in plant communities are complicated by physical and physiological feedback mechanisms between leaves and their environment. Some common approaches, and the current challenge of integrating leaf-atmosphere interactions into models of global relevance, are also briefly addressed. 相似文献
48.
Organisms inhabiting metal-contaminated areas can be stressed by metal exposure and are possibly subject to selection, resulting
in increased metal tolerance and changes in growth and/or reproduction characteristics. In a previous study it was found that
in the terrestrial isopod Porcellio scaber, sampled from the vicinity of a zine smelter, the body size was small and the brood size was large compared to isopods from
a reference area. To assess whether these differences were due to genetic differentiation between strains, isopods collected
from a reference wood, a zinc smelter area and a lead mine were cultured on non-polluted food, while growth, reproduction
and metal concentrations were studied in first and second laboratory generations. The isopods from the three populations differed
in age and weight at first reproduction, although there were hardly any differences in growth. The females of the mine and
the smelter population started to reproduce earlier, at a lower weight, which resulted in fewer young per female. However,
reproductive allocation (=wight of young relative to the weight of the mother) was higher in mine and smelter isopods. We
conclude that the isopods at the metal-contaminated sites have been selected for early reproduction and increased reproductive
allocation. The results indicate that populations inhabiting metal-polluted sites have probably undergone evolutionary changes.
This study showed that growth and reproduction characteristics of different populations under laboratory conditions may provide
information on selection processes in the field. 相似文献
49.
J. W. Kadereit 《Plant biology (Stuttgart, Germany)》1994,107(6):369-373
Various explanations can be offered for the incongruence between phylogenetic hypotheses resulting from morphological and molecular data sets. Of these, the possibility that incongruence may result from the mutation of major morphogenetic genes leading to dramatic morphological divergence unaccompanied by equivalent change of the phylogenetic marker molecule(s) used is discussed in detail. As evidence for this hypothesis, several examples for such incongruence are surveyed. It seems possible that in many cases the genetic basis of the morphological characters responsible for the incongruence found may be simple, and that the genes involved may be homologous to genes known from mutant systems. It is suggested that: 1. the systematic documentation of incongruence between molecular and morphological phylogenies may help to assess the frequency of evolutionary change through the mutation of major morphogenetic genes, and that 2. the identification of major morphological characters distinguishing closely related taxa with mutant phenotypes known from mutant systems eventually may allow an experimental approach to the problem of evolutionary change resulting from major genes. Natural taxa suspected to be the result of such processes could be changed morphologically through transformation with the relevant genes. 相似文献
50.
《Journal of Asia》2023,26(2):102057
In this study, we investigated the morphological and biological characteristics of a serious insect pest of conifer cones, the western conifer seed bug (WCSB), Leptoglossus occidentalis Heidemann (Heteroptera: Coreidae), in Korea. WCSB adults shared external morphology with overseas populations, including zig-zag fascia on the coria, dilation of the hind tibiae, and color pattern on the abdominal dorsum. The average body length and width of WCSB adults were 17.20 and 4.85 mm, respectively, in males and 19.05 and 5.55 mm in females. The average body weights were 111.27 and 169.89 mg in adult males and females, respectively. On average, eggs were 2.12 mm long and 1.17 mm wide. The average body lengths of the first- to fifth-instar nymphs were 3.07, 4.76, 8.77, 13.56, and 15.53 mm, respectively. For biological characteristics, developmental durations of the egg and five nymphal instar stages were 8.6, 3.7, 7.6, 5.8, 5.8, and 9.1 days, respectively, in laboratory conditions. Nymphal mortality was 47.5% in total. The average life expectancy of WCSB adults was 58.2 and 63.9 days in males and females, respectively. Oviposition of the adult females started 23.4 days after emergence, and the total number of eggs laid by a single adult female was 68.6 on average. The results of this study offer basic morphological and biological information that can be used to design an effective control strategy to manage the WCSB population in Korea. 相似文献