全文获取类型
收费全文 | 8522篇 |
免费 | 928篇 |
专业分类
9450篇 |
出版年
2023年 | 39篇 |
2022年 | 89篇 |
2021年 | 196篇 |
2020年 | 100篇 |
2019年 | 124篇 |
2018年 | 142篇 |
2017年 | 125篇 |
2016年 | 242篇 |
2015年 | 400篇 |
2014年 | 405篇 |
2013年 | 470篇 |
2012年 | 567篇 |
2011年 | 615篇 |
2010年 | 381篇 |
2009年 | 345篇 |
2008年 | 469篇 |
2007年 | 442篇 |
2006年 | 438篇 |
2005年 | 419篇 |
2004年 | 370篇 |
2003年 | 327篇 |
2002年 | 279篇 |
2001年 | 172篇 |
2000年 | 155篇 |
1999年 | 135篇 |
1998年 | 89篇 |
1997年 | 73篇 |
1996年 | 62篇 |
1995年 | 65篇 |
1994年 | 64篇 |
1993年 | 72篇 |
1992年 | 124篇 |
1991年 | 102篇 |
1990年 | 112篇 |
1989年 | 92篇 |
1988年 | 91篇 |
1987年 | 79篇 |
1986年 | 61篇 |
1985年 | 72篇 |
1984年 | 62篇 |
1983年 | 61篇 |
1982年 | 44篇 |
1981年 | 33篇 |
1979年 | 54篇 |
1978年 | 48篇 |
1977年 | 47篇 |
1976年 | 46篇 |
1974年 | 36篇 |
1973年 | 38篇 |
1969年 | 40篇 |
排序方式: 共有9450条查询结果,搜索用时 0 毫秒
81.
82.
T P Haverty M Watanabe E G Neilson C J Kelly 《Journal of immunology (Baltimore, Md. : 1950)》1989,143(4):1133-1141
83.
Human papillomavirus type 48. 总被引:3,自引:1,他引:2
The cloning and partial characterization of the genome of human papillomavirus (HPV) type 48 is presented. Hybridization and short DNA sequence analyses permitted the alignment of the genome to the HPV genetic map. 相似文献
84.
Minimum qualifications for directors: DNA-based genetic-testing laboratories. DNA Testing Subcommittee, Quality Assurance Committee, Council of Regional Networks for Genetic Services. 下载免费PDF全文
85.
86.
87.
This paper evaluates and criticises the developmental systems conception of evolution and develops instead an extension of the gene's eye conception of evolution. We argue (i) Dawkin's attempt to segregate developmental and evolutionary issues about genes is unsatisfactory. On plausible views of development it is arbitrary to single out genes as the units of selection. (ii) The genotype does not carry information about the phenotype in any way that distinguishes the role of the genes in development from that other factors. (iii) There is no simple and general causal criterion which distinguishes the role of genes in development and evolution. (iv) There is, however, an important sense in which genes but not every other developmental factor represent the phenotype. (v) The idea that genes represent features of the phenotype forces us to recognise that genes are not the only, or almost the only, replicators. Many mechanisms of replication are involved in both development and evolution. (vi) A conception of evolutionary history which recognises both genetic and non-genetic replicators, lineages of replicators and interactors has advantages over both the radical rejection of the replicator/interactor distinction and the conservative restriction of replication to genetic replication. 相似文献
88.
An ecomorphological analysis of the tallgrass prairie of central North America divided representative species of the native grassland flora into eight guilds or groups of species with similar life-form, phenology, and ecology. The guilds, segregated by multivariate analysis, are: (1) warm-season graminoids with Kranz anatomy and the Hatch-Slack photosynthetic pathway (C4 grasses); (2) cool-season graminoids without Kranz anatomy, but with the common Calvin or C3 photosynthetic pathway (C3 grasses and sedges); (3) annuals and biennial forbs; (4) ephemeral spring forbs; (5) spring forbs; (6) summer/fall forbs; (7) legumes; and (8) woody shrubs. The study was based on 158 plant species indigenous to three upland prairie sites in northeastern Kansas. Each species was scored for 32 traits which fall into five broad categories: plant habit, leaf characteristics, stem structures, root structures, and reproductive traits, including phenology. A multivariate, detrended correspondence analysis sorted the 158 species into the eight principal groups or guilds. These groups were further supported by a cluster analysis and discriminant function analysis of the same data set. The discriminant function analysis determined that 94.3% of the species were correctly classified in their respective guilds, and that the guilds were statistically different. Results indicate that guild analysis offers a basis for detailed classification of grassland vegetation that is more ecologically focused than species composition, as the myriad of species (about 1,000 prairie species on the central plains of North America) vary in presence, cover, and importance with their individualistic distribution.Abbreviations C3=
C3 photosynthesis
- C4=
C4 photosynthesis
- LSD=
least significant difference 相似文献
89.
90.
Kelly D 《Trends in ecology & evolution》1994,9(12):465-470
The past seven years have seen a revolution in understanding the causes of mast seeding In perennial plants. Before 1987, the two main theories were resource matching (i.e. plants vary their reproductive output to match variable resources) and predator satiation (i.e. losses to predators are reduced by varying the seed crop). Today, resource matching is restricted to a proximate role, and predator satiation is only one of many theories for the ultimate advantage of masting. Wind pollination, prediction of favourable years for seedling establishment, animal pollination, animal dispersal of fruits, high accessory costs of reproduction and large seed size have all been advanced as possible causes of masting. Of these, wind pollination, predator satiation and environmental prediction are important in a number of species, but the other theories have less support. In future, Important advances seem likely from quantifying synchrony within a population, and examining species with very constant reproduction between years. 相似文献