全文获取类型
收费全文 | 3002篇 |
免费 | 292篇 |
国内免费 | 167篇 |
出版年
2023年 | 23篇 |
2022年 | 19篇 |
2021年 | 39篇 |
2020年 | 63篇 |
2019年 | 85篇 |
2018年 | 90篇 |
2017年 | 113篇 |
2016年 | 105篇 |
2015年 | 80篇 |
2014年 | 131篇 |
2013年 | 177篇 |
2012年 | 63篇 |
2011年 | 112篇 |
2010年 | 69篇 |
2009年 | 136篇 |
2008年 | 124篇 |
2007年 | 166篇 |
2006年 | 155篇 |
2005年 | 118篇 |
2004年 | 99篇 |
2003年 | 106篇 |
2002年 | 119篇 |
2001年 | 94篇 |
2000年 | 99篇 |
1999年 | 80篇 |
1998年 | 83篇 |
1997年 | 71篇 |
1996年 | 75篇 |
1995年 | 69篇 |
1994年 | 83篇 |
1993年 | 61篇 |
1992年 | 44篇 |
1991年 | 47篇 |
1990年 | 55篇 |
1989年 | 38篇 |
1988年 | 41篇 |
1987年 | 34篇 |
1986年 | 42篇 |
1985年 | 45篇 |
1984年 | 25篇 |
1983年 | 15篇 |
1982年 | 23篇 |
1981年 | 30篇 |
1980年 | 27篇 |
1979年 | 23篇 |
1978年 | 23篇 |
1977年 | 10篇 |
1976年 | 7篇 |
1974年 | 5篇 |
1973年 | 6篇 |
排序方式: 共有3461条查询结果,搜索用时 15 毫秒
911.
Genetic relationships among Spanish sheep using microsatellites 总被引:23,自引:0,他引:23
Five indigenous Spanish breeds of sheep, Churra, Latxa, Manchega, Rasa–Aragonesa and Merino, with Awassi sheep as a reference breed were genotyped for 19 DNA microsatellites. Allele frequencies and mean heterozygosities revealed the greatest genetic variation in Merino sheep and the lowest in Awassis. Differences in variability were not great in the other breeds studied. The dendrograms obtained based on genetic distances showed a large differentiation between Awassi sheep and the Spanish breeds, as was to be expected from their distinct genetic origin. Merinos appeared separated from the other four breeds, of which, according to a classification based on the fleece characteristics, Churra and Latxa belong to the churro type and Manchega and Rasa–Aragonesa to the so called entrefino type, though no clear separation was evident between the two types. These results suggest that morphological data alone are insufficient for determining relationships between breeds and that studies involving genetic markers may be of great assistance. 相似文献
912.
Seiichi Mori 《Environmental Biology of Fishes》1998,52(1-3):243-250
Multidimensional scaling analysis of nest site location, nesting interval, and aggressive and mating interactions revealed that male three-spined sticklebacks, Gasterosteus aculeatus, formed nesting clusters comprising five patterns of dyadic relationships. These patterns were defined by intervening variables such as size difference between the two males, nest site locations, synchronization of nesting cycles (egg presence in the nest), distance between two nests, and nest raiding behaviours. Several patterns of dyadic relationships were integrated among nesting males and their constellations indicated an expression of the social structure. Through an understanding the social structure, I described effects of social context or situations on individual reproductive success. 相似文献
913.
Bruce A. Ford Julian R. Starr D. A. Ross McQueen Robert F. C. Naczi 《Plant Systematics and Evolution》1998,212(1-2):31-51
Allozyme variation was examined inCarex sect.Phyllostachys (Cyperaceae) to provide insight into phylogenetic relationships hypothesized in an earlier study and to determine the degree of genetic differentiation within and between taxa. Genetic identity values are concordant with the morphological differences found between species. The lowest values are found between species with the greatest morphological dissimilarity. Conversely, the highest values are associated with species pairs distinguished by relatively few morphological differences. Conspecific populations possess high genetic identities, although interpopulation differentiation has characterized the evolutionary history of some species. Geographic patterning is also evident within species, with geographically proximate populations often having the highest identity values. Phylogenetic trees produced using different cladistic methods were poorly supported and varied in their depiction of relationships among species. One cladogram produced using presence/absence allelic data is more or less congruent with a topology recovered from an earlier analysis utilizing molecular and morphological data. The wide- and narrow-scaled clades are maintained as are the sister species pairsC. backii/C. saximontana, C. basiantha/C. superata, andC. jamesii/C. juniperorum. Contrary to the finding of our previous study, however,C. willdenowii is aligned withC. jamesii/C. juniperorum. 相似文献
914.
Structure, above-ground biomass and dynamics of mangrove ecosystems: new data from French Guiana 总被引:5,自引:0,他引:5
The article presents new results on the structure and the above-ground biomass of the various population types of mangroves
in French Guiana. Nine mangrove stands were studied, each composed of three to ten adjoining plots with areas that varied
depending on the density of the populations. Structural parameters and indices were calculated. Individuals representative
of the three groups of taxa present were felled:Avicennia germinans (L) Stearn, Rhizophora spp., and Laguncularia racemosa (L) Gaertn. The trunks, branches and leaves were sorted and weighed separately. The biomass was obtained by determining the
allometric relationships, the general equation selected being of the type y = a
o
x
a1, where the diameter (x) is the predictive variable. The total above-ground biomass varied from 31 t ha−1 for the pioneer stages to 315 t ha−1 for mature coastal mangroves, but with large variations depending on the structural characteristics at each site. The results
place the Guianese mangroves among those with high biomass, although lower than those reported for Asia. Based on the relationships
between structural parameters and standing biomass, in particular with the use of the “self-thinning rule”, population dynamics
models are proposed.
Received: 16 August 1996 / Accepted 17 January 1998 相似文献
915.
David P. Watts 《Primates; journal of primatology》1998,39(1):71-78
Visually attending to conspecifics can give group-living primates important ecological information, help them to anticipate
the behavior of others and to regulate interactions with them, and provide other valuable social information. Variation in
the importance and quality of social relationships should influence the way individuals selectively attend to fellow group
members. Preliminary data on visual monitoring of conspecifics by wild female mountain gorillas (Gorilla gorilla beringei) show that selective attention mirrors variation in social relationships. Social bonds between males and females are central
to gorilla society; correspondingly, females are more likely to stop feeding, and focus their attention on males who walk
into view than on females, especially when males give dislays. Females are more likely to focus on other females with whom
they have antagonistic relationships than those (mostly close relatives) with whom they have affiliative, cooperative ones.
Further research on the context and consequences of visual monitoring could help to address questions about the regulation
of social relationships and about social cognition in gorillas. 相似文献
916.
飞播造林作为沙地大面积绿洲化的有效的手段,已在毛乌素沙地中运用并取得了显著的成效。本文根据伊金霍洛旗4个播区飞播后十余年的调查资料,分析了杨柴灌丛群落动态变化的特点以及演替的机制,并指出沙地土壤水分生境的改变是群落发生演替的主要原因。随着生境的变化,群落的演替可分为4个阶段:先锋植物群落、杨柴群落、油蒿群落和黑格兰、沙地柏、蒙古莸和柠条等中旱生灌丛群落阶段。为了防止杨柴群落的进一步演替或退化,确保这一人工草场的质量,对群落演替的合理人为干扰,是必要的。 相似文献
917.
Lignans are widely distributed in angiosperms and gymnosperms. The range of their structures and biological activities is broad. Various lignans are known to have anti-tumour, antimitotic and antiviral activity and to specifically inhibit certain enzymes. Toxicity to fungi, insects and vertebrates is observed for some lignans and a variety of physiological activities have been documented. This review summarizes what is presently known about the biological activities of lignans. 相似文献
918.
Simulated distribution of vegetation types in response to climate change on the Tibetan Plateau 总被引:1,自引:0,他引:1
Abstract. Questions: What is the relationship between alpine vegetation patterns and climate? And how do alpine vegetation patterns respond to climate changes? Location: Tibetan Plateau, southwestern China. The total area is 2500000 km2 with an average altitude over 4000 m. Methods: The geographic distribution of vegetation types on the Tibetan Plateau was simulated based on climatology using a small set of plant functional types (PFTs) embedded in the biogeochemistry‐biography model BIOME4. The paleoclimate for the early Holocene was used to explore the possibility of simulating past vegetation patterns. Changes in vegetation patterns were simulated assuming continuous exponential increase in atmospheric CO concentration, based on a transient ocean‐atmosphere simulation including sulfate aerosol effects during the 21st century. Results: Forest, shrub steppe, alpine steppe and alpine meadow extended while no desert vegetation developed under the warmer and humid climate of the early Holocene. In the future climate scenario, the simulated tree line is farther north in most sectors than at present. There are also major northward shifts of alpine meadows and a reduction in shrub‐dominated montane steppe. The boundary between montane desert and alpine desert will be farther to the south than today. The area of alpine desert would decrease, that of montane desert would increase. Conclusions: The outline of changes in vegetation distribution was captured with the simulation. Increased CO2 concentration would potentially lead to big changes in alpine ecosystems. 相似文献
919.
MARÍA R. ALONSO SARA L. GOODACRE BRENT C. EMERSON MIGUEL IBÁÑEZ RAINER HUTTERER KLAUS GROH 《Biological journal of the Linnean Society. Linnean Society of London》2006,89(1):169-187
Five new species of land snail (family Enidae) are described from La Gomera (Canary Islands) of which the majority, on the basis of anatomy alone, could be incorporated within a new supraspecific taxon. In addition to the morphological study of these new species, a region of the 16S mitochondrial gene is sequenced from three of the new species and a range of species of Napaeus from within its two subgenera ( Napaeinus and Napaeus ) . There is a disparity between the morphological and preliminary molecular phylogenetic data. Possible explanations for this conflict are discussed, as well as the evolutionary relationships among these different taxa, and it is suggested that this group may be an excellent model for further studies of adaptation and diversification. © 2006 The Linnean Society of London, Biological Journal of the Linnean Society , 2006, 89 , 169–187. 相似文献
920.
During two surveys of beet root crops in South Kazakhstan and Central Asia conducted in 1988 and 1989, 465 of 990 samples were found to contain beet mosaic virus (BMV) by double-antibody sandwich (DAS) ELISA. BMV infection was widely scattered in the area surveyed, and its incidence varied considerably, reaching 100% in some fields adjacent to beet seed crops. BMV isolates from Kazakhstan, Kirgizia, Uzbekistan and Ukraine were found to be serologically closely related in DAS-ELISA test. Beet yellows virus (BYV) was not detected in any location surveyed in South Kazakhstan and Central Asia. BYV spread into the area is probably prevented by its geographical isolation. 相似文献