全文获取类型
收费全文 | 13102篇 |
免费 | 1430篇 |
国内免费 | 1257篇 |
出版年
2024年 | 29篇 |
2023年 | 215篇 |
2022年 | 259篇 |
2021年 | 461篇 |
2020年 | 463篇 |
2019年 | 571篇 |
2018年 | 404篇 |
2017年 | 502篇 |
2016年 | 556篇 |
2015年 | 504篇 |
2014年 | 602篇 |
2013年 | 781篇 |
2012年 | 639篇 |
2011年 | 561篇 |
2010年 | 527篇 |
2009年 | 752篇 |
2008年 | 732篇 |
2007年 | 809篇 |
2006年 | 651篇 |
2005年 | 645篇 |
2004年 | 544篇 |
2003年 | 506篇 |
2002年 | 453篇 |
2001年 | 456篇 |
2000年 | 429篇 |
1999年 | 357篇 |
1998年 | 307篇 |
1997年 | 284篇 |
1996年 | 220篇 |
1995年 | 202篇 |
1994年 | 178篇 |
1993年 | 141篇 |
1992年 | 153篇 |
1991年 | 150篇 |
1990年 | 99篇 |
1989年 | 92篇 |
1988年 | 65篇 |
1987年 | 69篇 |
1986年 | 55篇 |
1985年 | 63篇 |
1984年 | 53篇 |
1983年 | 27篇 |
1982年 | 48篇 |
1981年 | 33篇 |
1980年 | 36篇 |
1979年 | 24篇 |
1978年 | 15篇 |
1977年 | 14篇 |
1975年 | 13篇 |
1973年 | 12篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
151.
Phytoplankton species groups were studied in a eutrophic reservoir at different time scales (daily, weekly, monthly and yearly). Four strategic groups along the r-K continuum were defined and their seasonal time courses were followed. Their temporal distribution of relative biomass reflected resource partitioning because each strategic group dominated the phytoplankton community at different times in the year. However, the relationships between strategic groups changed with the time scale involved. At the daily scale an inverse relationship between r- and K-groups occurred whereas at supradaily scales such relationships did not hold. Species groups reflected strategic groups. No relationship between population growth and losses was found, suggesting that both were not counterbalanced. In the long term (supraannual) the phytoplankton community changed very much in this reservoir, its ecological memory being small as compared with that of lakes. 相似文献
152.
The potato tuber moth (PTM),Phthorimaea operculella (Zeller) (Lepidoptera: Gelechiidae), is a major pest of processing tomatoes,Lycopersicon esculentum Mill. (Solanaceae), in Israel. The larvae penetrate the tomato fruit through the stem end and present a serious threat to
crop quality. Foliage and fruit samples were taken in nine commercial tomato fields located in Israel's three main tomato
growing areas, two of which are potato growing areas as well. PTM was not found where potatoes were absent. Potato harvest
in nearby fields was found to be the most significant factor affecting seasonal trends in PTM population density in tomatoes.
All four larval instars were found in foliage on all sampling dates. Significantly higher proportions of first instars were
found during the population density increase which followed potato harvest. Damaged fruits did not contain first instar larvae,
indicating that PTM never undergoes complete development within tomato fruit. Fruit damage levels at harvest were positively
correlated to the peak mean population densities on foliage and the date they were observed. In tomato fields not adjacent
to potatoes, infestation was first observed at the edge of the field. Both before and after the potato harvest in nearby fields,
population density at the edge of the field was significantly higher than at the center. In tomato fields adjacent to potatoes,
no significant differences were found between population densities at the edge and center before the potatoes were harvested.
After the potato harvest, population density at the center of tomato fields was higher than at the edge.
Deceased, October 1988 相似文献
153.
Hitoshi Suzuki Kimiyuki Tsuchiya Mitsuru Sakaizumi Shigeharu Wakana Susumu Sakurai 《Journal of molecular evolution》1994,38(2):107-112
An analysis by restriction endonuclease digestion of ribosomal DNA (rDNA) was carried out in natural populations of Apodemus speciosus, a field mouse that is endemic to Japan. Two restriction sites, the EcoRI (E3) and DraI (D4) sites, in the nontranscribed spacer region downstream from the gene for 28S RNA showed polymorphism within and between individuals in the populations from the Japanese main islands. By contrast, populations from the small adjoining islands which are thought to have separated from the main islands 1–2 × 104 years ago showed relatively low levels of polymorphism within and between individuals, i.e., one of the polymorphic bands in the case of each enzyme was predominant in these populations, irrespective of the variants. These results indicate that the rate of fixation of site variations depends on population size and that the direction of fixation is random. Furthermore, each polymorphic restriction site seems to be fixed independently.Correspondence to: H. Suzuki 相似文献
154.
M. S. Mayer P. S. Soltis 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1994,87(7):773-781
A restriction-site analysis of chloroplast DNA (cpDNA) variation in Lens was conducted to: (1) assess the levels of variation in Lens culinaris ssp. culinaris (the domesticated lentil), (2) identify the wild progenitor of the domesticated lentil, and (3) construct a cpDNA phylogeny of the genus. We analyzed 399 restriction sites in 114 cultivated accessions and 11 wild accessions. All but three accessions of the cultivar had identical cpDNAs. Two accessions exhibited a single shared restriction-site loss, and a small insertion was observed in the cpDNA of a third accession. We detected 19 restriction-site mutations and two length mutations among accessions of the wild taxa. Three of the four accessions of L. culinaris ssp. orientalis were identical to the cultivars at every restriction site, clearly identifying ssp. orientalis as the progenitor of the cultivated lentil. Because of its limited cpDNA diversity, we conclude that either the cultivated lentil has passed through a genetic bottleneck during domestication and lost most of its cytoplasmic variability or else was domesticated from an ancestor that was naturally depauperate in cpDNA restriction-site variation. However, because we had access to only a small number of populations of the wild taxa, the levels of variation present in ssp. orientalis can only be estimated, and the extent of such a domestication bottleneck, if applicable, cannot be evaluated. The cpDNA-based phylogeny portrays Lens as quite distinct from its putative closest relative, Vicia montbretii. L. culinaris ssp. odemensis is the sister of L. nigricans; L. culinaris is therefore paraphyletic given the current taxonomic placement of ssp. odemensis. Lens nigricans ssp. nigricans is by far the most divergent taxon of the genus, exhibiting ten autapomorphic restriction-site mutations. 相似文献
155.
J. Crossa R. Vencovsky 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1994,89(7-8):936-942
The concept of variance effective population size [Ne(v)] and other expressions are reviewed and described for specific sampling steps in germplasm collection and regeneration of monoecious species. Special attention is given to procedures for computing the variance of the number of contributed gametes [V(k)] to the next generation. Drift, as it occurs between generations, was considered to contain a component due to the sampling of parents and a subsequent component due to the sampling of gametes. This demonstrates that drift, caused by reduction of seed viability, damages the genetic integrity of accessions stored in germplasm banks. The study shows how mating designs, such as plant-to-plant or chain crossings with additional female gametic control, can partially alleviate this problem. Optimal procedures for increasing Ne(v) when collecting germplasm in the field are also discussed. The effect of different female and male gametic control strategies on Ne(v) is considered under several situations. Practical examples illustrating the use of V(k) and Ne(v) expressions are given. 相似文献
156.
157.
Genetic variation in an isolated brown bear population of the Western Carpathians was studied by electrophoretic analysis of 51 presumptive allozyme loci. In spite of a severe population bottleneck at the beginning of the 1930s (40 survivors), average heterozygosity (H=5.3%) was within the range commonly found in mammals and the mean number of alleles per polymorphic locus (Ap=3, over five polymorphic loci) was very high. Possible reasons for the maintenance of high allelic variation are discussed. 相似文献
158.
DNA sequencing with arbitrary primer pairs 总被引:8,自引:0,他引:8
159.
160.
Genetic variation of microsatellite loci in a bottlenecked species: the northern hairy-nosed wombat Lasiorhinus krefftii 总被引:12,自引:0,他引:12
We investigate the utility of hypervariable microsatellite loci to measure genetic variability remaining in the northern hairy-nosed wombat, one of Australia's rarest mammals. This species suffered a dramatic range and population reduction over the past 120 years and now exists as a single colony of about 70 individuals at Epping Forest National Park, central Queensland. Because our preliminary research on mitochondrial DNA and multilocus DNA fingerprints did not reveal informative variation in this population, we chose to examine variation in microsatellite repeats, a class of loci known to be highly polymorphic in mammals. To assess the suitability of various wombat populations as a reference for comparisons of genetic variability and subdivision we further analysed mitochondrial DNA cytochrome b sequence, using phylogenetic methods. Our results show that appreciable levels of variation still exist in the Epping Forest colony although it has only 41% of the heterozygosity shown in a population of a closely-related species. From museum specimens collected in 1884, we also assessed microsatellite variation in an extinct population of the northern hairy-nosed wombat, from Deniliquin, New South Wales, 2000 km to the south of the extant population. The apparent loss of variation in the Epping Forest colony is consistent with an extremely small effective population size throughout its 120-year decline. 相似文献