全文获取类型
收费全文 | 450446篇 |
免费 | 51865篇 |
国内免费 | 1003篇 |
专业分类
503314篇 |
出版年
2018年 | 4342篇 |
2017年 | 4135篇 |
2016年 | 5912篇 |
2015年 | 8577篇 |
2014年 | 9959篇 |
2013年 | 13330篇 |
2012年 | 15811篇 |
2011年 | 16205篇 |
2010年 | 10706篇 |
2009年 | 9724篇 |
2008年 | 14091篇 |
2007年 | 14514篇 |
2006年 | 13602篇 |
2005年 | 12971篇 |
2004年 | 13016篇 |
2003年 | 12115篇 |
2002年 | 11762篇 |
2001年 | 18999篇 |
2000年 | 18918篇 |
1999年 | 15143篇 |
1998年 | 5661篇 |
1997年 | 5677篇 |
1996年 | 5395篇 |
1995年 | 5185篇 |
1994年 | 4876篇 |
1993年 | 4883篇 |
1992年 | 12505篇 |
1991年 | 12466篇 |
1990年 | 12232篇 |
1989年 | 11689篇 |
1988年 | 10851篇 |
1987年 | 10222篇 |
1986年 | 9744篇 |
1985年 | 9512篇 |
1984年 | 7886篇 |
1983年 | 6835篇 |
1982年 | 5141篇 |
1981年 | 4633篇 |
1980年 | 4315篇 |
1979年 | 7360篇 |
1978年 | 5970篇 |
1977年 | 5294篇 |
1976年 | 4960篇 |
1975年 | 5773篇 |
1974年 | 6304篇 |
1973年 | 6125篇 |
1972年 | 5462篇 |
1971年 | 5080篇 |
1970年 | 4275篇 |
1969年 | 4206篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
991.
M. S. ISLAM J. M. TAO Q. F. GENG C. L. LIAN T. HOGETSU 《Molecular ecology resources》2006,6(4):1111-1113
Kandelia candel is an important mangrove tree species of family Rhizophoraceae. Here we isolated eight codominant compound microsatellite simple sequence repeat (SSR) loci from K. candel. Our isolated loci provided compound SSR markers with polymorphism of three to 11 alleles per locus. The expected and observed heterozygosities ranged from 0.230 to 0.887 and from 0.083 to 1.00, respectively. These markers would be the useful tools for analysing questions concerning population genetic structure and mating system of K. candel. 相似文献
992.
993.
Growth of 2659 Atlantic cod Gadus morhua aged 4 to 9 years examined in Placentia Bay, Newfoundland, peaked in most cases in June and was at a minimum in October or November. Water temperature, partial fullness index ( I P ) and gonado‐somatic index ( I G ) explained between 31 and 52% of the monthly variability in growth. Temperature and I P of capelin Mallotus villosus had significant effects on growth of all age groups and explained most of the variance for ages 6–8 and 4–5 years, respectively. The I P of large invertebrates (ages 4 to 7 years), sandlance ( Ammodytes sp. age 6 years) and demersal fishes (age 9 years) had age‐specific effects in the model. Overall, amphipods, decapods and echinoderms dominated the Atlantic cod diet in most seasons, but fish consumption by Atlantic cod was high in June and July, particularly on capelin. The rapid increase in somatic mass during June and July occurred despite cold water temperatures ( < 3° C at 50 m) and moderate to high gonado‐somatic index. The findings of this study suggest that when food was not a limiting factor, growth tended to increase even when Atlantic cod occupied colder waters, but when food was limiting, the opposite may have occured. 相似文献
994.
995.
In the scolex ganglia of the cestode Pelichnibothrium speciosum uni - and multipolar neurons can be found. Their neuroplasm is rich in free ribosomes. The ergastoplasmas membranes are frequently arranged in compact finger print-like structures. The nerve cells processes form tight junctions which cannot be interpreted as synaptic contacts. 相似文献
996.
In this study it is shown that the higher molecular weight previously reported for tiger shark carbonic anhydrase (carbonate hydro-lyase, EC 4.2.1.1) compared to other carbonic anhydrases is decreased to a normal value around 30 000 after disulfide reduction of the enzyme. This difference in molecular weight is at least partly due to the existence of disulfide-linked glutathione and cysteine residues. Approx. 3 mol glutathione and a similar amount of cysteine are shown to be bound per mol enzyme. The presence of these factors also has effects on the enzyme activity. 相似文献
997.
The debate about the biological species concept - a review 总被引:1,自引:0,他引:1
C. L. HÄUSER 《Journal of Zoological Systematics and Evolutionary Research》1987,25(4):241-257
The importance of the species concept in biology has led to a continuing debate about the definition of species. This paper summarizes the recent literature in relation to the ‘biological species concept’ (MAYR 1942). Among the general attributes demanded, possible limitations of the universality and applicability of a species definition are discussed. Three different areas of criticism of the biological species concept are considered: 1. The impracticability of the criterion of reproductive isolation. The demand for more practical criteria is rejected, because reproductive isolation is seen as the factor that produces and maintains species as discrete entities in nature. 2. The inapplicability to non-bisexual organisms. A brief survey of modes of uniparental reproduction and their relative importance suggests that obligatory apomicts are of little evolutionary significance. 3. The inapplicability to multidimensional situations. Despite practical difficulties, the biological species concept is held to apply to organisms separated in space. The impossibility to delimit species in time by reproductive isolation is recognized. Out of two ways to divide continuous evolutionary lineages in time, the phylogenetic approach, which considers only speciation events (cladogenesis), is preferred as it is more objective. A list of recently published alternative definitions of species, none of which is found acceptable, is given. It is concluded that the biological species concept needs not be changed or dismissed on the basis of the discussed criticisms. 相似文献
998.
999.
The formation of R-prime plasmids was selected in crosses involving soybean microsymbionts with genomic Tn5 insertions and carrying plasmid pJB3JI (with one IS2) copy as donors and Escherichia coli HB101 as recipient. Whereas the parent plasmid was 60 kb, recombinant plasmids between 76 kb and 121 kb were obtained. Restriction and Southern analyses confirmed the mobilization of Tn5 on four R-primes from Bradyrhizobium japonicum I-110 and on an R-prime plasmid from Rhizobium fredii HH303. The largest R-prime plasmid was obtained from the rescue of two symbiotically defective R. fredii mutant strains that required adenosine.Non-standard abbreviation TDP
transposon donor pool
Scientific article number A-4728 and contribution number 7724 of the Maryland Agricultural Experiment Station 相似文献
1000.