全文获取类型
收费全文 | 65617篇 |
免费 | 1180篇 |
国内免费 | 10163篇 |
出版年
2022年 | 329篇 |
2021年 | 438篇 |
2020年 | 424篇 |
2019年 | 998篇 |
2018年 | 1304篇 |
2017年 | 711篇 |
2016年 | 804篇 |
2015年 | 1002篇 |
2014年 | 1494篇 |
2013年 | 2397篇 |
2012年 | 1673篇 |
2011年 | 1185篇 |
2010年 | 989篇 |
2009年 | 1564篇 |
2008年 | 1801篇 |
2007年 | 2470篇 |
2006年 | 2691篇 |
2005年 | 2763篇 |
2004年 | 2981篇 |
2003年 | 2701篇 |
2002年 | 2673篇 |
2001年 | 2392篇 |
2000年 | 1780篇 |
1999年 | 2055篇 |
1998年 | 1795篇 |
1997年 | 1351篇 |
1996年 | 1312篇 |
1995年 | 1381篇 |
1994年 | 1462篇 |
1993年 | 1303篇 |
1992年 | 1122篇 |
1991年 | 1112篇 |
1990年 | 912篇 |
1989年 | 768篇 |
1988年 | 805篇 |
1987年 | 622篇 |
1986年 | 541篇 |
1985年 | 2017篇 |
1984年 | 3042篇 |
1983年 | 1999篇 |
1982年 | 2383篇 |
1981年 | 2023篇 |
1980年 | 1894篇 |
1979年 | 1679篇 |
1978年 | 1546篇 |
1977年 | 1415篇 |
1976年 | 1303篇 |
1975年 | 1101篇 |
1974年 | 1087篇 |
1973年 | 1021篇 |
排序方式: 共有10000条查询结果,搜索用时 921 毫秒
971.
Vibration and sound communication in solitary bees and wasps 总被引:2,自引:0,他引:2
ABSTRACT. Females of solitary bees ( Colletes cunicularius L.) and of digger wasps ( Bembix rostrata L.) produce buzzing sounds and are known to secrete volatile odours when digging their way from the subterranean nests to the soil surface. The odours allow patrolling males to determine the approximate position of the digging virgin female. The buzzes are measured as substrate-borne sound (soil buzz vibrations) and as air-borne sound (soil buzz sounds). Play-back experiments suggest that the soil buzzes are used by the males as additional cues for localization. Faint buzz sounds are emitted regularly by the male during genital contact in copulation. They may serve to change the receptivity of the female. Intense and broadband buzz sounds are produced by bees of either sex, if restrained from moving, perhaps serving to deter predators. 相似文献
972.
Chemical defence in chrysomelid eggs and neonate larvae 总被引:2,自引:0,他引:2
ABSTRACT. Eggs and neonate larvae of chrysomelid beetles (sub-tribes Chrysomelina and Phyllodectina) were investigated for the presence of defensive substances.
The two isoxazolinone glucosides (compounds 1 and 2), characteristic of the adult defence secretion, were detected in the eggs of all studied species. Compound 2, containing a nitropropionate, is always present in concentrations (above 10-2 M), which are highly deterrent to the ant Myrmica rubra. This compound is not at all or only slightly toxic to ants at 10-2 M. Compound 1, devoid of nitropropionate, is a minor constituent, and is neither deterrent nor toxic to ants.
The five Chrysomela species studied and Phratora vitellinae also sequester salicin in their eggs in amounts highly deterrent and toxic to ants. A single Chrysomela egg often contains enough salicin to kill an ant. While the isoxazolinones are discarded with the egg shells, salicin is used by neonate larvae as a precursor for the production of salicylaldehyde in the thoracic defence glands, already functional at hatching. No salicin could be detected in the eggs of those species whose larvae produce cyclopentanoid monoterpenes, even if they feed on Salicaceae. No larva of any species seems to be able to produce detectable amounts of monoterpenes at birth. A very early defence, possible only in those species using salicin as the precursor for their defensive secretion, could be highly advantageous in protecting the clustered larvae during the long process of hatching and in avoiding cannibalism between siblings.
Only trace amounts of oleic acid were found in the eggs of Gastrophysa viridula , in contrast to previous reports on its presence in large quantities in the American G. cyanea. 相似文献
The two isoxazolinone glucosides (compounds 1 and 2), characteristic of the adult defence secretion, were detected in the eggs of all studied species. Compound 2, containing a nitropropionate, is always present in concentrations (above 10
The five Chrysomela species studied and Phratora vitellinae also sequester salicin in their eggs in amounts highly deterrent and toxic to ants. A single Chrysomela egg often contains enough salicin to kill an ant. While the isoxazolinones are discarded with the egg shells, salicin is used by neonate larvae as a precursor for the production of salicylaldehyde in the thoracic defence glands, already functional at hatching. No salicin could be detected in the eggs of those species whose larvae produce cyclopentanoid monoterpenes, even if they feed on Salicaceae. No larva of any species seems to be able to produce detectable amounts of monoterpenes at birth. A very early defence, possible only in those species using salicin as the precursor for their defensive secretion, could be highly advantageous in protecting the clustered larvae during the long process of hatching and in avoiding cannibalism between siblings.
Only trace amounts of oleic acid were found in the eggs of Gastrophysa viridula , in contrast to previous reports on its presence in large quantities in the American G. cyanea. 相似文献
973.
Growth form parameters ofCapsella bursa-pastoris populations, including a wide range of different environments, have been analyzed from random block field and growth chamber experiments. Changes in one character are often correlated with changes in another. Of special interest are correlations detected with the onset of first flowering. Variation in each of the characters is clearly influenced by both phenotypic and genotypic components. However, genotype — environment interactions are also subject to variation. Therefore, the adaptive significance of a given parameter is not found to be constant over the entire geographical range of the genus. Alpine populations tend to shift from annual to biannual life cycles.Part of a series Adaptation in life history traits of colonizing plant species. Part of a doctoral thesis by the first author. Dedicated to Prof. Dr.Karl Mägdefrau on the occasion of his 80th birthday. 相似文献
974.
Osbertia, a stoloniferous group confined to the montane regions of Mexico and adjacent Guatemala, was first proposed as a genus byGreene (1895), but most workers have retained the taxon as part ofHaplopappus. It is clearly closer toNoticastrum, Erigeron orHeterotheca than it is toHaplopappus sensu stricto. The present treatment recognizes two species, a widespread highly variableOsbertia stolonifera and a newly describedO. chihuahuana from northwestern Mexico. Distribution maps, distinguishing features, full synonymy and illustrations are presented. 相似文献
975.
Abstract The first step of aerobactin biosynthesis, oxidation of an aliphatic primary amino group to an N -hydroxy-amino compound seems to be involved in the biosynthesis of most of the hydroxamatetype siderophores which are widely distributed among bacteria and fungi. Therefore, the first step of aerobactin biosynthesis, oxidation of lysine to N 6 -hydroxylysine was studied as a model reaction using a strain of Escherichia coli that contains the first gene aerA of aerobactin synthesis on a multi-copy plasmid and which is lacking the gene for the subsequent step in the pathway. In addition, culture conditions are described which lead to the secretion of N 6 -hydroxylysine into the medium in amounts that can easily be quantitatively determined by a simple, reliable chemical assay. This assay can be used for screening inhibitors of the oxidation of α-amino groups, which should interfere with the biosynthesis of siderophore hydroxamates and thus should create bacteriostatic conditions. 相似文献
976.
Abstract The anaerobic, Gram-positive coccus Staphylococcus aureus ssp. anaerobius and its aerobic mutant MVF-SR, when kept under anaerobic conditions, excreted coproporphyrin (mainly type III) into the medium and enriched uroporphyrin (mainly type I) within the cells.
The rate of porphyrin synthesis stayed practically unaltered when the growth medium was supplemented with 50 μ g/ml 5-aminolevulinic acid (ALA), but was significantly enhanced upon supplementation with hemin (0.5 μ g/ml). When hemin and ALA were given simultaneously, a more than two-fold increase in porphyrin production compared to normal growth medium was observed. These observations indicate a stimulation of porphyrin synthesis in S. aureus by hemin.
An as yet unidentified violet pigment with an intense red-violet fluorescence under UV light ( λ = 366 nm) was found to be present in considerable amounts in cells of S. aureus ssp. anaerobius , whereas the supernatant medium of aerobically grown cells of the mutant MVF-SR contained an equally unidentified blue, non-fluorescing pigment. 相似文献
The rate of porphyrin synthesis stayed practically unaltered when the growth medium was supplemented with 50 μ g/ml 5-aminolevulinic acid (ALA), but was significantly enhanced upon supplementation with hemin (0.5 μ g/ml). When hemin and ALA were given simultaneously, a more than two-fold increase in porphyrin production compared to normal growth medium was observed. These observations indicate a stimulation of porphyrin synthesis in S. aureus by hemin.
An as yet unidentified violet pigment with an intense red-violet fluorescence under UV light ( λ = 366 nm) was found to be present in considerable amounts in cells of S. aureus ssp. anaerobius , whereas the supernatant medium of aerobically grown cells of the mutant MVF-SR contained an equally unidentified blue, non-fluorescing pigment. 相似文献
977.
Genes for human U4 small nuclear RNA 总被引:10,自引:0,他引:10
978.
Cucurbitaceae are characterized by a high copy number for nuclear ribosomal RNA genes. We have investigated the genomic ribosomal DNA (rDNA) of four closely related species of this family with respect to structure, length heterogeneity, and evolution. InCucumis melo (melon) there are two main length variants of rDNA repeats with 10.7 and 10.55kb.Cucumis sativus (cucumber) shows at least three repeat types with 11.5, 10.5, and 10.2kb.Cucurbita pepo (zucchini) has two different repeat types with 10.0 and 9.3kb. There are also two different repeat types inCucurbita maxima (pumpkin) of about 11.2 and 10.5kb. Restriction enzyme mapping of the genomic rDNA of these four plants and of cloned repeats ofC. sativus shows further heterogeneities which are due to methylation or point mutations. By comparison of the restriction enzyme maps it was possible to trace some evolutionary events in the family ofCucurbitaceae. Some aspects of regulation and function of the middle repetitive rRNA genes (here between 2000 and 10000 copies) are discussed. 相似文献
979.
Ken-ichi Miyaji 《Ecological Research》1986,1(3):303-322
In order to demonstrate in detail the relationship between the longevity and productivity of leaves within a canopy, a new
life table approach, the ‘bioeconomic life table’, was applied to the leaves of kidney bean plants (Phaseolus vulgaris L.) in relation to planting density and their position within the canopy.
The net photosynthetic rate for upper leaves under full daylight tended to decline gradually due to leaf senescence from about
20 days after leaf emergence, and for the lower leaves the decrease was very rapid due to both shading and senescence about
10 days after emergence.
Analysis of the survivorship curves and daily surplus production of leaves suggested that the lower and middle leaves, especially
the latter, survived without surplus production of dry matter after they had reached mean longevity, and while the upper leaves
at high density had a much shorter mean longevity, they had very large values of daily surplus production throughout the survival
period.
For the total foliage, the summed value of accumulated surplus production during the survival period was about five times
as large as the summed value of the dry weight of the dead leaves, regardless of planting density.
The daily rate of canopy leaf respiration was almost proportional to that of canopy gross photosynthesis for the various leaf
area indices of the canopy, so that there was no optimum leaf area index that maximized canopy daily surplus production. 相似文献
980.
The gibberellin (GA) 2-hydroxylases in mature and immature seeds of Pisum sativum have been partially purified and characterised. The enzymes are unstable when stored below pH 7.0 or in the absence of a thiol reagent. The optimum assay pH is between 7.4 and 7.8 and activity is dependent upon the presence of -ketoglutarate, Fe2+ and ascorbate. The 2-hydroxylase activities for GA1, GA4, GA9 and GA20 are chromatographically inseparable and correspond to a protein of Mr 44000. The rate of GA 2-hydroxylation varies according to substrate and some evidence indicates that the 2-hydroxylase activities for GA1 and GA4 and for GA9 and GA20 may reside in different proteins. During pea seed maturation, the specific activity of the enzyme(s) increases dramatically and reaches a maximum at a time when endogenous GA9, GA20, GA29 and GA51 are also at their greatest concentration. This correlation is not the result of substrate induction of enzyme activity. Since the GA 2-hydroxylases operate at maximal rate at low substrate concentrations they are incapable of rapidly 2-hydroxylating excessive quantities of (exogenously applied) GA1 or GA20. On the basis of the kinetic parameters of the GA 2-hydroxylase activities, a generalised model is discussed for the control of the steady-state levels of bioactive hormone under normal physiological conditions.Abbreviations DTE
dithioerythritol
- EDTA
ethylenediaminetetraacetic acid
- GAn
gibberellin An
- HPLC
high-performance liquid chromatography
- HSS
high-speed supernatant
- LSS
low-speed supernatant
- PMSF
phenylmethane sulphonyl fluoride 相似文献