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BRYCE C. LAKE JOEL A. SCHMUTZ MARK S. LINDBERG CRAIG R. ELY WILLIAM D. ELDRIDGE & FRED J. BROERMAN 《Ibis》2008,150(3):527-540
We studied body mass of prefledging Emperor Geese Chen canagica at three locations across the Yukon–Kuskokwim Delta, Alaska, during 1990–2004 to investigate whether large-scale variation in body mass was related to interspecific competition for food. From 1990 to 2004, densities of Cackling Geese Branta hutchinsii minima more than doubled and were c . 2–5× greater than densities of Emperor Geese, which were relatively constant over time. Body mass of prefledging Emperor Geese was strongly related (negatively) to interspecific densities of geese (combined density of Cackling and Emperor Geese) and positively related to measures of food availability (grazing lawn extent and net above-ground primary productivity (NAPP)). Grazing by geese resulted in consumption of ≥ 90% of the NAPP that occurred in grazing lawns during the brood-rearing period, suggesting that density-dependent interspecific competition was from exploitation of common food resources. Efforts to increase the population size of Emperor Geese would benefit from considering competitive interactions among goose species and with forage plants. 相似文献
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JOHN C. BEIER FRED K. ONYANGO MUTALIB RAMADHAN JOSEPH K. KOROS CHARLES M. ASIAGO ROBERT A. WIRTZ DAVY K. KOECH CLIFFORD R. ROBERTS 《Medical and veterinary entomology》1991,5(1):63-70
The number of malaria sporozoites in the salivary glands was determined microscopically for 1137 wild, naturally infected Anopheles from western Kenya. Infective Anopheles gambiae Giles sensu lato (n = 874) contained a geometric mean (GM) of 962 sporozoites and An.funestus Giles (n = 263) contained 812. No significant differences were detected in geometric mean numbers of sporozoites between species, collection techniques or sites. Of the infective An.gambiae, 1.7% (15/874) contained more than 41,830 sporozoites, the maximum observed for An.funestus. Microscopic techniques were found to be more sensitive than enzyme-linked immunosorbent assays (ELISA) for detecting low-grade sporozoite infections in salivary glands. Salivary gland sporozoites from 83.6% of the 1137 gland infections were identified by ELISA as either Plasmodium falciparum Welch (n = 910), P.ovale Stephens (n = 7), P.malariae Grassi & Feletti (n = 3) or mixed (n = 30). The 187 gland infections which could not be identified by ELISA contained significantly fewer sporozoites (GM = 242) than those which could be identified (GM = 1200). 相似文献
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JAMES F. SMOTHERS MALAVI T. MADIREDDI FRED D. WARNER C. DAVID ALLIS 《The Journal of eukaryotic microbiology》1997,44(2):79-88
ABSTRACT. Programmed DNA rearrangements, including DNA degradation, characterize the development of the soma from the germline in a number of developmental systems. Pddl p (programmed DNA degradation 1 protein), a development-specific polypeptide in Tetrahymena , is enriched in developing macronuclei (anlagen) and has been implicated in DNA elimination and nucleolar biogenesis. Here, immunocytochemistry and fluorescent in situ hybridization (FISH) were employed to follow Pddl p and two nucleolar markers (Nopp52 and rDNA) during macronuclear development. Both Pdd 1p and Nopp52 localize to subnuclear structures, each of which resemble nucleoli. However, while true nucleoli form and persist during development, Pdd 1p-positive structures are only present for a brief period of macronuclear differentiation. Accordingly, two distinct organelles can be recognized in anlagen: (1) Pdd 1p-positive structures, which lack Nopp52 and rDNA, and (2) developing nucleoli which contain rDNA and Nopp52 but lack Pdd 1p. Taken together with recent data corroborating Pdd 1p's role in DNA elimination, we favor the hypothesis that Pdd 1p structures are unique, short-lived organelles, likely to function in programmed DNA degradation and not in nucleolar biogenesis. 相似文献
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Stereospecific Glucocorticoid Binding to Subcellular Fractions of the Sensitive and Resistant Lymphosarcoma P1798 总被引:5,自引:0,他引:5
WE have shown that in vitroexposure of cells of these PI798 tumours to physiological doses of glucocorticoids produces significant inhibition of DNA, RNA and protein metabolism1–3 and glucose uptake2, 4 only in the glucocorticoid-sensi-tive tumour. These glucocorticoid-induced inhibitions can be correlated with the inhibition of growth and subsequent regression and dissolution of the sensitive tumour. In this study we have carried out competition experiments using the two 11-hydroxy isomers of Cortisol which have been shown to have the requisite stereospecific restraints necessary to show specific glucocorticoid binding5. It would be reasonable to suppose that both the 11α and 11β-isomers of Cortisol would compete almost equally for nonspecific binding sites of triamcinolone acetonide. But since only 11β-cortisol has glucocorticoid activity, only this isomer could compete with triamcinolone acetonide for specific binding. Thus, the difference in displacement caused by these two 11-hydroxy isomers is an indication of the specific binding of 3H-triamcinolone acetonide to different fractions of the PI 798 tumour. 相似文献
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FRED R. GANDERS 《Botanical journal of the Linnean Society. Linnean Society of London》1972,65(2):211-221
Sporophytes from natural populations of Thelypteris palustris and Onoclea sensibilis were subjected to breeding tests to reveal the presence of recessive lethal alleles in their genotypes. All sporophytes were found to be heterozygous for recessive sporophytic lethals. Despite the capability for producing completely homozygous genotypes by intragametophytic self-fertilization, ferns in natural populations seem to retain considerable genetic loads of deleterious recessive alleles. It is suggested that natural selection operates to strongly favour heterozygous rather than homozygous sporophytes under most conditions. 相似文献