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31.
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Abstract Effects of bednets impregnated with permethrin 200 mg and 500mg/m2 on pyrethroid resistant and susceptible strains of Anopheles stephensi and their F hybrid progeny were studied, using free-flying female mosquitoes of these three genotypes, in a room with a human subject under a polyester net, having one of his arms in contact with the treated netting. Unexpectedly an apparently higher feeding rate, but lower knockdown and mortality rates, of mosquitoes were obtained for each of the three genotypes with the higher concentration of 500mg/m2 compared with the lower dose of 200mg/m2. At the lower dose there was 100% mortality 24 h after exposure of all three genotypes, suggesting that there would not be selection for resistance at this dose. However, at the higher dose there was significantly higher mortality of the susceptible strain than of the F hybrids, suggesting incomplete recessiveness of this resistance and that there would therefore be effective selection for resistance by this dose.
When female mosquitoes were confined in bioassay cones on treated netting, the resistant strain of An.stephensi showed significantly less irritability (scored as the time until first flight take-off) in response to each dose, as compared with the susceptible strain and F, hybrids. The higher dose provoked more irritation of each genotype; this could explain the greater knockdown and mortality rates of mosquitoes exposed to the lower dose which was less irritating and hence more effectively insecticidal. Thus a dose of 200mg/m2 is preferable to 500mg/m2 for malaria vector control.  相似文献   
33.
GABA and Prolonged Spinal Inhibition   总被引:2,自引:0,他引:2  
TWO explanations have been provided for the relatively long latency and prolonged (often exceeding 100 ms) inhibition of firing of spinal motoneurones which is caused by repetitive impulses produced by electrical or natural stimulation1–4 in muscle and cutaneous afferent fibres. This prolonged inhibitory process is exemplified by the reduction in the amplitude of monosynaptic excitatory synaptic potentials (EPSPs) and reflexes of extensor motoneurones by tetanic stimulation of group I afferents of flexor motoneurones2. In contrast with “direct” inhibition, the prolonged inhibition is not reduced by strychnine but is diminished by Picrotoxin4,6.  相似文献   
34.
Thermal stress is one environmental parameter that has greatlyinfluenced the migration of crustaceans from the sea to land.Since a greater number of species of terrestrial crabs are foundin the tropics than in the temperate zone, comparative studiesof the influence of temperature on latitudinally separated populationswere undertaken. Two tropical species, U. rapax and U. thayeri,may occur as far north as St. Augustine, Florida, or, followinga severe winter, may be rare north of Cape Kennedy. The lethaleffect of the low temperatures recorded during one severe winter(1957–58) is supported by laboratory studies in whichLD50, deaths occurred in 4.5 days at 10°C for U. rapax acclimatedto 18°C. The experiment demonstrates that U. rapax cannotacclimate to and survive low temperatures. This contrasts markedlywith the situation in semi-terrestrial crabs of the temperatezone, which are able to acclimate to cold. The distribution of Uca around Cape Cod Bay correlates wellwith the coastal hydrographic thermal gradient and supportsPassano's suggestion that temperatures below 20° may belimiting as they inhibit proecdysis in U. pugnax. Such an inhibitionis found experimentally in U. pugilator and in the tropicalspecies, U. rapax. It is hypothesized that a shift in the thermodynamicsof the processes underlying molting has not occurred in Ucaof the temperate zone. The paucity of semi-terrestrial Brachyurain the temperate zone may be due to the failure of many speciesto evolve capacity-adaptations to carry out all requisite lifeprocesses at temperatures below 20°, or the resistance-adaptationsnecessary to survive the low temperatures of winter.  相似文献   
35.
1. Mature chloroplasts are able to synthesize a wide variety of compounds of low molecular weight in addition to carbohydrates. 2. Mature chloroplasts from higher plants can synthesize fatty acids from acetate, and galactolipids from UDP-galactose; but, thus far, there is no direct evidence that chloroplasts can produce their principal fatty acid, α-linolenate, independently of the rest of the cell. 3. Chloroplasts possess the enzymic machinery necessary to generate most of the common amino acids from inorganic nitrogen plus appropriate a-keto analogs of amino acids. However, the plastids do not appear able to synthesize many α-keto carbon compounds from the initial products of photosynthetic carbon dioxide fixation. 4. Whether chloroplasts can generate their own supply of acetate remains in doubt. 5. There is little evidence for or against the existence of chloroplastic enzymes catalysing synthesis of purines and pyrimidines. 6. Recent evidence confirms that immature plastids possess the complement of enzymes required for synthesis of protochlorophyllide from 8-aminolaevulinic acid but leaves open the possibility that extrachloroplastic cofactors may be involved in protochlorophyllide biosynthesis. 7. The weight of the available evidence suggests that, despite its great metabolic versatility and possible reproductive autonomy, the chloroplast of the higher plant is not metabolically autonomous or nutritionally independent of the remainder of the plant cell. Therefore, if there is any validity to the oft-repeated speculation that chloroplasts have evolved from ancient free-living procaryotes, it appears that the evolution of the chloroplast has led to a considerable loss of nutritional autonomy concomitant with the development or preservation of photosynthetic competence.  相似文献   
36.
BIOSYNTHESIS OF SMALL MOLECULES IN CHLOROPLASTS OF HIGHER PLANTS   总被引:1,自引:0,他引:1  
1. Chloroplasts of higher plants contain enzymes which permit them to synthesize many kinds of small molecules in addition to carbohydrates. 2. Either aqueous or non-aqueous techniques may be used to isolate chloroplasts. Aqueous methods permit the isolation of chloroplasts showing high rates of photosynthesis; the organelles can be purified by means of density gradients. Non-aqueously isolated chloroplasts cannot photosynthesize, but show good retention of low-molecular-weight substances and soluble enzymes. 3. Whole cells photoassimilating 14CO2 show considerable formation of 14C-labelled amino acids and lipids, but isolated chloroplasts exhibit very poor synthesis of amino acids and lipids from 14CO2. 4. Chloroplasts play an important rôle in reducing nitrate to ammonia. There is controversy about the presence in chloroplasts of nitrate reductase and about the mechanism of the light-dependent reduction of nitrate to nitrite; however, it is generally agreed that non-cyclic electron transport directly supports reduction of nitrite to ammonia via a chloroplastic nitrite reductase. 5. Chloroplasts actively assimilate inorganic nitrogen into amino acids. The assimilation reaction is either the reductive amination of α-ketoglutarate to glutamate or the ATP-dependent conversion of glutamate to glutamine. The enzyme glutamate synthase has recently been found to be present in chloroplasts and may play an important function in nitrogen assimilation. 6. Numerous transaminases (aminotransferases) are present in chloroplasts. 7. The source of α-keto-acid precursors of chloroplastic amino acids is unknown. It remains to be established whether chloroplasts import the required keto acids or whether some of them might be generated via an incomplete tricarboxylic-acid cycle located in the chloroplast. 8. Chloroplasts contain characteristically high levels of mono and digalactosyl diglycerides, sulpholipid and phosphatidyl glycerol. They also have large amounts of polyunsaturated fatty acids. 9. Fatty acids are synthesized by the concerted action of fatty-acid synthetase, elongases and desaturases. Two pathways have been implicated for the formation of α-linolenic acid. 10. The galactosyldiglycerides are synthesized by successive galactosylation of diglyceride. The enzymes responsible are probably located in the chloroplastic envelope. 11. The other major chloroplastic acyl lipids (sulpholipid, phosphatidylglycerol and phosphatidylcholine) have not been, as yet, synthesized de novo by means of isolated chloroplast fractions. However, indirect evidence indicates that the first two are probably formed there. 12. Chlorophyllide synthesis involves the formation of δ-aminolaevulinic acid (δALA) followed by conversion of δALA to protoporphyrin IX, which is then transformed into protochlorophyll. 13. Recent evidence favours the view that δALA synthesis is not mediated by δALA synthetase but by another pathway in which δALA can be derived from α-ketoglutarate or glutamate. It has not been established whether this pathway is localized in plastids. 14. Conversion of δALA to protoporphyrin IX is mediated by soluble enzymes of the plastid stroma. Membrane-bound enzymes mediate the conversion of protoporphyrin to protochlorophyll. 15. Carotenoids are synthesized from acetyl CoA via geranylgeranyl-pyrophosphate and phytoene intermediates. Evidence has been obtained for both neurosporene and lycopene as precursors of the cyclic carotenoids. 16. The overall pathway of carotenoid formation is subject to photoregulation, particularly during the development of the chloroplast. 17. Carotenes are precursors of xanthophylls, the inserted oxygen being derived from molecular oxygen. 18. Chloroplasts may synthesize or interconvert gibberellin hormones.  相似文献   
37.
Asymmetrin was isolated as the sodium salt from culture filtrateof Penicillium thomii. By comparison with hadacidin (N-formylhydroxyamino-acetic acid) we concluded that the two compoundsare identical. 1Journal Paper No. 2329 of the Purdue Agricultural ExperimentStation. Supported in part by Grant G-20989 from the NationalScience Foundation to R. W. CURTIS. 2Present address: United Fruit Company, Norwood, Massachusetts.  相似文献   
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39.
Forest harvesting and wildfire were widespread in the upper Great Lakes region of North America during the early 20th century. We examined how long this legacy of disturbance constrains forest carbon (C) storage rates by quantifying C pools and fluxes after harvest and fire in a mixed deciduous forest chronosequence in northern lower Michigan, USA. Study plots ranged in age from 6 to 68 years and were created following experimental clear‐cut harvesting and fire disturbance. Annual C storage was estimated biometrically from measurements of wood, leaf, fine root, and woody debris mass, mass losses to herbivory, soil C content, and soil respiration. Maximum annual C storage in stands that were disturbed by harvest and fire twice was 26% less than a reference stand receiving the same disturbance only once. The mechanism for this reduction in annual C storage was a long‐lasting decrease in site quality that endured over the 62‐year timeframe examined. However, during regrowth the harvested and burned forest rapidly became a net C sink, storing 0.53 Mg C ha−1 yr−1 after 6 years. Maximum net ecosystem production (1.35 Mg C ha−1 yr−1) and annual C increment (0.95 Mg C ha−1 yr−1) were recorded in the 24‐ and 50‐year‐old stands, respectively. Net primary production averaged 5.19 Mg C ha−1 yr−1 in experimental stands, increasing by < 10% from 6 to 50 years. Soil heterotrophic respiration was more variable across stand ages, ranging from 3.85 Mg C ha−1 yr−1 in the 6‐year‐old stand to 4.56 Mg C ha−1 yr−1 in the 68‐year‐old stand. These results suggest that harvesting and fire disturbances broadly distributed across the region decades ago caused changes in site quality and successional status that continue to limit forest C storage rates.  相似文献   
40.
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