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61.
Tree seed rain and seed removal,but not the seed bank,impede forest recovery in bracken (Pteridium aquilinum (L.) Kuhn)‐dominated clearings in the African highlands
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Considerable areas dominated by bracken Pteridium aquilinum (L.) Kuhn occur worldwide and are associated with arrested forest recovery. How forest recovery is impeded in these areas remains poorly understood, especially in the African highlands. The component processes that can lead to recruitment limitation—including low seed arrival, availability and persistence—are important determinants of plant communities and offer a potential explanation for bracken persistence. We investigated key processes that can contribute to recruitment limitation in bracken‐dominated clearings in the Bwindi Impenetrable National Park, Uganda. We examined if differences in seed rain (dispersal limitation), soil seed bank, or seed removal (seed viability and persistence) can, individually or in combination, explain the differences in tree regeneration found between bracken‐dominated areas and the neighboring forest. These processes were assessed along ten 50‐m transects crossing the forest–bracken boundary. When compared to the neighboring forest, bracken clearings had fewer seedlings (bracken 11,557 ± 5482 vs. forest 34,515 ± 6066 seedlings/ha), lower seed rain (949 ± 582 vs. 1605 ± 335 tree seeds m?2 year?1), comparable but sparse soil seed bank (304 ± 236 vs. 264 ± 99 viable tree seeds/m2), higher seed removal (70.1% ± 2.4% vs. 40.6% ± 2.4% over a 3‐day interval), and markedly higher rodent densities (25.7 ± 5.4 vs. 5.0 ± 1.6 rodents per 100 trapping sessions). Camera traps revealed that rodents were the dominant animals visiting the seeds in our seed removal study. Synthesis: Recruitment limitation contributes to both the slow recovery of forest in bracken‐dominated areas, and to the composition of the tree species that occur. Low seed arrival and low persistence of unburied seeds can both explain the reduced density of seedlings found in bracken versus neighboring forest. Seed removal, likely due to rodents, in particular appears sufficient to constrain forest recovery and impacts some species more severely than others. 相似文献
62.
Fredrick H. Bahls Dennis G. Emery Philip G. Haydon 《Invertebrate neuroscience : IN》1995,1(2):123-131
In this study, we have further characterized the morphology and physiology of the neuroglandular synapse between the identified
buccal neuron, B4, and the salivary gland ofHelisoma. We demonstrate that the coupling coefficient between salivary cells within an individual acinus is approximately 1.0. We
also demonstrate that synapses within the salivary gland are located near a superficial muscle layer. We examine the effects
of glutamate on the salivary gland and on the B4-salivary gland EPSP.l-glutamate produces a transient, rapid onset depolarization of salivary gland cells. The response is mimicked by high concentrations
ofl-homocysteic acid, but not by NMDA,l-aspartate,d-glutamate or kainate. The response is blocked by the presence ofl- ord-glutamate in the bath, but not by CNQX, DNQX, DGG,d-AP5, orl-AP3. The depolarization is primarily dependent on the presence of calcium in the bathing solution. When eitherl- ord-glutamate is present in the bathing solution, the amplitude of the B4-salivary gland EPSP is reversibly reduced. The similar
pharmacological properties of the response of the salivary gland to glutamate and the B4 epsp indicate thatl-glutamate is a strong candidate for the fast excitatory neurotransmitter at theHelisoma neuroglandular synapse. 相似文献
63.
Clement Zeh Seth C. Inzaule Pascale Ondoa Lillian G. Nafisa Alex Kasembeli Fredrick Otieno Hilde Vandenhoudt Pauli N. Amornkul Lisa A. Mills John N. Nkengasong 《PloS one》2016,11(2)
Objective
To identify unique characteristics of recent versus established HIV infections and describe sexual transmission networks, we characterized circulating HIV-1 strains from two randomly selected populations of ART-naïve participants in rural western Kenya.Methods
Recent HIV infections were identified by the HIV-1 subtype B, E and D, immunoglobulin G capture immunoassay (IgG BED-CEIA) and BioRad avidity assays. Genotypic and phylogenetic analyses were performed on the pol gene to identify transmitted drug resistance (TDR) mutations, characterize HIV subtypes and potential transmission clusters. Factors associated with recent infection and clustering were assessed by logistic regression.Results
Of the 320 specimens, 40 (12.5%) were concordantly identified by the two assays as recent infections. Factors independently associated with being recently infected were age ≤19 years (P = 0.001) and history of sexually transmitted infections (STIs) in the past six months (P = 0.004). HIV subtype distribution differed in recently versus chronically infected participants, with subtype A observed among 53% recent vs. 68% chronic infections (p = 0.04) and subtype D among 26% recent vs. 12% chronic infections (p = 0.012). Overall, the prevalence of primary drug resistance was 1.16%. Of the 258 sequences, 11.2% were in monophyletic clusters of between 2–4 individuals. In multivariate analysis factors associated with clustering included having recent HIV infection P = 0.043 and being from Gem region P = 0.002.Conclusions
Recent HIV-1 infection was more frequent among 13–19 year olds compared with older age groups, underscoring the ongoing risk and susceptibility of younger persons for acquiring HIV infection. Our findings also provide evidence of sexual networks. The association of recent infections with clustering suggests that early infections may be contributing significant proportions of onward transmission highlighting the need for early diagnosis and treatment as prevention for ongoing prevention. Larger studies are needed to better understand the structure of these networks and subsequently implement and evaluate targeted interventions. 相似文献64.
65.
Semienclosed Tube Cultures of Bean Plants (Phaseolus vulgaris L.) for Enumeration of Rhizobium phaseoli by the Most-Probable-Number Technique 总被引:4,自引:4,他引:0
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Ricardo S. Araujo Jaime Maya-Flores Deborah Barnes-McConnell Charles Yokoyama Frank B. Dazzo Fredrick A. Bliss 《Applied microbiology》1986,52(4):954-956
The semienclosed tube culture technique of Gibson was modified to permit growth of common bean (Phaseolus vulgaris L.) roots in humid air, enabling enumeration of the homologous (nodule forming) symbiont, Rhizobium phaseoli, by the most-probable-number plant infection method. A bean genotype with improved nodulation characteristics was used as the plant host. This method of enumeration was accurate when tubes were scored 3 weeks after inoculation with several R. phaseoli strains diluted from aqueous suspensions, peat-based inoculants, or soil. A comparison of population sizes obtained by most-probable-number tube cultures and plate counts indicated that 1 to 3 viable cells of R. phaseoli were a sufficient inoculant to induce nodule formation. 相似文献
66.
The ability of reactive dyes based on chloro-s-triazine to reactwith the primary and secondary hydroxyl groups of glucose, formingcovalent bonds, has been used to dye the storage polyglucosidesof the algae, Cytmidium caldarium, Oscillatoria princeps, Rhodymeniapertusa and Spirogyra setiformis, prior to electrophoresis oncellulose polyacetate strips in borate buffer. The techniqueresolves mixtures of glucans and defines some of the parametersof the internal structure of these glucans. The similarity ofthe glucans from Cyanidium caldarium and Oscillatoria princepsis evident using this method. The two components of the "starch"from Spirogyra setiformis have been studied and identified bytheir reactions with the dye and the mobilities of the dyedcomponents during electrophoresis. Inferences as to the biphyleticevolutionary pathways leading from the Cyanophyceae to the Rhodophyceaeand the Chlorophyceae can be made based on the data. (Received June 27, 1973; ) 相似文献
67.
Fredrick J. Seil 《The Western journal of medicine》1972,116(5):25-33
An animal model for acute multiple sclerosis (ms) is experimental allergic encephalomyelitis (eae). eae is produced by intradermal injection of a protein component of central nervous system (cns) myelin. Ultrastructural studies of eae and of a peripheral nerve analog, experimental allergic neuritis (ean), have revealed an orderly sequence of cellular events leading to the destruction and removal of myelin with sparing of axons (primary demyelination). Acute ms has not been studied electron microscopically, but the ultrastructural similarities between ean and a case of acute Landry-Guillain-Barré syndrome, a primary demyelinating disease of the peripheral nervous system, suggest that a similar sequence of events might be found in acute ms. While the pathological findings support a cellmediated or delayed hypersensitivity response, there is also evidence for the pathogenetic role of circulating antibodies. Among such evidence is included the finding that sera from animals with eae and humans with acute ms rapidly produce a reversible block of complex (polysynaptic) electrical activity when applied to cns tissue cultures, which suggests a possible mechanism for transient symptoms in ms. Epidemiological and other studies link ms with a viral cause, although no direct evidence that ms is caused by a virus exists. Viral and immunological mechanisms are not mutually exclusive in considering pathogenetic possibilities for ms, for it can be postulated that a viral infection of the central nervous system acts as a triggering agent for a series of immune responses, including production of a bioelectric blocking antibody and demyelination mediated by sensitized cells, the combination of which ultimately produces the total clinical picture of ms. 相似文献
68.
69.
Animals use a variety of escape mechanisms to increase the probability of surviving predatory attacks. Antipredator defenses can be elaborate, making their evolutionary origin unclear. Trap-jaw ants are known for their rapid and powerful predatory mandible strikes, and some species have been observed to direct those strikes at the substrate, thereby launching themselves into the air away from a potential threat. This potential escape mechanism has never been examined in a natural context. We studied the use of mandible-powered jumping in Odontomachus brunneus during their interactions with a common ant predator: pit-building antlions. We observed that while trap-jaw ant workers escaped from antlion pits by running in about half of interactions, in 15% of interactions they escaped by mandible-powered jumping. To test whether escape jumps improved individual survival, we experimentally prevented workers from jumping and measured their escape rate. Workers with unrestrained mandibles escaped from antlion pits significantly more frequently than workers with restrained mandibles. Our results indicate that some trap-jaw ant species can use mandible-powered jumps to escape from common predators. These results also provide a charismatic example of evolutionary co-option, where a trait that evolved for one function (predation) has been co-opted for another (defense). 相似文献
70.
Fredrick Nyongesa Kassilly 《African Journal of Ecology》2003,41(2):187-189