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1.
Andrea J Webster Andy Purvis 《Proceedings. Biological sciences / The Royal Society》2002,269(1487):143-149
Many methods are available for estimating ancestral values of continuous characteristics, but little is known about how well these methods perform. Here we compare six methods: linear parsimony, squared-change parsimony, one-parameter maximum likelihood (Brownian motion), two-parameter maximum likelihood (Ornstein-Uhlenbeck process), and independent comparisons with and without branch-length information. We apply these methods to data from 20 morphospecies of Pleistocene planktic Foraminifera in order to estimate ancestral size and shape variables, and compare these estimates with measurements on fossils close to the phylogenetic position of 13 ancestors. No method produced accurate estimates for any variable: estimates were consistently less good as predictors of the observed values than were the averages of the observed values. The two-parameter maximum-likelihood model consistently produces the most accurate size estimates overall. Estimation of ancestral sizes is confounded by an evolutionary trend towards increasing size. Shape showed no trend but was still estimated very poorly: we consider possible reasons. We discuss the implications of our results for the use of estimates of ancestral characteristics. 相似文献
2.
Sieve element pores in Nicotiana pith culture 总被引:3,自引:0,他引:3
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Cryptococcus neoformans was instilled intranasally into mice which were periodically sacrificed to determine the course of infection. Cryptococci persisted within the nasal passages throughout the 90 day study. Extranasal dissemination began 14–28 days after instillation and was still demonstrable 90 days post-exposure. Ten percent mortality was observed in mice receiving 106 cryptococci, while no mortality was observed in mice exposed to 103 or 104 cryptococci. Our research suggests that nasal colonization with C. neoformans can precede pulmonary and systemic cryptococcosis by weeks or months. 相似文献
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Thalamic stimulation for control of movement disorders 总被引:5,自引:0,他引:5
O J Andy 《Applied neurophysiology》1983,46(1-4):107-111
Chronic recurrent thalamic stimulation has been effective in alleviating a variety of movement disorders. In contrast to thalamic lesions, it is preferred for the treatment of intractable motor disorders in low-risk elderly patients and patients with diffuse brain lesions secondary to trauma. Abnormal diencephalic electrical discharges have been observed and thought to be associated, in some way, with either generating or sustaining the movement abnormalities. The beneficial effects are ascribed to an electrophysiologic functional ablation of the discharging systems. This interpretation is based on the observation that the diencephalic discharges are attenuated by the applied stimulation and that the beneficial effects are reversible even after several months of applied therapeutic stimulation. 相似文献
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Summary TIBA has been applied laterally to actively growing stems of uprightAcer rubrum seedlings. The frequency of initiation of tracheary elements is reduced and a complete ring of tension wood is developed in the stem locally below the TIBA application site. Rings of tension wood were never formed above the TIBA treatment site. In regard to anatomy, lignin distribution and peroxidase activity, the tension wood fibers formed as a result of TIBA treatment are identical to those which can be induced by bending.In the region of the stem above the site of TIBA application there is a particularly strong alteration in the development of tracheary elements.Application of IAA, NAA, or 2,4-D to the TIBA treatment site suppresses the capacity of TIBA to induce the development of tension wood and at the same time generally increases the frequency of initiation of tracheary elements.The effect of auxin alone on theAcer rubrum system has been studied. The secondary xylem formed during treatment with auxins (especially 2,4-D and NAA) at the stated concentrations is generally characterized by large groups of tracheary elements with a conspicuous angular outline in transverse section.The evidence suggests that auxins are involved in the regulatory systems which bring about the orderly development of the secondary xylem in arborescent angiosperms.This material was included in a doctoral thesis submitted by P. R.Morey to the graduate school of Yale University, New Haven. 相似文献