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941.
Age determination on the basis of ring count holds great potential for the study of dry woodland ecosystem dynamics. Discs from stems of Acacia tortilis from Marsabit District, northern Kenya, were collected for dendrochronological analysis. Problems in the identification of growth rings are summarized. Regression analysis is used to develop age–ring count and ring count–size relationships. The time seedlings spend in a browse‐suppressed state is also estimated. It is concluded that there is insufficient evidence for the development of statistically valid age–size relationships. Problems that current techniques have yet to overcome are identified. Future studies are recommended using cambial injury techniques, particular emphasis being placed on the potential effects of browsing on cambial activity. 相似文献
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DAVID TAYLOR FABIO FALSETTI FERGAL O'BRIEN PETER O'REILLY EOIN BANVILLE T. CLIVE LEE 《Computer methods in biomechanics and biomedical engineering》2013,16(5):413-420
A theoretical model has been developed for predicting the extent of bone adaptation as a function of exercise level. The model works by estimating the amount of fatigue damage which will occur for a given cyclic strain. This allowed us to predict that there would be a difference in the strengths of bones taken from sedentary individuals compared to those from more active individuals. The predicted difference, expressed as the ratio of the fatigue strengths of the two bones, was 1.53. Experimental tests were carried out using bones from chickens, comparing those kept in battery cages (who are unable to engage in any strenuous activity) with those kept in normal free-range conditions. A finite element model was used to ensure that the in vitro fatigue tests were conducted at strain levels within the physiological range. Significant differences in both static and fatigue strength were found for the radius and tibia bones. These differences could be accurately predicted by the theoretical model, after a slight difference in weight between the two groups was allowed for. Additional evidence for these predictions can also be found in the literature. This work supports the concept that bone adaptation is driven by the need to optimise fatigue damage accumulation. 相似文献
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DAVID S. WOODRUFF STEPHENJAY GOULD 《Biological journal of the Linnean Society. Linnean Society of London》1980,14(3-4):389-416
Cerion's exuberant morphological diversity (600 described 'species') combined with the extreme rarity of reproductive isolation among morphotypes (only one unambiguous case of sympatry in the Bahamas) has long made this genus an object of fascination for evolutionists and of frustration for taxonomists. We have pursued an integrated approach of genetic and morphometric study based on field investigations of ecology and biogeography in the Bahamas. Cerion's morphotypes are not distributed haphazardly, but show definite patterns of correlation with habitat and geography. Although all morphotypes interbreed, hybrid zones tend to be narrow and characterized by highly local genetic anomalies–unique alleles present in neither parental population. Different patterns of covariance in ontogeny, and habitat preferences, also indicate mat the morphotypes are distinctive, non-amalgamating entities (despite little difference in the frequencies of structural genes among them) that may be called species once a definition based on strict reproductive isolation is abandoned. Variation in structural genes, anatomy and morphology is non-concordant, but orderly for each criterion. Similar morphologies are often polyphyletic and evolved repeatedly as one possible ontogenetic route within a developmental program common to all Cerion. Although we cannot always distinguish among competing causes for observed patterns, we can establish genetic, morphological, anatomical, and biogeographic criteria for decisions when adequate evidence is available. The species of Cerion will be reduced by more than an order of magnitude from a list currently described. 相似文献
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ADELINE SOULIER-PERKINS DAVID OUVRARD MARC ATTIÉ THIERRY BOURGOIN 《Systematic Entomology》2007,32(2):305-311
Abstract. The association between the Lophopidae (Hemiptera, Fulgoromorpha) and their host plants was studied within a phylogenetic framework. Host plant use was optimized on Lophopidae phylogeny and the most parsimonious hypothesis is presented. This hypothesis describes the evolution of host plant use by the Lophopidae, and postulates the ancestral plant family used. This scenario is discussed within the biogeographical evolutionary context of the fulgoromorphan families, and is corroborated by information from both insect and host plant fossils. The association of the Lophopidae and their host plants is made by comparing the angiosperms and Lophopidae phylogenies, demonstrating at this level of comparison that the insects show 'taxonomic conservatism' for their host plants. 相似文献