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101.
SYNOPSIS. Cladistic analysis of mayflies suggests that dispersalwas very asymmetrical after the new land connection betweenNorth and South America. Twenty-one genera apparently movedfrom South to North and Central America, but there is good evidencefor only one North American genus moving into the south. Testablepredictions are possible once the boreal (Laurasian) or austral(Gondwanian) designations are made. For example, Paracloeodes,a genus of austral origin, was predicted to be found in SouthAmerica, and in North America in warm rivers north and eastof its known distribution. These predictions have been confirmed.Other characteristics of tropical mayflies, such as length oflarval period and emergence and mating patterns, may be usedto predict habitats and characteristics of present North Americangenera. Merger events and consequent dispersal of organismshave profound influences on distributional patterns, and fromsuch information, biologically useful generalities can be made. 相似文献
102.
T H Wise D Caton W W Thatcher A R Lehrer M J Fields 《Journal of reproduction and fertility》1982,66(2):513-518
Catheters were placed in the carotid artery via a facial artery (n = 12) and in the ovarian vein (n = 12), and, in conjunction, electromagnetic flow meters were placed around the ovarian artery (n = 6) in cyclic beef cows. Androstenedione was quantitatively the highest and dehydroepiandrosterone the lowest of the ovarian androgens measured. Ovarian androgens were correlated positively with each other (P less than 0.05) but not with ovarian blood flow or day of the cycle. There was a trend for spikes of androgen release (ovarian vein concentration x ovarian blood flow) from the ovary to be greatest during the period of decreasing progesterone and CL regression. However, only with testosterone were spikes of release different (Days--13 to--9 less than Days -8 to -4; P less than 0.05; Day 0 = oestrus). The dynamic changes in ovarian androgens noted in this study were compatible with the concept of continuous follicular development and atresia throughout the oestrous cycle. 相似文献
103.
Involvement of deoxyribonucleic acid polymerase III in W-reactivation in Bacillus subtilis 总被引:3,自引:3,他引:0 下载免费PDF全文
6-(p-Hydroxyphenylazo)-uracil, a purine analog that is known to specifically inhibit deoxyribonucleic acid polymerase III in gram-positive organisms, inhibited W-reactivation in Bacillus subtilis. On the other hand, W-reactivation proceeded normally in the presence of 6-(p-hydroxyphenylazo)-uracil when a strain possessing a resistant deoxyribonucleic acid polymerase III was used as the host. 相似文献
104.
George R. McGhee JR. 《Lethaia: An International Journal of Palaeontology and Stratigraphy》1976,9(2):111-136
Four benthic marine communities occur in the clastic facies of the prograding Upper Frasnian-Lower Famennian (Upper Devonian) Foreknobs Formation in the Central Appalachians along the Allegheny Front in Maryland, West Virginia, and Virginia. Deep-water, rapidly prograding environments were inhabited by the Ambocoelia-Chonetes Community, dominated by an epifauna of unattached brachiopods. Offshore bar environments were inhabited by the Cyrtospirifer-Camarotoechia Community, exhibiting adaptations to shallow-water, high-energy conditions and probably lowered salinities. Shallow-water, sublittoral environments were inhabited by the Atrypa-Cypricardella Community, a community in which existed a variety of life habitats and a diverse epifaunal and infaunal association of brachiopods and bivalve molluscs. The Leptodesma-Tylothyris Community flourished in nearshore bar-protected environments in the southern region of the study area, whereas in the north the Cyrtospirifer-Camarotoechia Community inhabited nearshore environments in conjunction with the onshore development of a large fluviodeltaic system. 相似文献
105.
Lipids were extracted from fresh, field-grown coastal bermudagrass (Cynodon dactylon (L.) Pers.) and the fatty acids determined by gas chromatography. Total fatty acid levels (dry weight basis) increased during the day and reached a single maximum at sunset in 2-week-old grass; whereas, in older grass, the fluctuations in fatty acid levels showed two maxima. The first maximum occurred 4 h after sunrise and the second maximum occurred at sunset. Total fatty acid levels, based on dry weight, decreased during the first 6 weeks of growth and changed very little after an additional 4 weeks' growth in bermudagrass leaf blades. Chlorophyll levels (dry weight basis) continuously decreased during the entire growth period (10 weeks). Chlorophyll alb ratios increased at sunset in 2- and 6-week-old grass, but this ratio did not change during the day in subsequent growth stages. The results of these experiments show that stages of maturity affected fatty acid fluctuations during the day as well as total fatty acid and chlorophyll levels in Coastal bermudagrass leaf blades. Chlorophyll alb ratios varied independently of fatty acid levels. 相似文献
106.
Franz Boas spent several weeks at Fort Rupert, British Columbia, at the end of 1894, when he saw the Kwakiutl hamatsa ritual in situ for the first time. Soon after his return east Boas posed for a series of photographs in the U.S. National Museum for a diorama of the hamatsa dance. These photographs, now published for the first time, are a sharp reminder of Boas' constant (and sometimes forced) collaboration with the limited number of anthropological institutions in America at the end of the century, and of his personal difficulties in establishing himself professionally in America. 相似文献
107.
Messenger RNA metabolism of animal cells: possible involvement of untranslated sequences and mRNA-associated proteins 总被引:11,自引:1,他引:10 下载免费PDF全文
The past several years have seen a virtual revolution in the study of eukaryotic mRNA. Among the notable recent achievements are the positive identification of mRNA precursors in HnRNA, the enumeration of the DNA sequences from which mRNA is transcribed, and the finding that mRNA in cultured cells is much more stable than was previously believed. One of most far-reaching discoveries has been the finding that mRNA in eukaryotes contains poly A. This discovery, aside from providing a powerful tool for mRNA isolation, has generated a large body of research into the properties and metabolism of poly A itself. In addition, the finding of a poly A-associated protein has given a renewed stimulus to the study of proteins associated with mRNA. This review is devoted to a discussion of these and related achievements, and some of their implications 相似文献
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