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The secretory organelles of Plasmodium knowlesi were studied ultrastructurally to examine their mode of action during invasion. The formation of lamellar structures in merozoite rhoptries within late stage schizonts is prevented by the protease inhibitors chymostatin and leupeptin. Under normal conditions vesicles lined by 6-nm membranes are formed in rhoptries during erythrocyte invasion. Stereoscopic viewing of tilted sections shows that where the merozoite apex contacts the parasitophorous vacuole (PV) membrane during invasion, a domed elevation of the PV surface lies within the mouth of the rhoptry duct in contact with the secretory matrix. The membrane of the early invasion pit is thinner (6 nm) than the red cell membrane elsewhere, and sheets of lamellar material are frequently present on the invasion pit surface. These findings support the proposal that the rhoptry-microneme complex is capable of generating membranous material and inserting it into the red cell surface in a controlled manner to create the parasitophorous vacuole. On the basis of this model, measurements from serial sections show that the rhoptries could provide enough material to create a membrane lining the parasitophorous vacuole, and, with the contribution of the microspheres, could double it to accommodate the early ring stage of the parasite. 相似文献
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The control of incompatibility in distylous Pulmonaria affinis Jordan (Boraginaceae) 总被引:1,自引:0,他引:1
A.J. RICHARDS JEAN MITCHELL 《Botanical journal of the Linnean Society. Linnean Society of London》1990,104(4):369-380
In P. affinis, pin pollen is shorter on average than thrum pollen. Pins have more coronal hooks on stigmatic papillae than thrums, but gaps between papillae are relatively smaller for thrums. Pin stigmas receive more pollen than thrum stigmas. Thrum stigmas receive more (dissortive) pin pollen, but pin stigmas are assortively pollinated. Pollen only germinates when trapped below papilla coronas. On thrum stigmas, most trapped pollen is pin. Pollen germination is better on thrum stigmas than pin stigmas, and thrum stigmas show a close relationship between numbers of legitimate pollen grains, numbers of germinating grains, and numbers of pollen tubes in the style. There is no inhibition of illegitimate pollen germination. Illegitimate pollen tubes are inhibited in the style. Incompatibility operates by a combination of dissortive pollination, dissortive pollen trapping, and stylar pollen tube inhibition. All heteromorphic features differing between pins and thrums are implicated in the inhibition of within-morph fertilization in thrums. 相似文献
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Effects of nitrogen on the response of loblolly pine to drought 总被引:3,自引:1,他引:2
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In field trials at sites of an outbreak of arabis mosaic nepovirus (AMV) in England and of raspberry ringspot nepovirus (RRV) in Scotland, the results of exposure of some new raspberry cultivars to natural infection with these viruses showed discrepancies from those obtained in graft inoculation tests using AMV-Lib and RRV-S, the Scottish type isolates. In particular, cv. Glen Prosen, which is immune to AMV-Lib and RRV-S, was infected with AMV and RRV in the field trials. Studies on these and other field isolates of AMV and RRV showed that they differed from the type isolates in Rubus host range and in symptomatology in herbaceous hosts. However, whereas four isolates of RRV found infecting Rubus were distinguishable by spur formation in gel double-diffusion serological tests, six AMV isolates were indistinguishable by this method. Immunoelectrophoresis of virus particles did not distinguish the six AMV isolates, but isolates RRV-MX and RRV-T were distinguishable from RRV-S and the English type isolate, RRV-E. Like the two RRV type isolates, RRV-MX contained a single electrophoretic component, but it migrated must faster whereas RRV-T contained two components, one with a migration rate similar to that of RRV-MX and the other similar to that of the type isolates. Polyacrylamide gel electrophoresis of protein preparations from highly purified virus particles of RRV isolates E, S and MX detected a single polypeptide of estimated mol. wt 54 × 103, 54 × 103 and 50 × 103 respectively but that of isolate T contained two polypeptides of estimated mol. wt 54 × 103 and 50 × 103. These data suggest that RRV-T is a mixture of two isolates. In laboratory tests the nematode Xiphinema diversicaudatum transmitted four isolates of AMV efficiently whereas two populations of the nematode Longidorus elongatus were less efficient vectors of four RRV isolates. Neither vector species transmitted virus to any of nine raspberry cultivars. The results are discussed in relation to the control of nepoviruses in raspberry and to the biology of these viruses. 相似文献
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CHARLES E. MITCHELL JÖRG MALETZ 《Lethaia: An International Journal of Palaeontology and Stratigraphy》1995,28(4):317-331
The base of the Undulograptus austrodentatus Biozone appears to be a synchronous event that is widely recognizable within graptolitic facies around the world. It occurs within an interval in which graptolite species ranges are now well known and in which there is a rapid turnover in the composition of graptolite faunas. This turnover reflects the rapid evolutionary radiation of the Diplograptacea simultaneously with the appearance of several distinctive pseudisograptid and glossograptid species. These events provide the basis for the recognition of two thin but widely applicable subzones; a lower Arienigraptus zhejiangensis Subzone and an upper U. sinicus Subzone. The occurrence of the lower boundary of the U. austrodentatus Biozone within a succession of first appearances also permits accurate and reliable identification of the boundary as well as assessment of stratigraphic completeness across the boundary interval in correlated sections. Diverse graptolite faunas of late Yapeenian and early Darriwilian age occur in association with the Histiodella altifrons Biozone of the North American midcontinent conodont zonation and the Paroistodus originalis and Microzarkodina parva biozones of the North Atlantic conodont zonation. They also occur in association with the shelly-fossil zonations developed for several different continents. These features of the base of the U. austrodentatus Biozone make it a suitable level for use as the boundary level for a global stage. Its stratigraphic position within the Ordovician System relative to other likely global stages as well as its coincidence with one of the major events in graptolite evolutionary history suggest that this level also may be a suitable level for the base of a global Middle Ordovician Series.Ordovician System, Ordovician stages, graptolite zonation, chronostratigraphy, international correlation. Charles E. Mitchell and Jörg Maletz, Department of Geology, State University of New York at Buffalo, Buffalo, New York 14260-1550, USA; 13th July, 1994; revised 22nd May, 1995. 相似文献