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LARRY G. MARSHALL ZOFIA KIELAN-JAWOROWSKA 《Lethaia: An International Journal of Palaeontology and Stratigraphy》1992,25(4):361-374
A cladistic analysis of the major groups of early metatherian mammals shows that the Deltatheroida and South American dog-like Borhyaenoidea are closely related and are placed in a new supercohort Deltatheralia. while all other metatherians are placed in the supercohort Marsupialia. A reassessment of Early Cretaceous mammals with tribosphenic molars shows that metatherians and eutherians apparently evolved independently from a peramuran-like ancestor, and an entoconid. distinctly basined talonid and large protocone were acquired separately in each group. Consequently. the Tribosphenida ( sensu McKenna) is apparently paraphyletic. Aegialodontia are regarded as structurally ancestral to Metatheria but not Eutheria. The known fossil record supports the view that differentation of the stocks which gave rise to metatherians and eutherians apparently occurred in Late Jurassic or Early Cretaceous time. 相似文献
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Measurements of the photosynthesis-light response of flag leavesin a winter wheat crop were made during the period from maximumelongation until complete senescence. Immediately followingleaf elongation, the maximum rates of photosynthesis and thevalues of efficiency at low light were in the range 2.83.6g CO2 m2 h1 and 811 µg CO2 J1respectively. The shape of the photosynthesis-light responseremained constant throughout and was close to a Blackmantype response rather than a rectangular hyperbola. The resultswere analysed, therefore, using a more recent model which isa non-rectangular hyperbola. Stomatal and internal resistanceswere equally important in limiting the maximum rate of photosynthesis. 相似文献
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Influence of boron on the reproductive growth of white clover (Trifolium repens L.) cultivars 总被引:1,自引:0,他引:1
The effect of boron on the reproductive growth of four contrasting white clover cultivars was examined in a controlled environment and glasshouse experiment. Plants grown under a nutrient regime excluding boron and those receiving 0.5 mg/litre boron had fewer stolons, reproductive stolons and inflorescences than those receiving 1, 2 or 3 mg/litre. Inflorescences were also smaller and had fewer seeds per floret. A boron concentration of 1 mg/litre gave optimum reproductive growth, higher levels of boron giving no further increase. Peduncle length and nectar quantity, but not nectar quality, were also influenced by boron indicating a possible beneficial effect of this element on pollinator activity. Cultivars differed in both vegetative and reproductive growth. Cv. Gwenda, a small leaved cultivar, had more stolons but fewer inflorescences than cv. Menna and smaller inflorescences than cv. Olwen, a large leaved cultivar. Generally, these cultivars responded similarly to boron. The implications of these results for the reproductive growth of white clover are discussed. 相似文献