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141.
142.
约当重心定理(1869年)提出,一棵树的重心不是由一个单一的顶点组成,就是由一对相邻的顶点组成-这篇文章提出了一个证明这个定理的新方法- 相似文献
143.
Normal 0 false false false EN-US X-NONE X-NONE MicrosoftInternetExplorer4 /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-qformat:yes; mso-style-parent:""; mso-padding-alt:0in 5.4pt 0in 5.4pt; mso-para-margin:0in; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:11.0pt; font-family:"Calibri","sans-serif"; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-fareast-font-family:"Times New Roman"; mso-fareast-theme-font:minor-fareast; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin; mso-bidi-font-family:"Times New Roman"; mso-bidi-theme-font:minor-bidi;} Hybridization between the pocket gophers Thomomys bottae and T. townsendii at Gold Run Creek in Honey Lake Valley, Lassen County, California, is examined by electromorphic and morphologic characters and by the distribution of ectoparasitic mallophagan lice. Hybrid formation is minimal (about 12% of the total sample of 104 individuals examined), but both F 1 hybrids and a few presumptive backcross individuals are apparent. Nevertheless, no evidence of genic introgression based on five diagnostic allozyme loci is present in parental populations of either taxon within a mile of the hybrid zone. Similarly, louse species unique to each parental gopher host do not penetrate beyond the geographic limits of the genetic hybrid zone into the range of the opposite gopher species. A narrow zone of hybridization is thus concordantly defined by genetic, morphologic, and ectoparasitic parameters. These two gopher taxa are thus genetically if not reproductively isolated and should be considered separate biological species. 相似文献
144.
The progress of bud development was studied in limber pines growing in the mountains of northeastern Utah. Initiation of new bud scales began in mid-June, several weeks after elongation of the current shoot had begun. Needle primordia first appeared in September and continued to form through the winter, until all were present in May. This winter activity is believed to be fostered by surface temperatures on the terminal buds considerably higher than ambient air temperatures. 相似文献
145.
Ronald M. Lanner 《西北部美国博物学家》2011,40(3)
Controlled pollinations were performed on four pinyons ( Pinus edulis Engelm.) to compare the results of selfing and outcrossing. Final cone size was the same under both treatments. There was no significant difference in number of seeds per cone. Filled-seed yields averaged 14.4 percent in selfings and 90.5 percent in outcrossings. Relative self-fertility averaged about 15 percent, a level comparable with that of other pine species studied. 相似文献
146.
147.
Kupfer A Müller H Antoniazzi MM Jared C Greven H Nussbaum RA Wilkinson M 《Nature》2006,440(7086):926-929
Although the initial growth and development of most multicellular animals depends on the provision of yolk, there are many varied contrivances by which animals provide additional or alternative investment in their offspring. Providing offspring with additional nutrition should be favoured by natural selection when the consequent increased fitness of the young offsets any corresponding reduction in fecundity. Alternative forms of nutrition may allow parents to delay and potentially redirect their investment. Here we report a remarkable form of parental care and mechanism of parent-offspring nutrient transfer in a caecilian amphibian. Boulengerula taitanus is a direct-developing, oviparous caecilian, the skin of which is transformed in brooding females to provide a rich supply of nutrients for the developing offspring. Young animals are equipped with a specialized dentition, which they use to peel and eat the outer layer of their mother's modified skin. This new form of parental care provides a plausible intermediate stage in the evolution of viviparity in caecilians. At independence, offspring of viviparous and of oviparous dermatotrophic caecilians are relatively large despite being provided with relatively little yolk. The specialized dentition of skin-feeding (dermatophagous) caecilians may constitute a preadaptation to the fetal feeding on the oviduct lining of viviparous caecilians. 相似文献
148.
Sub1A is an ethylene-response-factor-like gene that confers submergence tolerance to rice 总被引:10,自引:0,他引:10
Xu K Xu X Fukao T Canlas P Maghirang-Rodriguez R Heuer S Ismail AM Bailey-Serres J Ronald PC Mackill DJ 《Nature》2006,442(7103):705-708
Most Oryza sativa cultivars die within a week of complete submergence--a major constraint to rice production in south and southeast Asia that causes annual losses of over US 1 billion dollars and affects disproportionately the poorest farmers in the world. A few cultivars, such as the O. sativa ssp. indica cultivar FR13A, are highly tolerant and survive up to two weeks of complete submergence owing to a major quantitative trait locus designated Submergence 1 (Sub1) near the centromere of chromosome 9 (refs 3, 4, 5-6). Here we describe the identification of a cluster of three genes at the Sub1 locus, encoding putative ethylene response factors. Two of these genes, Sub1B and Sub1C, are invariably present in the Sub1 region of all rice accessions analysed. In contrast, the presence of Sub1A is variable. A survey identified two alleles within those indica varieties that possess this gene: a tolerance-specific allele named Sub1A-1 and an intolerance-specific allele named Sub1A-2. Overexpression of Sub1A-1 in a submergence-intolerant O. sativa ssp. japonica conferred enhanced tolerance to the plants, downregulation of Sub1C and upregulation of Alcohol dehydrogenase 1 (Adh1), indicating that Sub1A-1 is a primary determinant of submergence tolerance. The FR13A Sub1 locus was introgressed into a widely grown Asian rice cultivar using marker-assisted selection. The new variety maintains the high yield and other agronomic properties of the recurrent parent and is tolerant to submergence. Cultivation of this variety is expected to provide protection against damaging floods and increase crop security for farmers. 相似文献
149.
Janzen V Forkert R Fleming HE Saito Y Waring MT Dombkowski DM Cheng T DePinho RA Sharpless NE Scadden DT 《Nature》2006,443(7110):421-426
Stem-cell ageing is thought to contribute to altered tissue maintenance and repair. Older humans experience increased bone marrow failure and poorer haematologic tolerance of cytotoxic injury. Haematopoietic stem cells (HSCs) in older mice have decreased per-cell repopulating activity, self-renewal and homing abilities, myeloid skewing of differentiation, and increased apoptosis with stress. Here we report that the cyclin-dependent kinase inhibitor p16INK4a, the level of which was previously noted to increase in other cell types with age, accumulates and modulates specific age-associated HSC functions. Notably, in the absence of p16INK4a, HSC repopulating defects and apoptosis were mitigated, improving the stress tolerance of cells and the survival of animals in successive transplants, a stem-cell-autonomous tissue regeneration model. Inhibition of p16INK4a may ameliorate the physiological impact of ageing on stem cells and thereby improve injury repair in aged tissue. 相似文献
150.
Ronald Suleski 《杭州师范学院学报(社会科学版)》2012,34(3)
通过那些流传在民间社会的抄本,我们可以了解中国平民社会的生活状态.大约成书于1944年的唐家家谱就是这样一种民间抄本.仔细研究这本家谱,我们会在字里行间发现很多关于执笔者的故事:它的历史脉络、经济地位、婚姻网络等等.由此,通过家谱的编写,人们把历史融人到了自己的日常生活之中,并由此实现对文化的重新阐述.所有这些,又折射出民国时期中国的平民文化. 相似文献