全文获取类型
收费全文 | 147篇 |
免费 | 3篇 |
出版年
2011年 | 1篇 |
2009年 | 2篇 |
2008年 | 3篇 |
2007年 | 2篇 |
2006年 | 2篇 |
2004年 | 1篇 |
2003年 | 4篇 |
2001年 | 2篇 |
2000年 | 1篇 |
1999年 | 4篇 |
1998年 | 3篇 |
1997年 | 4篇 |
1996年 | 4篇 |
1995年 | 7篇 |
1994年 | 3篇 |
1993年 | 4篇 |
1992年 | 2篇 |
1991年 | 3篇 |
1990年 | 4篇 |
1989年 | 1篇 |
1988年 | 2篇 |
1987年 | 3篇 |
1986年 | 2篇 |
1985年 | 1篇 |
1982年 | 3篇 |
1981年 | 1篇 |
1980年 | 4篇 |
1979年 | 3篇 |
1978年 | 1篇 |
1975年 | 1篇 |
1973年 | 2篇 |
1972年 | 2篇 |
1971年 | 1篇 |
1970年 | 2篇 |
1969年 | 2篇 |
1966年 | 1篇 |
1965年 | 1篇 |
1962年 | 1篇 |
1959年 | 4篇 |
1958年 | 17篇 |
1957年 | 5篇 |
1956年 | 5篇 |
1955年 | 4篇 |
1954年 | 2篇 |
1953年 | 7篇 |
1952年 | 4篇 |
1951年 | 3篇 |
1950年 | 4篇 |
1949年 | 1篇 |
1947年 | 1篇 |
排序方式: 共有150条查询结果,搜索用时 15 毫秒
1.
JON MULLER 《American anthropologist》1972,74(4):952-953
Book reviewed in this article:
Archeology: Computer Analysis of Chronological Seriation . FRANK HOLE and MARY SHAW
Archeology: Archéologie et calculateurs: problèmes sémi-ologiques et mathématiques . CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE 相似文献
Archeology: Computer Analysis of Chronological Seriation . FRANK HOLE and MARY SHAW
Archeology: Archéologie et calculateurs: problèmes sémi-ologiques et mathématiques . CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE 相似文献
2.
3.
ANDRÉ PICARD JEAN-CLAUDE LABBÉ GÉRARD PEAUCELLIER FRANÇOISE LE BOUFFANT CHRISTIAN LE PEUCH MARCEL DOREE 《Development, growth & differentiation》1987,29(1):93-103
Many studies suggest that MPF activation depends on protein phosphorylation or that MPF is itself a protein kinase. In the present report, cyclic variations of MPF activity have been correlated in vivo with changes in the extent of protein phosphorylation or in vitro with changes of a major protein kinase during the first cell cycles of fertilized starfish eggs. This cycling protein kinase neither requires cAMP nor Ca2+ . Neither colchicine nor aphidicoline, which inhibits cleavage and chromosome replication respectively, was found to suppress the synchronous and cyclic variations of both MPF and protein kinase activities. Protein synthesis was found to be required for both MPF and protein kinase activities to reappear after their simultaneous drop at the time of mitotic or meiotic cleavages. Production of either MPF or protein kinase activities is not the immediate result of protein synthesis since there is a delay at each cell cycle between the time when protein synthesis is required and the time when both MPF and protein kinase activities are produced. This suggests that both MPF and protein kinase activities might involve some post-translational modification of a precursor protein synthesized during the preceeding cell cycle. 相似文献
4.
5.
6.
Melanocytes in human skin reside both in the epidermis and in the matrix and outer root sheath of anagen hair follicles. Comparative study of melanocytes in these different locations has been difficult as hair follicle melanocytes could not be cultured. In this study we used a recently described method of growing hair follicle melanocytes to characterize and compare hair follicle and epidermal melanocytes in the scalp of the same individual. Three morphologically and antigenically distinct types of melanocytes were observed in primary culture. These included (1) moderately pigmented and polydendritic melanocytes derived from epidermis; (2) small, bipolar, amelanotic melanocytes; and (3) large, intensely pigmented melanocytes; the latter two were derived from hair follicles. The three sub-populations of cells all reacted with melanocyte-specific monoclonal antibody. Epidermal and amelanotic hair follicle melanocytes proliferated well in culture, whereas the intensely pigmented hair follicle melanocytes did not. Amelanotic hair follicle melanocytes differed from epidermal melanocytes in being less differentiated, and they expressed less mature melanosome antigens. In addition, hair follicle melanocytes expressed some antigens associated with alopecia areata, but not antigens associated with vitiligo, whereas the reverse was true for epidermal melanocytes. Thus, antigenically different populations of melanocytes are present in epidermis and hair follicle. This could account for the preferential destruction of hair follicle melanocytes in alopecia areata and of epidermal melanocytes in vitiligo. 相似文献
7.
This study was undertaken to determine which of the two NO3? fluxes (influx or efflux) across plasma membranes of root cells is the target of those amino acids which have been shown to inhibit net NO3? uptake (Muller & Touraine 1992, Journal of Experimental Botany 43 , 617–623). Parallel experiments were performed to mea-sure either the time course of 15NO3? release from roots of soybean seedlings previously labelled with this isotope into non-labelled solution, or the time course of 15N accumulation from labelled 15NO3? solution in non-labelled seedlings. Focusing on the fate of 15NO3? in the cytoplasmic compartment, a model is developed to describe the time courses of the accumulation and release of tracer across the plasma membranes of root cells. Both time courses can be described by the sum of an exponential plus a linear term. In our material, the linear part of the accumulation time course is obscured by the NO3? fluxes exiting the cytoplasm, and the curve thus appears to be quasilinear over several minutes. However, we show that the use of the net tracer accumulation rate during this time period as an estimate of NO3? influx does not provide accurate estimates of influx and efflux. By contrast, 15NO3? efflux analysis permits calculation of the unidirectional fluxes across plasma membranes of root cells and the kinetic parameters of the cytoplasmic NO3? pool. Under our experimental conditions, efflux accounted for 30 to 50% of influx, and the cytoplasmic NO3? content was found to be in the 70–400nmol g?1 fw range. Using this methodology, the effect of amino acid accumulation on unidirectional fluxes of nitrate was then examined. Pretreatments of the seedlings with an amino acid which has been shown to inhibit net NO3? uptake led to concomitant decreases in net accumulation rates of 15NO3? and of reduced 15N in roots and total 15N in cotyledons. NO3? influx was markedly inhibited by these treatments, while NO3? efflux remained essentially unaffected, or even decreased. It is concluded that the target of the regulation of NO3? uptake by phloemtranslocated amino acids is the influx system. 相似文献
8.
Differential Stimulation of PEP-carboxylation in Guard Cells and Mesophyll Cells by Ammonium or Fusicoccin 总被引:1,自引:0,他引:1
Dark CO2-fixation in guard cells of Vicia faba was much moresensitive to ammonium than in mesophyll cells. Addition of ammonium(5.0 mol m3; pH0 7.6) caused up to a 7-fold increasein dark CO2-fixation rates in guard cell protoplasts (GCP),whereas in leaf slices, mesophyll cells, and mesophyll protoplaststhe increase was only about 1.4-fold. In both cell or tissuetypes, total CO2-fixation rates were higher in the light (212-foldhigher in GCP and 28-fold in mesophyll); these rates were onlyslightly changed by ammonium treatment. However, separationof 14C-labelled products after fixation of CO2 in the lightby GCP revealed a large ammonium-induced shift in carbon flowfrom starch and sugars to typical products of C4-metabolism(mainly malate and aspartate). In contrast, in mesophyll cellsamino acid and malate labelling was only moderately increasedby ammonium at the expense of sucrose. The data suggest thatin vivo ammonium might facilitate stomatal opening and/or delaystomatal closing through an increased production of organicacids. Key words: PEP-carboxylation, guard cell protoplasts, ammonium, fusicoccin 相似文献
9.
LANG JUDITH C.; LASKER HOWARD R.; GLADFELTER ELIZABETH H.; HALLOCK PAMELA; JAAP WALTER C.; LOSADA FREDDY J.; MULLER ROBERT G. 《Integrative and comparative biology》1992,32(6):696-706
Western Atlantic coral reefs were differentially affected bya mass bleaching (discoloration) event in 1987. We periodicallyassessed the "appearance" of zooxanthellate organisms betweenDecember 1987 and June 1988 at nine conspicuously affected sitesin the Bahamas, Florida, St. Croix, and Venezuela, using a standardizedpoint-count technique. Three to four months after the localinitiation of the event, the "bleached" state was still presentin one to three of the most abundant reef coral taxa and ina few of the less common species (n = 5 sites). "Recovery" occurredsomewhat faster at shallower depths, at least in the Bahamasand Florida. Scleractinian corals which were "prolonged bleachers"had foliaceous or massive, rather than branching, morphologies."Bleached" points disappeared from the point counts after $6to $8 months. Long-term field data on spatial and temporal variability inthe dynamics of zooxanthellate organisms would help us to understandthe ecological consequences of bleaching. More generally, weneed to distinguish anthropogenic changes in the structure andfunctioning of reef ecosystems from those which occur naturally.Point-count techniques are well suited for collaborative studiesinvolving rapid quantification of coloration states and healthin reef corals. 相似文献
10.
Interaction between atmospheric and pedospheric nitrogen nutrition in spruce (Picea abies L. Karst) seedlings 总被引:4,自引:2,他引:4
The effect of NO2 fumigation on root N uptake and metabolism was investigated in 3-month-old spruce (Picea abics L. Karst) seedlings. In a first experiment, the contribution of NO2 to the plant N budget was measured during a 48 h fumigation with 100mm3m?3 NO2. Plants were pre-treated with various nutrient solutions containing NO2 and NH4+, NO3? only or no nitrogen source for 1 week prior to the beginning of fumigation. Absence of NH4+ in the solution for 6d led to an increased capacity for NO3? uptake, whereas the absence of both ions caused a decrease in the plant N concentration, with no change in NO3? uptake. In fumigated plants, NO2 uptake accounted for 20–40% of NO3? uptake. Root NO3? uptake in plants supplied with NH4+plus NO3? solutions was decreased by NO2 fumigation, whereas it was not significantly altered in the other treatments. In a second experiment, spruce seedlings were grown on a solution containing both NO2 and NH4+ and were fumigated or not with 100mm3m?3 NO2 for 7 weeks. Fumigated plants accumulated less dry matter, especially in the roots. Fluxes of the two N species were estimated from their accumulations in shoots and roots, xylem exudate analysis and 15N labelling. Root NH4+ uptake was approximately three times higher than NO3? uptake. Nitrogen dioxide uptake represented 10–15% of the total N budget of the plants. In control plants, N assimilation occurred mainly in the roots and organic nitrogen was the main form of N transported to the shoot. Phloem transport of organic nitrogen accounted for 17% of its xylem transport. In fumigated plants, neither NO3? nor NH4+ accumulated in the shoot, showing that all the absorbed NO2 was assimilated. Root NO3? reduction was reduced whereas organic nitrogen transport in the phloem increased by a factor of 3 in NO2-fimugated as compared with control plants. The significance of the results for the regulation of whole-plant N utilization is discussed. 相似文献