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121.
In light of the Central American Initiative for the control of Chagas disease, efforts were made on the part of Costa Rican and Nicaraguan teams, working separately, to determine the present status of Rhodnius pallescens in areas close to the common border of the two countries, where the insect has appeared within the last few years. The opportunity was also used to establish whether R. prolixus, a vector present in some areas of Nicaragua, has been introduced in recent years into Costa Rica with Nicaraguan immigrants. It became evident that wild adults of R. pallescens are common visitors to houses in different towns of a wide area characterized as a humid, warm lowland, on both sides of the frontier. Up to the present, this bug has been able to colonize a small proportion of human dwellings only on the Nicaraguan side. There was strong evidence that the visitation of the adult bug to houses is related to the attraction of this species to electric lights. There were no indications of the presence of R. prolixus either in Nicaragua or in Costa Rica in this area of the Caribbean basin. Triatoma dimidiata, a widespread domestic species in both countries, was totally absent in the explored areas of Costa Rica but occasionally occurs on the Nicaraguan side. Serological surveys in children of both areas showed that transmission of Chagas disease takes place in a rather small degree in Costa Rica and more commonly in Nicaragua, indicating that R. pallescens could be a potential threat as a vector in this particular region. 相似文献
122.
Differentiation and cell division in the mammalian thymus 总被引:20,自引:0,他引:20
123.
Effects of forest clearing and succession on the carbon and nitrogen content of soils in Puerto Rico and US Virgin Islands 总被引:8,自引:0,他引:8
Soil samples from mature and secondary forests and agricultural sites in three subtropical life zones of Puerto Rico and the
US Virgin Islands were collected to determine the effects of forest conversion to agriculture and succession on soil organic
carbon (C) and nitrogen (N) contents. Site characteristics that may affect soil C and N (slope, elevation, aspect, and texture)
were as uniform as possible. Carbon contents (to 50 cm depth or bedrock) of cultivated sites, as a percent of corresponding
mature forests, were lower in the wet (44%) and moist (31%) than in the dry (86%) life zones whereas N contents were relatively
high regardless of life zone (60–130% of the mature forests). Conversion of forests to pasture resulted in less soil C and
N loss than conversion to crops. The time for recovery of soil C and N during succession was approximately the same in all
three life zones, about 40–50 yr for C about 15–20 yr for N. However, the rate of recovery of soil C was faster in the wet
and moist life zone, whereas N appeared to recover faster in the dry life zone. Evidence for loss of soil C during cultivation
and gain during succession to soil depths of 50–100 cm is presented. 相似文献
124.
The tyrosine kinase P210 is the gene product of the rearranged BCR-ABL locus on the Philadelphia chromosome (Ph1), which is found in leukemic cells of patients with chronic myelogenous leukemia. It has a weakly oncogenic effect in immature murine hematopoietic cells and does not transform NIH 3T3 cells. We have found that P210 has a strikingly different effect in Rat-1 cells, another line of established rodent fibroblasts. Stable expression of P210 in Rat-1 cells caused a distinct morphological change and conferred both tumorigenicity and capacity for anchorage-independent growth. The introduction of v-myc into Rat-1 cells expressing P210 led to complete morphological transformation and enhanced tumorigenicity. No such interaction took place in NIH 3T3 cells. Thus, Rat-1 cells can be used to detect cooperation between BCR-ABL and other oncogenes and may prove useful for the identification of secondary oncogenic events in chronic myelogenous leukemia. 相似文献
125.
Long-term studies are needed to understand the dynamics of tropical forests, particularly those subject to periodic disturbances such as hurricanes. We studied a flood plain Prestoea montana palm forest in the Luquillo Mountains of Puerto Rico over a 15-yr period (1980–1995), which included the passage of Hurricane Hugo in September 1989. The passage of the hurricane caused the dominant species to become more dominant and created low instantaneous tree mortality (1% of stems) and reductions in tree biomass (-16 Mg/ha/yr) and density, although not in basal area. Five years after the hurricane, the palm flood plain forest had exceeded its prehurricane aboveground tree biomass, tree density, and basal area. Aboveground tree biomass accumulated at a rate of 9.2 Mg/ha/yr, 76 percent of which was due to palms. Before the hurricane this rate was on the order of 3 Mg/ha/yr. Forest floor litter decreased to prehurricane levels (6.7 Mg/ha), within 5 yr, mostly due to the disappearance of woody litter. Thirteen tree species not represented in the canopy entered the forest by regeneration, and 2 species suffered almost 20 percent/yr mortality over a 5-yr period after the storm (floodplain average of 2%/yr). Delayed tree mortality was twice as high as instantaneous tree mortality after the storm and affected dicotyledonous trees more than it did palms. Regencration of dicotyledonous trees, palms, and tree ferns was influenced by a combination of factors including hydroperiod, light, and space. Redundancy Data Analysis showed that the area near the river channel was the most favorable for plant regeneration. Palm regeneration was higher in locations with longer hydroperiods, while regeneration of dicotyledonous trees was higher in areas with low risk of flooding. This study shows how a periodic disturbance provides long-term opportunities for species invasions and long-term ecosystem response at the patch scale of < 1 ha. 相似文献
126.
Pilon-Smits E; de Souza M; Lytle C; Shang C; Lugo T; Terry N 《Journal of experimental botany》1998,49(328):1889-1892
The poplar hybrid Populus tremula x
alba was shown to volatilize significant amounts of
selenium. The volatilization rates were 230-fold higher from
selenomethionine compared to selenite, and 1.5-fold higher from selenite
than from selenate. X-ray absorption spectroscopy showed that selenate was
metabolized slowly, but selenite was quickly converted to organic
selenium. 相似文献
127.
Soil oxygen availability and biogeochemistry along rainfall and topographic gradients in upland wet tropical forest soils 总被引:12,自引:5,他引:7
We measured soil oxygen concentrations at 10 and 35 cm depths and indices of biogeochemical cycling in upland forest soils along a rainfall and elevation gradient (3500–5000 mm y–1; 350–1050 masl) and along topographic gradients (ridge to valley, 150 m) in the Luquillo Experimental Forest, Puerto Rico. Along the rainfall gradient, soil O2 availability decreased significantly with increasing annual rainfall, and reached very low levels (<3%) in individual chambers for up to 25 consecutive weeks over 82 weeks of study. Along localized topographic gradients, soil O2 concentrations were variable and decreased significantly from ridges to valleys. In the valleys, up to 35% of the observations at 10–35 cm depth were <3% soil O2. Cross correlation analyses showed that soil O2 concentrations were significantly positively correlated along the topographic gradient, and were sensitive to rainfall and hydrologic output. Soil O2 concentrations in valley soils were correlated with rainfall from the previous day, while ridge sites were correlated with cumulative rainfall inputs over 4 weeks. Soils at the wettest point along the rainfall gradient had very high soil methane concentrations (3–24%) indicating a strong influence of anaerobic processes. We measured net methane emission to the atmosphere at the wettest sites of the rainfall gradient, and in the valleys along topographic gradients. Other measures of biogeochemical function such as soil organic matter content and P availability were sensitive to chronic O2 depletion along the rainfall gradient, but less sensitive to the variable soil O2 environment exhibited at lower elevations along topographic gradients. 相似文献
128.
Rehabilitation of Tropical Lands: A Key to Sustaining Development 总被引:10,自引:0,他引:10
Land rehabilitation is proposed as a management strategy to reverse the negative consequences of tropical deforestation and land degradation. We first define the concepts associated with ecosystem modification—conversion, damage, and degradation—and those associated with ecosystem repair—restoration, rehabilitation, and reclamation. We then present a scheme of sustainable land use in the tropics, with illustrations of how rehabilitation and restoration activities fit into the overall scheme of the use of land. Because damaged lands cannot contribute effectively to sustained economic development, land rehabilitation is a necessary step for increasing the chances of attaining sustainability. Approaches for rehabilitating ecosystems are discussed, including the management of stressors and subsidies in relation to their point of interaction in the ecosystem. Finally, we illustrate the concepts of ecosystem rehabilitation of damaged, degraded, and derelict lands with examples of case studies from dry to humid life zones in island and continental situations throughout the tropics. The case studies demonstrate that opportunities for success exist, even with severely degraded lands, but a considerable amount of research remains to be done before we have a full understanding of the complexity of the task facing us. 相似文献
129.
130.
Rhizospheric soil samples were taken from Puna native grasses along an altitudinal gradient. Biodiversity of arbuscular mycorrhizal
fungi (AMF) and associated bacteria was analyzed considering altitude and grasses photosynthetic pathways (metabolic type
C3, C4). Cultivation-dependent approaches were applied to obtain further information about the phylogeny of the dominating cultivable
aerobic–heterotrophic bacteria communities present in rhizospheric soil samples. In average, the bacterial count ranged between
1.30 × 102 and 8.66 × 104 CFU g−1 of dry weight of soil. Individual bacterial colonies of aerobic heterotrophic bacteria grown on R2A medium were morphologically
grouped and identified as typical soil bacteria belonging to the genera Bacillus, Pseudomonas, and Arthrobacter. Ten AMF taxa were found: Acaulospora sp., A. laevis, A. spinosa, Gigaspora sp., Gi. ramisporophora, Glomus sp., Gl. aggregatum, Gl. ambisporum, Gl. sinuosum, and Scutellospora biornata. AMF diversity decreased with altitude. 相似文献