首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   47篇
  免费   2篇
  2020年   1篇
  2019年   3篇
  2018年   2篇
  2017年   1篇
  2016年   1篇
  2015年   3篇
  2014年   1篇
  2013年   1篇
  2011年   3篇
  2010年   1篇
  2009年   2篇
  2008年   1篇
  2006年   1篇
  2005年   1篇
  2004年   1篇
  1998年   2篇
  1997年   1篇
  1992年   1篇
  1991年   2篇
  1990年   1篇
  1984年   3篇
  1983年   4篇
  1982年   2篇
  1981年   1篇
  1979年   1篇
  1978年   4篇
  1977年   1篇
  1975年   2篇
  1974年   1篇
排序方式: 共有49条查询结果,搜索用时 609 毫秒
1.
The upper Tennessee River drainage, which includes portions of the States of Virginia, North Carolina, and Tennessee, supports an exceptionally diverse fish fauna. Recent reductions in abundance and geographic ranges of several freshwater fishes have promulgated the imposition of protective measures for about 115 species among the three states, with nearly half of those species occurring in the upper Tennessee River. Most protected species are darters (Percidae: Etheostomatinae) or minnows (Cyprinidae), and are typically small, benthic invertivores. Major impacts on the fish fauna have resulted from dams, introduced species, toxic spills, mining and agriculture. An important cumulative effect of these impacts is fragmentation of the watershed; nearly 40% of the riverine habitat in major tributaries is either impounded or altered by tailwater discharges. The isolation and stress imposed on tributaries of the river have caused and will continue to cause extirpations of fishes, mussels and other aquatic fauna. Numerous federal, state, and private organizations are co-operating in efforts to protect rare species and habitats, improve agricultural and coal-producing practices, and enforce regulations for industrial and municipal effluents.  相似文献   
2.
3.
Average height of the free population in the United States born in the mid-1830s began to decline despite growing per capita incomes. Explanations for this "antebellum puzzle" revolve around a possibly deteriorating disease environment promoted by urban agglomeration and increases in the relative price of protein-rich foods. However, several groups were immune to the effect, including members of the middle class, whose income was high enough, and increased enough to overcome the adverse developments and maintain their nutritional status. Although at the opposite end of the social spectrum, the height of male slaves also increased, as it was in their owners' interest to raise their slaves' food allotments. The height of Tennessee convicts, analyzed in this article, also increased in the late-1830s, being the third exception to the "antebellum puzzle." Mid-19th century Tennessee was integrated into interstate commerce in cotton and tobacco and experienced considerable movement of people who would have brought with them diseases from elsewhere, hence, it would have been integrated into the US disease pool, and the fact that heights did not decline in the 1830s is therefore an indication that the antebellum puzzle cannot be explained exclusively by the spread of diseases. Yet, Tennessee's economy was quite different to that of the rest of the country. Although it did export live swine to the South, these exports did not increase during the antebellum decades. Hence, Tennessee remained self-sufficient in pork, and consumption of pork did not decline. Thus, the evidence presented here is consistent with the economic interpretation of the "antebellum puzzle": self-sufficiency in protein production protected even the members of the lower-classes of Tennessee from the negative externalities associated with the onset of industrialization.  相似文献   
4.
The concentration of CO2 in stream water is a product of not only instream metabolism but also upland, riparian, and groundwater processes and as such can provide an integrative measure of whole catchment soil respiration. Using a 5-year dataset of pH, alkalinity, Ca2+, and Mg2+ in surface water of the West Fork of Walker Branch in eastern Tennessee in conjunction with a hydrological flowpath chemistry model, we investigated how CO2 concentrations and respiration rates in stream, bedrock, and soil environments vary seasonally and interannually. Dissolved inorganic carbon concentration was highest in summer and autumn (P < 0.05) although the proportion as free CO2 (pCO2) did not vary seasonally (P > 0.05). Over the 5 years, pCO2 was always supersaturated with respect to the atmosphere ranging from 374 to 3626 ppmv (1.0- to 10.1-fold greater than atmospheric equilibrium), and CO2 evasion from the stream to the atmosphere ranged from 146 to 353 mmol m−2 d−1. Whereas pCO2 in surface water exhibited little intra-annual or interannual variation, distinct seasonal patterns in soil and bedrock pCO2 were revealed by the catchment CO2 model. Seasonally, soil pCO2 increased from a winter low of 8167 ppmv to a summer high of 27,068 ppmv. Driven by the seasonal variation in gas levels, evasion of CO2 from soils to the atmosphere ranged from 83 mmol m−2 d−1 in winter to 287 mmol m−2 d−1 in summer. The seasonal variation in soil CO2 tracked soil temperature (r 2= 0.46, P < 0.001) and model-derived estimates of CO2 evasion rate from soils agreed with previously reported fluxes measured using chambers (Pearson correlation coefficient = 0.62, P < 0.05) supporting the model assumptions. Although rates of CO2 evasion were similar between the stream and soils, the overall rate of evasion from the channel was only 0.4% of the 70,752 mol/d that evaded from soils due to the vastly different areas of the two subsystems. Our model provides a means to assess whole catchment CO2 dynamics from easily collected and measured stream-water samples and an approach to study catchment scale variation in soil ecosystem respiration. Received 24 July 1997; accepted 14 November 1997.  相似文献   
5.
Abstract: We used a 60-yr forest simulation of the Cherokee National Forest, Tennessee, USA, to model the effects of timber harvest and natural disturbance upon habitat availability for 6 songbird species: Acadian flycatcher (Epidonax virescens), blue-headed vireo (Vireo solitarius), chestnut-sided warbler (Dendroica pensylvanica), tufted titmouse (Parus bicolor), yellow-billed cuckoo (Coccyzus americanus), and yellow-throated warbler (Dendroica dominica). Forest simulations, based on expected harvest intensities and historic levels of natural disturbance, were used to update a stand inventory database at 10-yr intervals between 1993 and 2053. Habitat models for the 6 bird species were applied to the updated stand inventory and available habitat quantified for each decade. Late-successional species showed substantial increases in habitat availability over the 60-yr period at most harvest intensities, whereas habitat for early-successional species was stable or declined at most harvest intensities. Acadian flycatcher, yellow-throated warbler, and blue-headed vireo habitat increased by 200%, 213%, and 40%, respectively, whereas tufted titmouse habitat remained relatively constant at expected harvest levels. Chestnut-sided warbler habitat was stable at expected harvest levels but declined at lower harvest intensities, and yellow-billed cuckoo habitat declined by 37% at expected harvest levels. Natural disturbance had little effect on habitat availability for any bird species compared to the effects of timber harvests and increasing forest age. Our models suggest that anthropogenic disturbance, and lack thereof, can play a definitive role determining habitat availability and population viability for forest songbirds.  相似文献   
6.
Treponemal disease is known to be associated with the compromised community health of permanent village settlement. This association explains its high visibility in the village‐based, arguably chiefdom level, agriculturalist societies of late prehistoric (AD 1300–1600) North America. Within chiefdom‐level societies, health differences have often been demonstrated between mortuary‐defined “elite” and “nonelite” individuals. This theoretically should predict status‐based differences in treponemal disease visibility. The prediction is tested in a five‐site osteological sample (N = 650) from the Dallas phase (AD 1300–1550), a simple mortuary‐defined two‐tiered presumptive chiefdom level maize agriculturalist socioeconomic context from lower east Tennessee. The Dallas phase results affirm a general pre‐Colombian North American pattern of no sex differences and display comparable adult to subadult frequencies. The study also reveals that given a sufficient sample size, “elites” do indeed exhibit a significantly lower frequency of tertiary stage treponemal disease. This can be attributed to better baseline health, which has been previously demonstrated in this sample. It may also be affected by the mortuary inclusion of achieved status individuals whose good health may have facilitated sociopolitical advancement. Another pattern that emerged is an apparent young adult age bias in disease visibility. This suggests that tertiary treponemal disease morbidity may either directly or synergistically factor in early adult age at death. Future research will address the veracity of this association. Am J Phys Anthropol, 2011. © 2010 Wiley‐Liss, Inc.  相似文献   
7.
The gregarine Cosmetophilus vonones , n. g., n. sp. (Sporozoasida: Actinocephalidae: Acanthosporinae) from the intestine and intestinal caeca of the harvestman Vonones sayi (Simon) (Opiliones: Cosmetidae) is described. The new internal parasite is the first recorded from a harvestman in the New World and the second from the opilion suborder Laniatores. In addition to the records from the type locality in western Texas, additional collections are reported from the same host in central and eastern Texas and Tennessee.  相似文献   
8.
Half of each ovary of seven rabbits was removed and deep-frozen in liquid nitrogen for two to three months before being thawed and reimplanted in the original animal. Three of the animals with cryopreserved demi-ovaries bred but did not conceive and these three plus one other had surviving ovarian tissue and normal uterine morphology at autopsy. In one of these cases the ovaries were smooth surfaced but, in two animals, follicles were present, and, in one, corpora haemoragica were observed. The three remaining rabbits would not breed after their frozen-thawed demi-ovaries were reimplanted and their uteri were atrophied at autopsy. No surviving ovarian tissue was detected. In the four cases where the organs survived cryopreservation, three sets of demi-ovaries were fast-frozen by a series of three plunges, respectively, into alcohol baths at ?7° C and ?38° C and then into liquid nitrogen at ?196° C. The fourth set was frozen at slower rates of 1° C per minute to ?7° C and then at 0.3° C per minute to ?38° C before being plunged into liquid nitrogen. One set of demi-ovaries was intact and the other three had the medulla cored out before freezing.We conclude that several protocols suffice to deep-freeze rabbit ovaries and permit the organs to return to partial function when reimplanted.  相似文献   
9.
An outbreak of blastomycosis in Eastern Tennessee   总被引:1,自引:1,他引:0  
Most cases of blastomycosis are sporadic and only nine outbreaks representing a total of 112 cases have previously been reported. Less than half of these have been culture proven cases. Outbreaks have previously occurred in North Carolina, Minnesota, Illinois, Wisconsin and Virginia. We report three culturally confirmed cases of blastomycosis from Elizabethton, Tennessee, who had onset of illness within a one-week span of time. The patients presented with fever, chest pain, weight loss, poor appetite and myalgia. Each initially had a dry cough which became productive of purulent sputum as the illness progressed. Mild hemoptysis occurred during each patient's course. Serologic testing by immunodiffusion and enzyme immunoassay were positive and testing by complement fixation was negative in each case. The diagnosis was made by histopathology on transbronchial biopsy or transthoracic needle aspiration material. Each patient improved on ketoconazole therapy.  相似文献   
10.
The effects of the second generation sulfonylurea, glyburide, on the pyruvate dehydrogenase multienzyme complex (PDC) of rat myocardial tissue were examined using rat ventricular slices and isolated mitochondria. Therapeutic concentrations (10(-7) to 10(-6)M) of glyburide produced a 30% increase in the decarboxylation of [1(-14)C] pyruvate by the PDC of ventricular tissue. Addition of glyburide to intact rat heart mitochondria stimulated activity of the PDC in a time- and concentration-dependent manner. Half-maximal stimulation of the enzyme occurred with 6 X 10(-5)M glyburide and maximal activation of the enzyme was achieved with 1 X 10(-4)M glyburide. At the height of stimulation, PDC activities were 6-fold greater than those observed under control conditions with succinate alone. When mitochondria were disrupted by sonication or freeze-thawing, glyburide produced no stimulation of pyruvate decarboxylation. We conclude that glyburide directly stimulates the decarboxylation of pyruvate by the PDC of the myocardium. Furthermore, the presence of intact mitochondria is necessary for the stimulatory action of glyburide on the PDC.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号