全文获取类型
收费全文 | 5976篇 |
免费 | 656篇 |
国内免费 | 1444篇 |
专业分类
8076篇 |
出版年
2024年 | 45篇 |
2023年 | 156篇 |
2022年 | 186篇 |
2021年 | 217篇 |
2020年 | 250篇 |
2019年 | 300篇 |
2018年 | 314篇 |
2017年 | 245篇 |
2016年 | 238篇 |
2015年 | 237篇 |
2014年 | 291篇 |
2013年 | 333篇 |
2012年 | 246篇 |
2011年 | 296篇 |
2010年 | 254篇 |
2009年 | 354篇 |
2008年 | 366篇 |
2007年 | 387篇 |
2006年 | 370篇 |
2005年 | 314篇 |
2004年 | 287篇 |
2003年 | 236篇 |
2002年 | 201篇 |
2001年 | 151篇 |
2000年 | 152篇 |
1999年 | 172篇 |
1998年 | 161篇 |
1997年 | 118篇 |
1996年 | 118篇 |
1995年 | 89篇 |
1994年 | 99篇 |
1993年 | 96篇 |
1992年 | 82篇 |
1991年 | 88篇 |
1990年 | 88篇 |
1989年 | 71篇 |
1988年 | 69篇 |
1987年 | 53篇 |
1986年 | 45篇 |
1985年 | 53篇 |
1984年 | 49篇 |
1983年 | 28篇 |
1982年 | 43篇 |
1981年 | 38篇 |
1980年 | 25篇 |
1979年 | 18篇 |
1978年 | 12篇 |
1977年 | 14篇 |
1976年 | 5篇 |
1974年 | 6篇 |
排序方式: 共有8076条查询结果,搜索用时 15 毫秒
51.
James R. Hummert 《American journal of physical anthropology》1983,62(2):167-176
Cortical bone growth is analyzed for 174 children from a Medieval Christian population at Kulubnarti in the Batn el Hajar of Sudanese Nubia (550–1450 AD ). Using the tibia as a representative long bone, total subperiosteal area, cortical area, medullary area, and percent cortical area at midshaft were calculated. While growth in total and cortical areas, as well as in length, appear to be fairly well maintained, percent cortical area reveals unusual growth patterns which reflect excessive endosteal resorption. Compared to the relative reduction in bone mass which has been observed in malnourished living children, as well as with previously reported evidence for stress in the Kulubnarti population, the present data support an interpretation of nutritionally related stress and of no major diachronic dietary change. 相似文献
52.
Type 1 insulin-like growth factor receptor (IGF1R) is a membrane-spanning glycoprotein of the insulin receptor family that has been implicated in a variety of cancers. The key questions related to molecular mechanisms governing ligand recognition by IGF1R remain unanswered, partly due to the lack of testable structural models of apo or ligand-bound receptor complexes. Using a homology model of the IGF1R ectodomain IGF1RΔβ, we present the first experimentally consistent all-atom structural models of IGF1/IGF1RΔβ and IGF2/IGF1RΔβ complexes. Our explicit-solvent molecular dynamics (MD) simulation of apo-IGF1RΔβ shows that it displays asymmetric flexibility mechanisms that result in one of two binding pockets accessible to growth factors IGF1 and IGF2, as demonstrated via an MD-assisted Monte Carlo docking procedure. Our MD-generated ensemble of structures of apo and IGF1-bound IGF1RΔβ agrees reasonably well with published small-angle X-ray scattering data. We observe simultaneous contacts of each growth factor with sites 1 and 2 of IGF1R, suggesting cross-linking of receptor subunits. Our models provide direct evidence in favor of suggested electrostatic complementarity between the C-domain (IGF1) and the cysteine-rich domain (IGF1R). Our IGF1/IGF1RΔβ model provides structural bases for the observation that a single IGF1 molecule binds to IGF1RΔβ at low concentrations in small-angle X-ray scattering studies. We also suggest new possible structural bases for differences in the affinities of insulin, IGF1, and IGF2 for their noncognate receptors. 相似文献
53.
54.
Most mammals have deformable bodies, making it difficult to measure the size of living or freshly killed ones accurately. Because small rodents are common prey of many snakes, and because nearly all snakes swallow their prey whole, we explored four methods for determining the ingestible size (the smallest cross‐sectional area that the largest part of the rodent can be made into without breaking bones or dislocating joints) of 100 intact rodents, including 50 Musmusculus and 50 Rattus norvegicus. Cross‐sectional areas derived from maximal height and width of specimens at rest or the same specimens wrapped snout to pelvic girdle are roughly 1.5× higher than areas calculated either by the height and width of the same specimens rolled into cylinders or by volumetric displacement. Rolling rodents into cylinders reduces cross‐sectional area by straightening the vertebral column, lengthening the abdominal cavity, elevating the sternum, compressing the thoracic cavity, and protracting the shoulder joint, that is, changes similar to those seen in rodents eaten by snakes. Reduced major axis regression of the smallest attainable cross‐sectional area, y, on mass, x, shows that y (in log mm2) approximates 1.53x (in log grams)0.69 for rats and 1.63x0.64 for mice. Our results suggest that visual cues provided by live rodents might lead most predators, like snakes, to overestimate ingestible size and hence rarely attack prey too large to ingest. J. Morphol. 2012. © 2012 Wiley Periodicals, Inc. 相似文献
55.
We examined patterns of dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) loading to a small urban stream
during baseflow and stormflow. We hypothesized that lower DOC and TDN contributions from impervious surfaces would dilute
natural hydrologic flowpath (i.e., riparian) contributions during storm events in an urban watershed, resulting in lower concentrations
of DOC and TDN during storms. We tested these hypotheses in a small urban watershed in Portland, Oregon, over a 3-month period
during the spring of 2003. We compared baseflow and stormflow chemistry using Mann–Whitney tests (significant at p<0.05). We also applied a mass balance to the stream to compare the relative significance of impervious surface contributions
versus riparian contributions of DOC and TDN. Results showed a significant increase in stream DOC concentrations during stormflows
(median baseflow DOC = 2.00 mg l−1 vs. median stormflow DOC = 3.46 mg l−1). TDN streamwater concentrations, however, significantly decreased with stormflow (median baseflow TDN = 0.75 mg l−1 vs. median stormflow TDN = 0.56 mg l−1). During storms, remnant riparian areas contributed 70–74% of DOC export and 38–35% of TDN export to the stream. The observed
pattern of increased DOC concentrations during stormflows in this urban watershed was similar to patterns found in previous
studies of forested watersheds. Results for TDN indicated that there were relatively high baseflow nitrogen concentrations
in the lower watershed that may have partially masked the remnant riparian signal during stormflows. Remnant riparian areas
were a major source of DOC and TDN to the stream during storms. These results suggest the importance of preserving near-stream
riparian areas in cities to maintain ambient carbon and nitrogen source contributions to urban streams. 相似文献
56.
毕节试验区石漠化时空演变过程和演变特征 总被引:6,自引:0,他引:6
基于RS和GIS技术,解译了2000、2005和2010年3期毕节试验区石漠化数据,利用空间分析和数理统计分析方法,在探讨石漠化时空演变总体特征的基础上,选取演变方式、演变方向和演变速率等指标,重点分析了不同石漠化强度之间的内部转移特征,演绎了10年间毕节试验区石漠化演变过程。研究表明:(1)10年间,石漠化总面积呈现先增加后趋稳的演变态势,石漠化扩张趋势虽然初步遏制,但局部地区还在恶化,防治形势仍很严峻;(2)石漠化演变方式以渐变式为主,跳跃式为辅,返变式最少;(3)石漠化演变的方向既存在改善也存在恶化,且轻度改善和轻度恶化的面积均比较大,一边治理、一边破坏的现象还没有根本遏制;(4)非石漠化与石漠化相互转换非常活跃,潜在石漠化虽被定义为非石漠化,但并不稳定,容易转变为石漠化;(5)石漠化演变速率分为转入速率、转出速率和综合速率,中度石漠化综合速率最高,转入速率大于转出速率,潜在石漠化和轻度石漠化是中度石漠化增加的主要来源;(6)石漠化时空演变特征与生态环境建设及社会经济发展具有一定的相关性。该研究成果为喀斯特山区生态环境保护和石漠化防治提供了有益的参考。 相似文献
57.
周期性土壤干旱和叶片水势对气孔响应木质部ABA灵敏度的影响 总被引:5,自引:0,他引:5
研究了周期性土壤干旱期间气孔对木质部ABA响应的灵敏度的变化以及叶片水势对灵敏度的影响。实验结果证明了木质部ABA浓度是反映根系周围土壤水分状况的一个指标的结论。土壤周期性干旱不影响木质部ABA浓度对土壤水分状况的依赖关系,但显著地提高了气孔对木质部ABA 响应的灵敏度。根据对实测数据的数学模拟结果显示,引起气孔导度下降50% 所需的木质部ABA浓度从第一轮土壤干旱的750 nmol/L降至第二轮土壤干旱的550 nmol/L。分根实验的结果表明,叶片水分亏缺显著提高了气孔对木质部ABA 的响应的灵敏程度,全根干旱中引起气孔导度下降50 % 所需的木质部ABA 浓度比半根干旱的小2 ~4 倍。这表明,气孔对木质部ABA响应的灵敏度不是一个固定的特性,可随植物生长环境及许多其他因素的变化而表现出很大的差异 相似文献
58.
59.
TERESA SCHWARZKOPF SUSAN J. RIHA TIMOTHY J. FAHEY STEPHEN DEGLORIA 《Austral ecology》2011,36(3):280-289
Cloud forest vegetation structure and composition were studied in the Venezuelan Andes at three sites in Mérida State. Although the sites are within 10 to 30 km of each other, climatic, geologic and topographic differences are remarkable. The main purpose of the study was to determine the relationship of specific environmental variables to forest vegetation characteristics, including basal area, tree height, density and diversity, and leaf area index (LAI). At 51 plots, all trees' diameter at breast height >10 cm were recorded and identified. Although the environment at the three sites is distinctive, the tree species composition of the most abundant species was very similar. None of the measured environment variables were significantly correlated with the measured vegetation structure variables, except LAI, which was correlated with slope orientation; LAI showed higher values at south‐facing plots. Tree height was relatively uniform, while basal area was highly variable and reached very high values. Stem densities were in the range reported elsewhere in cloud forests. Multivariate analysis using structure or composition data shows segregation of the plots by site. Principal component analyses by site indicate a minor impact of environmental factors on forest variables. At each site, a particular group of species are correlated with the ordination axes. We conclude that species pools and forest dynamics add to the complexity of the structure of the studied cloud forests. 相似文献
60.