全文获取类型
收费全文 | 1931篇 |
免费 | 73篇 |
国内免费 | 3篇 |
专业分类
2007篇 |
出版年
2013年 | 42篇 |
2012年 | 59篇 |
2011年 | 62篇 |
2010年 | 84篇 |
2009年 | 81篇 |
2008年 | 70篇 |
2007年 | 102篇 |
2006年 | 47篇 |
2005年 | 51篇 |
2004年 | 32篇 |
2003年 | 25篇 |
2001年 | 16篇 |
1999年 | 19篇 |
1998年 | 33篇 |
1997年 | 48篇 |
1996年 | 55篇 |
1995年 | 42篇 |
1994年 | 27篇 |
1993年 | 32篇 |
1992年 | 37篇 |
1991年 | 35篇 |
1990年 | 42篇 |
1989年 | 46篇 |
1988年 | 38篇 |
1987年 | 39篇 |
1986年 | 22篇 |
1985年 | 32篇 |
1984年 | 35篇 |
1983年 | 23篇 |
1982年 | 28篇 |
1981年 | 18篇 |
1980年 | 31篇 |
1979年 | 32篇 |
1978年 | 25篇 |
1977年 | 22篇 |
1976年 | 21篇 |
1975年 | 36篇 |
1974年 | 31篇 |
1973年 | 22篇 |
1972年 | 37篇 |
1971年 | 43篇 |
1959年 | 15篇 |
1958年 | 19篇 |
1957年 | 35篇 |
1956年 | 26篇 |
1955年 | 22篇 |
1954年 | 15篇 |
1953年 | 16篇 |
1952年 | 16篇 |
1951年 | 22篇 |
排序方式: 共有2007条查询结果,搜索用时 8 毫秒
11.
12.
The Influence of Carbon Dioxide and Daily Photon-flux Density on Optimal Leaf Nitrogen Concentration and Root: Shoot Ratio 总被引:5,自引:1,他引:4
Using a cost-benefit model, the leaf nitrogen concentrationand root : shoot ratio that maximize whole-plant relative growthrate are determined as a function of the above-ground environment(integrated daily photon flux density and the concentrationof carbon dioxide at the site of fixation within the leaf).The major advantage of this approach is that it determines theadaptive significance of leaf physiology by considering thefunctional integration of leaves and roots. The predicted responseto increasing daily photon flux densities is an increase inoptimal leaf N concentration (Nopt) and a concomitant increasein root: shoot ratio. Increased carbon dioxide concentrations,on the other hand, reduce Nopt and only slightly change root:shoot ratio. The observed increase in leaf nitrogen concentrationfound in plants growing at high altitudes (low CO2 partial pressure)is also predicted. Since these responses to light and CO2 maximizethe whole-plant relative growth rate, the observed adjustmentsthat plants make to light and carbon dioxide concentration appearto be adaptive. We show that the relationship between photosynthesis and leafnitrogen concentration is complex and depends on the light andCO2 levels at which photosynthesis is measured. The shape ofthis function is important in determining Nopt and the oppositeresponse of leaf nitrogen to light and carbon dioxide is shownto be the result of the different effects of light and CO2 onthe photosynthesis-leaf nitrogen curve. Plant growth, photosynthesis, leaf nitrogen, biomass allocation, optimization, carbon dioxide light 相似文献
13.
Ruta graveotens subshrubs, known to contain a large fractionof their furanocoumarins on the leaf surface, were used forinvestigation of salt and acid rain effects on the plant surfaceand substances occurring there. Groups of shoots on each plantcovered by plastic bags were sprayed with either sulphuric acid,pII 2.5, saturated sodium chloride solution, or a distilledwater control. Uncovered shoots, an additional control, hadthe highest concentrations of furanocoumarins both on the surfaceand in the whole leaf. The upper leaves contained less furanocoumarinsin absolute amounts but were similar to the lower leaves ona percentage basis. Spraying caused a decrease in total furanocoumarinconcentrations, slight in the case of distilled water and greaterfor acid and salt, but the percentage on the surface after sprayingincreased. The upper and lower leaves reacted differently tosalt than to sulphuric acid and water: with salt, the percentagesof furanocoumarins on the surface of both kinds of leaves showedsimilar increases; with the other two sprays, upper leaves showeda similar increase, but the lower leaves had a much higher percentageon the surface owing to a high rate of extrusion. Simulated acid rain, salt sprays, plant surface, furanocoumarins, Ruta graveolens 相似文献
14.
15.
The hock (ankle) joint of horses is bistable: if placed in an intermediate position it springs either to full extension or to strong flexion. We have performed experiments to measure the torques involved and to elucidate the mechanism. The torques were much smaller in hocks of Przewalski's horse ( E. ferus ) and zebra ( E. grevyi ) than in those of domestic horses. We suggest that the bistability of the hock is too slight to have much importance for wild horses but may have been enhanced in the breeding of domestic horses by selection for elegance of gait. The elbow joint is also bistable but the stifle (knee) and carpal joints are not. 相似文献
16.
17.
The diversification of South American murid rodents: evidence from mitochondrial DNA sequence data for the akodontine tribe 总被引:9,自引:0,他引:9
MARGARET F. SMITH JAMES L. PATTON 《Biological journal of the Linnean Society. Linnean Society of London》1993,50(3):149-177
Phylogenetic relationships based on 801 base pairs (bp) of the mitochondrial cytochrome b gene are examined for eight genera and 28 species of the akodontine tribe of South American murid rodents. The akodontine tribe comprises some 35% of the total diversity of the subfamily Sigmodontinae, but the current taxonomy at virtually all levels is uncertain because of inadequate generic diagnoses and assessments of variation and trends in traditional morphological characters. Monophyly of the tribe cannot be resolved by the sequence data, based on comparisons to outgroup taxa in three other tribes (Oryzomyini, Phyllotini, and Thomasomyini). However, highly corroborated monophyletic units within the group are obtained in a variety of both parsimony and distance analyses. These include a redefined and numerically dominant genus Akodon (with Microxus and Hypsimys as synonyms), Bolomys, Lenoxus, Oxymycterus, and a strongly supported assemblage that includes the central Andean Chroeomys and 'Akodon' andinus and the southern Abrothrix, 'Akodon' olivaceus, and the long-clawed mice of the genera Notiomys, Geoxus, and Chelemys. Sequence divergence within species is typically less than 5%, although levels can reach 10% for some highly polytypic forms. Divergence among genera within the tribe reaches 35% in corrected estimates, a level that is as great as that among representatives of different tribes. Changes in the current classification of akodontines are suggested based on these data, and the timing and place of origin of the tribe and its radiation is discussed. 相似文献
18.
19.
20.