全文获取类型
收费全文 | 981篇 |
免费 | 24篇 |
专业分类
1005篇 |
出版年
2015年 | 16篇 |
2014年 | 9篇 |
2013年 | 23篇 |
2012年 | 16篇 |
2011年 | 15篇 |
2010年 | 31篇 |
2009年 | 24篇 |
2008年 | 35篇 |
2007年 | 50篇 |
2006年 | 18篇 |
2005年 | 25篇 |
2004年 | 18篇 |
2002年 | 16篇 |
1999年 | 14篇 |
1998年 | 11篇 |
1997年 | 17篇 |
1996年 | 13篇 |
1995年 | 12篇 |
1994年 | 12篇 |
1993年 | 19篇 |
1992年 | 11篇 |
1991年 | 21篇 |
1990年 | 17篇 |
1989年 | 14篇 |
1988年 | 12篇 |
1987年 | 17篇 |
1986年 | 13篇 |
1985年 | 16篇 |
1984年 | 14篇 |
1982年 | 13篇 |
1980年 | 12篇 |
1979年 | 11篇 |
1976年 | 12篇 |
1975年 | 14篇 |
1974年 | 17篇 |
1973年 | 19篇 |
1972年 | 16篇 |
1971年 | 10篇 |
1970年 | 8篇 |
1969年 | 9篇 |
1968年 | 10篇 |
1965年 | 36篇 |
1964年 | 19篇 |
1963年 | 32篇 |
1962年 | 23篇 |
1961年 | 33篇 |
1960年 | 21篇 |
1959年 | 27篇 |
1958年 | 27篇 |
1957年 | 13篇 |
排序方式: 共有1005条查询结果,搜索用时 15 毫秒
131.
Biomechanical and morphometric comparisons among coleoptilesfrom wheat seedlings differing in Rht gene-dosage (Rht = 0,2, 4 doses) are presented in an effort to evaluate the influenceof Rht on the mechanics of soil penetration by this organ. Rhtis known to reduce seedling establishment compared to the wildtype. Data from 37-day-old seedlings indicate that Rhtreduces tissue elastic modulus E, increases the second momentof area I, and decreases the slenderness ratio (l/r) of coleoptiles.Rht-relatedchanges in E and I are such that the flexural stiffness of coleoptilesfrom Rht plants does not differ significantly from the wildtype-hence the growing coleoptiles of all three genotypes haveequivalent biomechanical capacity to penetrate the soil. Rhtreduction of coleoptile slenderness ratios confers a capacityto safely sustain higher axial compressive loads compared tocoleoptiles with equivalent flexural stiffness but higher ratios.However, wild type seedlings produce longer coleoptiles andlonger subcrown internodes than Rht seedlings. Longer coleoptilesdeliver the crown node closer to the top of the soil beforethe crown node extends beyond the lateral confinement of thecoleoptile. This reduces the potential for buckling of the subcrowninternode and leaves due to the compressive loading of soil.Rht affects a variety of mechanical features whose influenceis dependent upon the stage of seedling growth and the degreeof soil compaction. However, at equivalent depths of burialwhich exceed the maximum length of coleoptiles and moderatesoil compaction, Rht is biomechanically disadvantageous to seedlingestablishment. Wheat, germination, biomechanics, Rht-gene 相似文献
132.
133.
Penelope Reimers Natalie Shenker Gillian Weaver Anna Coutsoudis 《International breastfeeding journal》2018,13(1):43
Background
Donor human milk is the World Health Organization’s recommendation for infant feeding when the mother’s own breast milk is unavailable. Breast milk has been shown to reduce morbidity and mortality and in low birthweight infants, donor milk reduces the incidence of necrotising enterocolitis, late onset sepsis and improves outcomes. There is a paucity of literature documenting outcomes of using donor human milk in older children who need additional support for a variety of health issues.Case presentation
A series of seven case studies is presented of orphaned and abandoned children, many of whom were either HIV exposed or positive. All children were fed with pasteurised donor human milk at a transition home and their progress reported.Conclusions
Although detailed medical records were not always available, the case studies provide anecdotal evidence of the protective effects of donor human milk against failure to thrive, diarrhoea, atopic dermatitis, and opportunistic infections.134.
Replication slippage may cause parallel evolution in the secondary structures of mitochondrial transfer RNAs 总被引:9,自引:4,他引:5
Presence of the dihydrouridine (D) stem in the mitochondrial cysteine tRNA
is unusually variable among lepidosaurian reptiles. Phylogenetic and
comparative analyses of cysteine tRNA gene sequences identify eight
parallel losses of the D-stem, resulting in D-arm replacement loops.
Sampling within the monophyletic Acrodonta provides no evidence for
reversal. Slipped-strand mispairing of noncontiguous repeated sequences
during replication or direct replication slippage can explain repeats
observed within cysteine tRNAs that contain a D-arm replacement loop. These
two mechanisms involving replication slippage can account for the loss of
the cysteine tRNA D-stem in several lepidosaurian lineages, and may
represent general mechanisms by which the secondary structures of
mitochondrial tRNAs are altered.
相似文献
135.
A flux analysis of glucose metabolism in the filamentous fungus Rhizopus oryzae was achieved using a specific radioactivity curve-matching program, TFLUX. Glycolytic and tricarboxylic acid cycle intermediates labeled through the addition of extracellular [U-14C]glucose were isolated and purified for specific radioactivity determinations. This information, together with pool sizes and the rates of glucose utilization and end product production, provided input for flux maps of the metabolic network under two different experimental conditions. Based upon the flux analysis of this system, a mutant of R. oryzae with higher lactate and lower ethanol yields than the parent was sought for and found. 相似文献
136.
137.
1. We examined the role of flooding on the leaf nutrient content of riparian trees by comparing the carbon : nitrogen : phosphorus (C : N : P) ratio of leaves and litter of Rio Grande cottonwood (Populus deltoides ssp. wislizenii) in flood and non‐flood sites along the Middle Rio Grande, NM, U.S.A. The leaf C : N : P ratio was also examined for two non‐native trees, saltcedar (Tamarix chinensis) and Russian olive (Elaeagnus angustifolia), and six species of dominant riparian arthropods. 2. Living leaves and leaf litter of cottonwoods at flood sites had a significantly lower leaf N : P ratio and higher %P compared with leaves and litter at non‐flood sites. A non‐flood site downstream from wastewater effluent had a significantly lower litter C : N ratio than all other sites, suggesting N fertilisation through ground water. The non‐native trees, saltcedar and Russian olive, had higher mean leaf N content, N : P ratio, and lower C : N ratio compared with cottonwoods across study sites. 3. Riparian arthropods ranged from 5.2 to 7.1 for C : N ratio, 56–216 for C : P ratio, and 8.9–34 for N : P ratio. C content ranged from 25 to 52% of dry mass, N content from 4.7 to 10.8%, and P content from 0.59 to 1.2%. Differences in stoichiometry between high C : nutrient leaf litter and low C : nutrient invertebrates suggests possible food‐quality constraints for detritivores. 4. These results suggest that spatial and temporal variation in the C : N : P ratio of cottonwood leaves and leaf litter is influenced by surface and subsurface hydrologic connection within the floodplain. Reach‐scale variation in the elemental composition of riparian organic matter inputs may have important implications for decomposition, nutrient cycling, and food webs in river floodplain systems. 相似文献
138.
139.
140.
SYNOPSIS. Supraspecific taxa of the ciliate Order Tintinnida Corliss, 1955 are indexed by 304 entries; genetic type species are cited with reference to the original citation. The accompanying bibliography of 133 entries includes those publications cited in the index. Authors for several type citations are corrected, as is authorship of some genera, and a new type designation is made for Parundella to replace the previous erroneous designation. Genera present in the fossil record are so indicated. 相似文献