全文获取类型
收费全文 | 1795篇 |
免费 | 208篇 |
出版年
2022年 | 16篇 |
2021年 | 20篇 |
2020年 | 10篇 |
2019年 | 19篇 |
2018年 | 26篇 |
2017年 | 19篇 |
2016年 | 36篇 |
2015年 | 58篇 |
2014年 | 68篇 |
2013年 | 68篇 |
2012年 | 80篇 |
2011年 | 107篇 |
2010年 | 73篇 |
2009年 | 61篇 |
2008年 | 80篇 |
2007年 | 111篇 |
2006年 | 86篇 |
2005年 | 85篇 |
2004年 | 83篇 |
2003年 | 71篇 |
2002年 | 81篇 |
2001年 | 64篇 |
2000年 | 59篇 |
1999年 | 53篇 |
1998年 | 25篇 |
1997年 | 27篇 |
1996年 | 22篇 |
1995年 | 19篇 |
1994年 | 18篇 |
1993年 | 14篇 |
1992年 | 29篇 |
1991年 | 21篇 |
1990年 | 24篇 |
1989年 | 22篇 |
1988年 | 26篇 |
1987年 | 25篇 |
1986年 | 18篇 |
1985年 | 27篇 |
1984年 | 18篇 |
1983年 | 17篇 |
1982年 | 15篇 |
1980年 | 13篇 |
1979年 | 11篇 |
1977年 | 15篇 |
1976年 | 12篇 |
1974年 | 13篇 |
1972年 | 8篇 |
1971年 | 14篇 |
1970年 | 11篇 |
1965年 | 8篇 |
排序方式: 共有2003条查询结果,搜索用时 31 毫秒
1.
Cyrtandra (Gesneriaceae) is a genus of flowering plants with over 800 species distributed throughout Southeast Asia and the Pacific Islands. On the Hawaiian Islands, 60 named species and over 89 putative hybrids exist, most of which are identified on the basis of morphology. Despite many previous studies on the Hawaiian Cyrtandra lineage, questions regarding the reconciliation of morphology and genetics remain, many of which can be attributed to the relatively young age and evidence of hybridization between species. We utilized targeted enrichment, high‐throughput sequencing, and modern phylogenomics tools to test 31 Hawaiian Cyrtandra samples (22 species, two putative hybrids, four species with two samples each, one species with four samples) and two outgroups for species relationships and hybridization in the presence of incomplete lineage sorting (ILS). Both concatenated and species‐tree methods were used to reconstruct species relationships, and network analyses were conducted to test for hybridization. We expected to see high levels of ILS and putative hybrids intermediate to their parent species. Phylogenies reconstructed from the concatenated and species‐tree methods were highly incongruent, most likely due to high levels of incomplete lineage sorting. Network analyses inferred gene flow within this lineage, but not always between taxa that we expected. Multiple hybridizations were inferred, but many were on deeper branches of the island lineages suggesting a long history of hybridization. We demonstrated the utility of high‐throughput sequencing and a phylogenomic approach using 569 loci to understanding species relationships and gene flow in the presence of ILS. 相似文献
2.
3.
Jill E. Schneider Joan M. Hamilton George N. Wade 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1987,157(1):39-44
Summary Mice selectively bred for either high or low levels of thermoregulatory nest building were cold-acclimated (5°C) for 3 weeks without nesting material; then body weight and food intake were measured. The mice selected for low nest building (Lows) of both sexes showed lower feed efficiencies than the high nest-building mice (Highs), although their body weights were not significantly different (Table 1). This adds to a large body of evidence which suggests that nest building and feed efficiency were influenced by a common mechanism (Lacy et al. 1978; Sulzbach and Lynch 1984; Lunch et al. 1981; Lynch and Roberts 1984).Brown adipose tissue mitochondrial GDP binding and cytochrome c oxidase activity were measured in the above mice. In females, the Lows had 100% higher levels of total GDP binding than the Highs, while no difference between the lines was seen in males (Fig. 2). Thus in the High females, lower energy expenditure through brown fat thermogenesis may account for their greater feed efficiency. In males, the genetic differences in feed efficiency must be due to differences in either thermogenesis in tissues other than brown fat, or mechanisms which reduce heat loss.Abbreviations
Highs
Mice from lines selectively bred for high levels of nest-building;Lows mice from the low nest-building selected lines 相似文献
4.
The coordinating properties of open-chain ligands containing alcoholic or ethereal oxygen donors are examined. Addition of oxygen donors usually leads to complex stabilisation for large metal ions (Pb2+, Cd2+) and to less favourable effects on complex stability for small metal ions (Cu2+, Ni2+). The formation constants of these metal ions with the set of ligands RN(CH2CHOH·CH3)2 where R is ---H, ---CH2CHOH·CH3, ---CH2CH2OCH3, ---CH2CH2OCH2CH2OH, and ---CH2---CHOCH2CH2CH2 are reported. The largest stabilisation for each case where R is an O-donor group relative to R = H occurs for Pb2+, the largest metal ion, while Cu2+, the smallest metal ion, shows the smallest stabilisation. The crystal structure of [Ni(HOCH2CH2NHCH2CH2NH2)2] (NO3)2 is reported. The space group is P
, with cell constants a = 13.098(3), b = 8.737(4), and c = 7.746(3) Å, β = 112.66(3), β = 90.65(3), and γ = 85.03(2), and Z = 2. Disorder of the nitrate anions hindered refinement, with the result that a final conventional R factor of 0.0903 was achieved. The Ni---N bond lengths average 2.06(1) (secondary nitrogen) and 2.10(2) (primary nitrogen). The Ni---O bond lengths are rather long, averaging 2.15(1) Å, which is used to support the idea that the steric effects are responsible for destabilising the complexes of small metal ions such as Ni(11) when neutral oxygen donors are present. 相似文献
5.
Serum prolactin (PRL) decreases in Syrian (Mesocricetus auratus) and Siberian (Phodopus sungorus sungorus) hamsters following short-photoperiod exposure. Both species also exhibit short-photoperiod-induced changes in body and lipid mass, but in opposite directions; Syrian hamsters increase and Siberian hamsters decrease their body weight, changes reflected nearly exclusively in their carcass lipid content. The purpose of these experiments was to determine whether the photoperiod-induced changes in PRL were responsible for the seasonal changes in energy balance in Syrian and Siberian hamsters by using the strategy of experimentally producing serum PRL levels opposite to those normally associated with the photoperiod in which the animals were housed. In long photoperiods serum PRL was reduced to short-day levels by subcutaneous (s.c.) CB-154 (bromoergocryptine, a dopamine agonist) injections. In short photoperiods, serum PRL was elevated to long-day levels in Syrian hamsters by ectopic pituitary explants, and in Siberian hamsters, serum PRL was elevated by chronic s.c. infusions of ovine PRL (oPRL). In both species, manipulations of serum PRL did not affect food intake, carcass composition, or the wet weight of various white and brown adipose tissue pads (WAT and BAT, respectively). Body weight increased in CB-154-treated Syrian hamsters and decreased in Siberian hamsters, an effect partially reversed by coadministration of oPRL in Syrian, but not Siberian, hamsters. Thus, lowering serum PRL to short-day levels in long-day-housed hamsters of both species mimicked the directional change in body weight appropriate for each species when they are exposed to short days. This effect of CB-154 on body weight may be a result of some as yet unidentified effect of dopaminergic stimulation on overall growth since 1) these changes in body weight were not reflected as changes in lipid mass, as occurs naturally following short-day exposure for each species, and 2) neither species exhibited a reciprocal change in body weight when serum PRL was experimentally elevated in short days. Alternatively, it may be that once the energetic responses to short-day exposure have been fully expressed, the ability of PRL to stimulate the target sites of action for PRL for these responses may be decreased. BAT protein content, cytochrome oxidase activity (measures of metabolic growth of this tissue), and retroperitoneal total and specific lipoprotein lipase (LPL) activities were increased by CB-154 treatment in Syrian hamsters.(ABSTRACT TRUNCATED AT 400 WORDS) 相似文献
6.
Visualization of endocytosed markers in freeze-fracture studies of toad urinary bladder 总被引:1,自引:0,他引:1
R A Coleman H W Harris J B Wade 《The journal of histochemistry and cytochemistry》1987,35(12):1405-1414
Antidiuretic hormone (ADH) stimulation increases the apical membrane water permeability of granular cells in toad urinary bladder. This response correlates closely with the fusion of tubular cytoplasmic vesicles with the membrane and delivery of intramembrane particle (IMP) aggregates from the tubules (aggrephores) to the apical membrane. These aggregates are believed to be associated with the channels responsible for the water permeability increase. Removal of ADH triggers apical membrane endocytosis and disappearance of aggregates from the apical membrane. However, it has been unclear whether aggregate disappearance is due to disassembly of aggregates within the apical membrane or to their endocytic retrieval as intact structures. Using colloidal gold and horseradish peroxidase to follow endocytosis from the apical surface after ADH removal, we have directly observed in cross-fractured bladder cells the intramembrane structure of intracellular vesicles that contain these fluid-phase markers. Under these conditions, intact aggregates can be identified in the membrane of tubular endocytosed vesicles. This directly demonstrates that conditions which lower apical membrane water permeability cause the tubular aggrephores to "shuttle" intact aggregates from the apical membrane back into the cytoplasm. An additional population of vesicles with tracer are found which are spherical and display structural features of the apical membrane, as well as occasional aggregates. These vesicles may be responsible for retrieval of aggregates from the surface apical membrane. 相似文献
7.
Rose C. Wade Janet V. Powers Cyril Ponnamperuma 《Origins of life and evolution of the biosphere》1989,19(2):199-220
This bibliography is the seventeenth annual supplement to the comprehensive bibliography on the same subject which was published in Space Life Sci. 2(1970), 225-295; 3(1972), 293-304; 4(1973), 309-329 and in Origins of Life 5(1974), 505-527; 6(1975), 285-300; 7(1976), 75-85; 8(1977), 59-66; 9(1978), 67-74; 10(1980), 69-87; 10(1980), 379-404; 11(1981), 273-288; 12(1982), 93-118; 13(1983), 61-80; 15(1984), 55-69; 17(1987), 171-184; 17(1987), 185-206. 相似文献
8.
The heptose-less mutant of Escherichia coli, D31m4, bound complement subcomponent C1q and its collagen-like fragments (C1qCLF) with Ka values of 1.4 x 10(8) and 2.0 x 10(8) M-1 respectively. This binding was suppressed by chemical modification of C1q and C1qCLF using diethyl pyrocarbonate (DEPC). To investigate the role of lipopolysaccharides (LPS) in this binding, biosynthetically labelled [14C]LPS were purified from E. coli D31m4 and incorporated into liposomes prepared from phosphatidylcholine (PC) and phosphatidylethanolamine (PE) [PC/PE/LPS, 2:2:1, by wt.]. Binding of C1q or its collagen-like fragments to the liposomes was estimated via a flotation test. These liposomes bound C1q and C1qCLF with Ka values of 8.0 x 10(7) and 2.0 x 10(7) M-1; this binding was totally inhibited after chemical modification of C1q and C1qCLF by DEPC. Liposomes containing LPS purified from the wild-strain E. coli K-12 S also bound C1q and C1qCLF, whereas direct binding of C1q or C1qCLF to the bacteria was negligible. Diamines at concentrations which dissociate C1 into C1q and (C1r, C1s)2, strongly inhibited the interaction of C1q or C1qCLF with LPS. Removal of 3-deoxy-D-manno-octulosonic acid (2-keto-3-deoxyoctonic acid; KDO) from E. coli D31m4 LPS decreases the binding of C1qCLF to the bacteria by 65%. When this purified and modified LPS was incorporated into liposomes, the C1qCLF binding was completely abolished. These results show: (i) the essential role of the collagen-like moiety and probably its histidine residues in the interaction between C1q and the mutant D31m4; (ii) the contribution of LPS, particularly the anionic charges of KDO, to this interaction. 相似文献
9.
Classification and ordination of macroinvertebrate assemblages to predict stream acidity in upland Wales 总被引:2,自引:2,他引:0
Macroinvertebrates were collected from riffles at 104 sites in upland Wales during April and July 1984. Species assemblages were ordinated by DECORANA, classified by TWINSPAN and related to stream chemistry and other environmental factors using correlation and multiple discriminant analysis. DECORANA axis 1 was most strongly correlated with pH and aluminium concentration whilst axis 2 correlated with stream gradient and flow. Four TWINSPAN site groups established in each season were also principally related to pH and aluminium concentration, and reflected overall taxon-richness; differences between groups were most apparent during spring, when catchment forest cover and taxon-richness were also related. A dichotomous key based on indicator species was established for each season with the coleopteran Hydraena gracilis Germar and the Ephemeroptera, including Baetis rhodani Pictet, important indicators at Level 1. We propose that these indicator systems may be used for the rapid detection and assessment of acid waters throughout Wales, and that the methodology is applicable generally. 相似文献
10.
OBJECTIVE--To investigate the current problems and needs of terminally ill cancer patients and their family members, and to discover their views of hospital, community, and support team services. DESIGN--Prospective study of patients and families by questionnaire interviews in the patients'' homes. SETTING--Inner London and north Kent (London suburbs). SUBJECTS--65 Patients, each with a member of their family or a career. MAIN OUTCOME MEASURES--Ratings of eight current problems and ratings and comments on three services-hospital doctors and nurses, general practitioners and district nurses, and the support team staff-obtained after a minimum of two weeks'' care from palliative care support teams. RESULTS--Effect of anxiety on the patient''s nearest career. and symptom control were rated as the most severe current problems by both patients and families; a few patients and families identified other severe problems. Families'' ratings of pain control, symptom control, and effect of anxiety on the patient were significantly worse than the patients'' ratings (p less than 0.05). Support teams received the most praise, being rated by 58 (89%) patients and 59 (91%) of family members as good as excellent. General practitioners and district nurses were rated good or excellent by 46 (71%) patients and 46 (71%) family members, but six (9%) in each group rated the service as poor or very bad, and ratings in the inner London district were significantly worse than those in the outer London district. Hospital doctors and nurses were rated good or excellent by 22 (34%) patients and 35 (54%) of family members, and 14 (22%) patients and 15 (23%) family members rated this service as poor or very bad. Negative comments referred to communication (especially at diagnosis), coordination of services, the attitude of the doctor, delays in diagnosis, and difficulties in getting doctors to visit at home. Family members were more satisfied with the services than were patients. CONCLUSIONS--Palliative care needs to include both the patient and family because the needs of the family may exceed those of the patient. Support teams and some hospital and community doctors and nurses met the perceived needs of dying patients and families, but better education and organisation of services are needed. 相似文献