全文获取类型
收费全文 | 64886篇 |
免费 | 4193篇 |
国内免费 | 9篇 |
专业分类
69088篇 |
出版年
2023年 | 497篇 |
2022年 | 490篇 |
2021年 | 1016篇 |
2020年 | 896篇 |
2019年 | 914篇 |
2018年 | 2020篇 |
2017年 | 1882篇 |
2016年 | 2447篇 |
2015年 | 3226篇 |
2014年 | 3230篇 |
2013年 | 4294篇 |
2012年 | 5172篇 |
2011年 | 4620篇 |
2010年 | 2895篇 |
2009年 | 2273篇 |
2008年 | 3569篇 |
2007年 | 3375篇 |
2006年 | 3149篇 |
2005年 | 2699篇 |
2004年 | 2634篇 |
2003年 | 2325篇 |
2002年 | 2171篇 |
2001年 | 1424篇 |
2000年 | 1397篇 |
1999年 | 1082篇 |
1998年 | 467篇 |
1997年 | 375篇 |
1996年 | 356篇 |
1995年 | 342篇 |
1994年 | 263篇 |
1993年 | 244篇 |
1992年 | 533篇 |
1991年 | 459篇 |
1990年 | 413篇 |
1989年 | 407篇 |
1988年 | 397篇 |
1987年 | 358篇 |
1986年 | 336篇 |
1985年 | 339篇 |
1984年 | 350篇 |
1983年 | 236篇 |
1982年 | 221篇 |
1980年 | 172篇 |
1979年 | 210篇 |
1978年 | 204篇 |
1975年 | 194篇 |
1974年 | 214篇 |
1973年 | 222篇 |
1972年 | 185篇 |
1969年 | 170篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
981.
Jesús Rodríguez-Calcerrada Jose Alberto Pardos Luis Gil Peter B. Reich Ismael Aranda 《Plant Ecology》2008,195(2):273-285
In order to understand better the ecology of the temperate species Quercus petraea and the sub-Mediterranean species Quercus pyrenaica, two deciduous oaks, seedlings were raised in two contrasting light environments (SH, 5.3% full sunlight vs. HL, 70% full
sunlight) for 2 years, and a subset of the SH seedlings were transferred to HL (SH–HL) in the summer of the second year. We
predicted that Q. pyrenaica would behave more as a stress-tolerant species, with lower specific leaf area (SLA), allocation to leaf mass, and growth
rate and less responsiveness to light in these metrics, than Q. petraea, presumed to be more competitive when resources, especially light and water, are abundant. Seedlings of Q. petraea had larger leaves with higher SLA, and exhibited a greater relative growth rate (RGR) in both SH and HL. They also displayed
a higher proportion of biomass in stems (SMF), and a lower root to shoot ratio (R/S) in HL than those of Q. pyrenaica, which sprouted profusely, and had higher rates of photosynthesis (An) and stomatal conductance (gwv), but lower whole-plant net assimilation rate (NAR). On exposure to a sudden increase in light, SH–HL seedlings of both species
showed a short period of photoinhibition, but fully acclimated photosynthetic features within 46 days after transference;
height, main stem diameter, RGR and NAR all increased at the end of the experiment compared to SH seedlings, with these increases
more pronounced in Q. petraea. Observed differences in traits and responses to light confirmed a contrasting ecology at the seedling stage in Q. petraea and Q. pyrenaica in consonance with differences in their overall distribution. We discuss how the characteristics of Q. petraea may limit the availability of suitable regeneration niches to microsites of high-resource availability in marginal populations
of Mediterranean climate, with potential negative consequences for its recruitment under predicted climatic changes. 相似文献
982.
983.
The fruit mineral contents (K, Ca, Ba, Br, Zn, Co, Cr, Fe, Na, Rb, Cs, and Sr) of four native and two exotic naturalized shrubs
growing in different areas of the Andean Patagonian region of Argentina were investigated. Native species Berberis darwinii, Berberis microphylla (Berberidaceae), Aristotelia chilensis (Elaeocarpaceae) and Ribes magellanicum (Saxifragaceae) produce small berries while the fruits of the exotic species Rosa rubiginosa and Rosa canina (Rosaceae) are aggregates of aquenes. They are used to prepare jams, tea, liquors, and ice creams. Native shrub fruits had
higher content of Br, Zn, Co, Cr, Fe, Mo, and Na than those of the exotic naturalized species. Rosa species showed the highest contents of Ca and Ba in their fruits (the mean content doubled those of the native plant fruits).
The fruit nutrient content found in the studied species was similar or higher than other values reported for fruits of temperate
and tropical species in the world. 相似文献
984.
985.
Sáez ME González-Sánchez JL Ramírez-Lorca R Martínez-Larrad MT Zabena C González A Morón FJ Ruiz A Serrano-Ríos M 《PloS one》2008,3(8):e2953
Cardiovascular disease is the leading cause of morbidity and mortality in the industrialized world. Familial aggregation of cardiovascular risk factors is a frequent finding, but genetic factors affecting its presentation are still poorly understood. The calpain 10 gene (CAPN10) has been associated with type 2 diabetes (T2DM), a complex metabolic disorder with increased risk of cardiovascular disease. Moreover, the CAPN10 gene has been associated with the presence of metabolic syndrome (MS) in T2DM and in polycystic ovary syndrome (PCOS). In this work, we have analysed whether the polymorphisms UCSNP44, -43, -19 and -63 are related to several cardiovascular risk factors in the context of MS. Molecular analysis of CAPN10 gene was performed in 899 individuals randomly chosen from a cross-sectional population-based epidemiological survey. We have found that CAPN10 gene in our population is mainly associated with two indicators of the presence of insulin resistance: glucose levels two hours after a 75-g oral glucose tolerance test (OGTT) and HOMA values, although cholesterol levels and blood pressure values are also influenced by CAPN10 variants. In addition, the 1221/1121 haplogenotype is under-represented in individuals that fulfil the International Diabetes Federation (IDF) diagnostic criteria for MS. Our results suggest that CAPN10 gene is associated with insulin resistance phenotypes in the Spanish population. 相似文献
986.
Quílez J Torres E Chalmers RM Hadfield SJ Del Cacho E Sánchez-Acedo C 《Applied and environmental microbiology》2008,74(19):6026-6031
To provide information on the transmission dynamics of cryptosporidial infections in domestic small ruminants and the potential role of sheep and goats as a source for human cryptosporidiosis, Cryptosporidium-positive isolates from 137 diarrheic lambs and 17 goat kids younger than 21 days of age were examined by using genotyping and subtyping techniques. Fecal specimens were collected between 2004 and 2006 from 71 sheep and 7 goat farms distributed throughout Aragón (northeastern Spain). Cryptosporidium parvum was the only species identified by restriction analyses of PCR products from small-subunit rRNA genes from all 154 microscopy-positive isolates and the sequencing of a subset of 50 isolates. Sequence analyses of the glycoprotein (GP60) gene revealed extensive genetic diversity within the C. parvum strains in a limited geographical area, in which the isolates from lambs exhibited 11 subtypes in two subtype families (IId and IIa) and those from goat kids displayed four subtypes within the family IId. Most isolates (98%) belonged to the subtype family IId, whereas only three isolates belonged to the most widely distributed family, IIa. Three of the four most prevalent subtypes (IIdA17G1a, IIdA19G1, and IIdA18G1) were previously identified in humans, and five subtypes (IIdA14G1, IIdA15G1, IIdA24G1, IIdA25G1, and IIdA26G1) were novel subtypes. All IId subtypes were identical to each other in the nonrepeat region, except for subtypes IIdA17G1b and IIdA22G1, which differed by a single nucleotide polymorphism downstream of the trinucleotide repeats. These findings suggest that lambs and goat kids are an important reservoir of the zoonotic C. parvum subtype family IId for humans. 相似文献
987.
M. Cristina García Kirkbride 《Brittonia》1979,31(3):313-332
All neotropical genera previously included in the tribe Isertieae A. Rich. ex DC. are considered. The tribe Isertieae is here delimited by the following characters: absence of rhaphides, valvate aestivation of the corolla, cordiform or discoid placenta, fleshy indehiscent fruit, and seed coat cells with pitted walls. As so delimited, the tribe definitely includes the neotropical generaIsertia Schreb.,Gonzalagunia Ruiz & Pav.,Amphidasya Standl.,Raritebe Wernh.,Sabicea Aubl., andMussaenda L. as well as some paleotropical genera not here considered. The following neotropical genera are excluded from tribe Isertieae:Hippotis Ruiz & Pav.,Pentagonia Benth.,Sommera Schlecht.,Tammsia Karst.,Coccocypselum P. Browne,Schradera Vahl,Didymochlamys Hook, f., andXerococcus Oerst. The position of four genera,Striolaria Ducke,Patima Aubl.,Habroneuron Standl., andNeosabicea Wernh., remains uncertain due to a lack of material. 相似文献
988.
María L. Lebrón 《Oecologia》1979,42(1):31-46
Summary The present study is an attempt to understand biology of Palicourea riparia Bentham relative to the process of recovery following disturbance in a tropical rain forest in Puerto Rico. In this species seed germination and seedling maintenance are favored by environmental conditions associated with disturbance, yet germination and seedling persistence are significant in undisturbed forest conditions. The pattern of seed germination indicates that (1) the seeds possess a dormancy mechanism, (2) dormany is not evenly distributed within a seed population, and (3) some seeds can remain viable if buried in the soil for at least three years. Results from reciprocal transplant studies indicate that there has been no differentiation into growth ecotypes in this species. Photosynthetic rates are very low under both field and laboratory conditions. In the laboratory, photosynthetic rates are consistently enhanced by increases in light intensity. It is concluded that P. riparia is a species of wide ecological amplitude that acts as a gap opportunist. 相似文献
989.
García-Fuentes E Gil-Villarino A Zafra MF García-Peregrín E 《Comparative biochemistry and physiology. Part B, Biochemistry & molecular biology》2002,133(2):269-275
For a better understanding of the hyperlipidemic function of saturated fat, we have studied the comparative effects of diet supplementation with 10 and 20% coconut oil on the main lipid classes of chick plasma. Changes in fatty acid composition of free fatty acid and triglyceride fractions were parallel to that of the experimental diet. Thus, the increase in the percentages of 12:0 and 14:0 acids may contribute to the hypercholesterolemic effects of coconut oil feeding. Plasma phospholipids incorporated low levels of 12:0 and 14:0 acids whereas 18:0, the main saturated fatty acid of this fraction, also increased after coconut oil feeding. The percentage of 20:4 n-6 was higher in plasma phospholipids than in the other fractions and was significantly decreased by our dietary manipulations. Likewise, minor increases were found in the percentages of 12:0 and 14:0 acids in plasma cholesterol esters. However, the percentage of 18:2 acid significantly increased after coconut oil feeding. Our results show a relationship between fatty acid composition of diets and those of plasma free fatty acid and triglyceride fractions, whereas phospholipids and cholesterol esters are less sensitive to dietary changes. 相似文献
990.
Inducible dimerization of FGFR1: development of a mouse model to analyze progressive transformation of the mammary gland 总被引:6,自引:0,他引:6
Welm BE Freeman KW Chen M Contreras A Spencer DM Rosen JM 《The Journal of cell biology》2002,157(4):703-714
To develop an inducible and progressive model of mammary gland tumorigenesis, transgenic mice were generated with a mouse mammary tumor virus-long terminal repeat-driven, conditional, fibroblast growth factor (FGF)-independent FGF receptor (FGFR)1 (iFGFR1) that can be induced to dimerize with the drug AP20187. Treatment of transgenic mice with AP20187 resulted in iFGFR1 tyrosine phosphorylation, increased proliferation, activation of mitogen-activated protein kinase and Akt, and lateral budding. Lateral buds appeared as early as 3 d after AP20187 treatment and initially consisted of bilayered epithelial cells and displayed apical and basolateral polarity appeared after 13 d of AP20187 treatment. Invasive lesions characterized by multicell-layered lateral buds, decreased myoepithelium, increased vascular branching, and loss of cell polarity were observed after 2-4 wk of treatment. These data indicate that acute iFGFR1 signaling results in increased lateral budding of the mammary ductal epithelium, and that sustained activation induces alveolar hyperplasia and invasive lesions. 相似文献