全文获取类型
收费全文 | 1319篇 |
免费 | 157篇 |
专业分类
1476篇 |
出版年
2022年 | 10篇 |
2021年 | 11篇 |
2018年 | 10篇 |
2015年 | 22篇 |
2014年 | 46篇 |
2013年 | 61篇 |
2012年 | 65篇 |
2011年 | 115篇 |
2010年 | 76篇 |
2009年 | 43篇 |
2008年 | 36篇 |
2007年 | 41篇 |
2006年 | 25篇 |
2005年 | 38篇 |
2004年 | 36篇 |
2003年 | 29篇 |
2002年 | 28篇 |
2001年 | 42篇 |
2000年 | 36篇 |
1999年 | 43篇 |
1998年 | 21篇 |
1997年 | 24篇 |
1995年 | 11篇 |
1994年 | 10篇 |
1993年 | 12篇 |
1992年 | 23篇 |
1991年 | 33篇 |
1990年 | 23篇 |
1989年 | 13篇 |
1988年 | 16篇 |
1987年 | 15篇 |
1986年 | 11篇 |
1985年 | 16篇 |
1983年 | 14篇 |
1982年 | 14篇 |
1981年 | 12篇 |
1980年 | 15篇 |
1979年 | 18篇 |
1978年 | 26篇 |
1977年 | 17篇 |
1976年 | 17篇 |
1975年 | 18篇 |
1974年 | 8篇 |
1973年 | 25篇 |
1972年 | 14篇 |
1971年 | 12篇 |
1969年 | 16篇 |
1968年 | 17篇 |
1967年 | 15篇 |
1966年 | 15篇 |
排序方式: 共有1476条查询结果,搜索用时 10 毫秒
81.
82.
The vascular endothelium in vivo is a remarkably quiescent cell layer that displays a highly differentiated and tissue-specific phenotype. Once established in culture, endothelial cells (EC) are phenotypically different from their in situ counterparts, displaying altered gene expression, increased mitotic index, and decreased cell density. To determine whether manipulating the microenvironment of cells in vitro would lead to a more differentiated phenotype, we cultured bovine aortic EC on floating collagen gels. EC cultured to confluence on floating gels for 24 or 48 hr display mitotic indices nearly identical to those of quiescent endothelium in vivo, nearly two log orders lower than that of EC cultured to confluence on plastic, and cell density on floating gels also resembles that observed for endothelium in vivo. Culture of EC on floating gels leads to decreased expression of platelet-derived growth factor-B, fibronectin, and fibronectin isoform ED-B, and increased levels of connexin40, relative to cells cultured on plastic. We conclude that culture of bovine aortic EC under standard culture conditions results in a phenotype reminiscent of development and/or wound healing, and that culturing them on a floating collagen gel leads to a more differentiated phenotype, reminiscent of that observed for large vessel EC in vivo. 相似文献
83.
Salt cress. A halophyte and cryophyte Arabidopsis relative model system and its applicability to molecular genetic analyses of growth and development of extremophiles 总被引:16,自引:0,他引:16 下载免费PDF全文
84.
Goodwin SM Kolosova N Kish CM Wood KV Dudareva N Jenks MA 《Physiologia plantarum》2003,117(3):435-443
Floral volatiles, which are small and generally water-insoluble, must move from their intracellular sites of synthesis through the outermost cuticle membrane before release from the flower surface. To determine whether petal cuticle might influence volatile emissions, we performed the first analysis of petal cuticle development and its association with the emission of flower volatiles using Antirrhinum majus L. (snapdragon) as a model system. Petal cuticular wax amount and composition, cuticle thickness and ultrastructure, and the amounts of internal and emitted methylbenzoate (the major snapdragon floral scent compound) were examined during 12 days, from flower opening to senescence. Normal ( n -) alkanes were found to be the major wax class of snapdragon petals (29.0% to 34.3%) throughout the 12 days examined. Besides n -alkanes, snapdragon petals possessed significant amounts of methyl branched alkanes (23.6–27.8%) and hydroxy esters (12.0–14.0%). Hydroxy esters have not been previously reported in plants. Changes in amount of methylbenzoate inside the petals followed closely with levels of methylbenzoate emission, suggesting that snapdragon petal cuticle may provide little diffusive resistance to volatile emissions. Moreover, clear associations did not exist between methylbenzoate emission and the cuticle properties examined during development. Nevertheless, the unique wax composition of snapdragon petal cuticles shows similarities with those of other highly permeable cuticles, suggesting an adaptation that could permit rapid volatile emission by scented flowers. 相似文献
85.
Capsaicinoids are the pungent compounds in Capsicum fruits (i.e., "hot" peppers). Peroxidases catalyze capsaicinoid oxidation and may play a central role in their metabolism. However, key kinetic aspects of peroxidase-catalyzed capsaicinoid oxidation remain unresolved. Using transient-state methods, we evaluated horseradish peroxidase compound I and II reduction by two prominent capsaicinoids (25 degrees C, pH 7.0). We determined rate constants approaching 2 x 10(7) and 5 x 10(5)M(-1)s(-1) for compound I and compound II reduction, respectively. We also determined k(app) values for steady-state capsaicinoid oxidation approaching 8 x 10(5)M(-1)s(-1) (25 degrees C, pH 7.0). Accounting for stoichiometry, these are in excellent agreement with constants for compound II reduction, suggesting that this reaction governs capsaicinoid-dependent peroxidase turnover. Ascorbate rapidly reduced capsaicinoid radicals, assisting in the determination of the kinetic constants reported. Because ascorbate accumulates in Capsicum fruits, it may also be an important determinant for capsaicinoid content and preservation in Capsicum fruits and related products. 相似文献
86.
Reynolds JD Goodwin NB Freckleton RP 《Philosophical transactions of the Royal Society of London. Series B, Biological sciences》2002,357(1419):269-281
We provide the first review of phylogenetic transitions in parental care and live bearing for a wide variety of vertebrates. This includes new analyses of both numbers of transitions and transition probabilities. These reveal numerous transitions by shorebirds and anurans toward uniparental care by either sex. Whereas most or all of the shorebird transitions were from biparental care, nearly all of the anuran transitions have been from no care, reflecting the prevalence of each form of care in basal lineages in each group. Teleost (bony) fishes are similar to anurans in displaying numerous transitions toward uniparental contributions by each sex. Whereas cichlid fishes have often evolved from biparental care to female care, other teleosts have usually switched from no care to male care. Taxa that have evolved exclusive male care without courtship-role reversal are characterized by male territoriality and low costs of care per brood. Males may therefore benefit from care through female preference of parental ability in these species. Primates show a high frequency of transitions from female care to biparental care, reflecting the prevalence of female care in basal lineages. In the numerous taxa that display live bearing by females, including teleosts, elasmobranchs, squamate reptiles and invertebrates, we find that live bearing has always evolved from a lack of care. Although the transition counts and probabilities will undoubtedly be refined as phylogenetic information and methodologies improve, the overall biases in these taxa should help to place adaptive hypotheses for the evolution of care into a stronger setting for understanding directions of change. 相似文献
87.
Goodwin NB Dulvy NK Reynolds JD 《Philosophical transactions of the Royal Society of London. Series B, Biological sciences》2002,357(1419):259-267
Selection for live bearing is thought to occur when the benefits of increasing offspring survival exceed the costs of reduced fecundity, mobility and the increased metabolic demands of carrying offspring throughout development. We present evidence that live bearing has evolved from egg laying 12 times in teleost (bony) fishes, bringing the total number of transitions to 21 to 22 times in all fishes, including elasmobranchs (sharks and rays). Live bearers produce larger offspring than egg layers in all of 13 independent comparisons for which data were available. However, contrary to our expectation there has not been a consistent reduction in fecundity; live bearers have fewer offspring in seven out of the 11 available comparisons. It was predicted that live bearers would have a larger body size, as this facilitates accommodation of developing offspring. This prediction was upheld in 14 out of 20 comparisons. However, this trend was driven by elasmobranchs, with large live bearers in seven out of eight comparisons. Thus, while the evolution of live bearing in elasmobranchs is correlated with predicted increases in offspring size and adult size, teleost live bearers do not have such a consistent suite of life-history correlates. This suggests that constraints or selection pressures on associated life histories may differ in live-bearing elasmobranchs and teleost fishes. 相似文献
88.
89.
Chen B Pogue BW Goodwin IA O'Hara JA Wilmot CM Hutchins JE Hoopes PJ Hasan T 《Radiation research》2003,160(4):452-459
In the present study, the effects of photodynamic therapy (PDT) with verteporfin on tumor blood flow and tumor regrowth were compared as verteporfin distributed in different compartments within the RIF-1 tumor. Tissue distribution of verteporfin was examined by fluorescence microscopy, and blood flow measurements were taken with a laser Doppler system. It was found that, at 15 min after drug administration, when verteporfin was mainly confined within the vasculature, PDT induced a complete arrest of blood flow by 6 h after treatment. PDT treatment at a longer drug-light interval (3 h), which allowed the drug to diffuse to the tumor interstitium, caused significantly less flow decrease, only to 50% of the initial flow in 6 h. A histological study and Hoechst 33342 staining of functional tumor vasculature confirmed the primary vascular damage and the decrease in tumor perfusion. The regrowth rate of tumors treated with 15-min interval PDT was 64% of that of the control group. However, when tumors were treated with 3-h interval PDT, the regrowth rate was not significantly different from that of the control, indicating that only the 15-min interval PDT caused serious damage to the tumor vascular bed. These results support the hypothesis that temporal pharmacokinetic changes in the distribution of the photosensitizer between the tumor parenchyma and blood vessels can significantly alter the tumor target of PDT. 相似文献
90.