全文获取类型
收费全文 | 1268篇 |
免费 | 85篇 |
出版年
2023年 | 8篇 |
2022年 | 15篇 |
2021年 | 43篇 |
2020年 | 31篇 |
2019年 | 44篇 |
2018年 | 31篇 |
2017年 | 30篇 |
2016年 | 49篇 |
2015年 | 59篇 |
2014年 | 53篇 |
2013年 | 71篇 |
2012年 | 100篇 |
2011年 | 100篇 |
2010年 | 68篇 |
2009年 | 55篇 |
2008年 | 68篇 |
2007年 | 50篇 |
2006年 | 55篇 |
2005年 | 50篇 |
2004年 | 48篇 |
2003年 | 40篇 |
2002年 | 46篇 |
2001年 | 36篇 |
2000年 | 27篇 |
1999年 | 16篇 |
1998年 | 17篇 |
1997年 | 8篇 |
1996年 | 5篇 |
1995年 | 5篇 |
1994年 | 12篇 |
1993年 | 9篇 |
1992年 | 15篇 |
1991年 | 13篇 |
1990年 | 7篇 |
1989年 | 8篇 |
1988年 | 4篇 |
1987年 | 6篇 |
1986年 | 10篇 |
1985年 | 6篇 |
1984年 | 8篇 |
1983年 | 3篇 |
1982年 | 4篇 |
1981年 | 3篇 |
1980年 | 3篇 |
1979年 | 3篇 |
1978年 | 4篇 |
1973年 | 1篇 |
1972年 | 1篇 |
1968年 | 1篇 |
1960年 | 1篇 |
排序方式: 共有1353条查询结果,搜索用时 15 毫秒
31.
Visitation rate of pollinators and nectar robbers to the flowers and inflorescences of Tabebuia aurea (Bignoniaceae): effects of floral display size and habitat fragmentation
下载免费PDF全文
![点击此处可从《Botanical journal of the Linnean Society. Linnean Society of London》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Jorge A. Lobo Davi de Lacerda Ramos Aline Cabral Braga 《Botanical journal of the Linnean Society. Linnean Society of London》2016,181(4):667-681
Large floral displays favour pollinator attraction and the import and export of pollen. However, large floral displays also have negative effects, such as increased geitonogamy, pollen discounting and nectar/pollen robber attraction. The size of the floral display can be measured at different scales (e.g. the flower, inflorescence or entire plant) and variations in one of these scales may affect the behaviour of flower visitors in different ways. Moreover, the fragmentation of natural forests may affect flower visitation rates and flower visitor behaviour. In the present study, video recordings of the inflorescences of a tree species (Tabebuia aurea) from the tropical savannah of central Brazil were used to examine the effect of floral display size at the inflorescence and tree scales on the visitation rate of pollinators and nectar robbers to the inflorescence, the number of flowers approached per visit, the number of visits per flower of potential pollinators and nectar robbers, and the interaction of these variables with the degree of landscape disturbance. Nectar production was quantified with respect to flower age. Although large bees are responsible for most of the pollination, a great diversity of flower insects visit the inflorescences of T. aurea. Other bee and hummingbird species are highly active nectar robbers. Increases in inflorescence size increase the visitation rate of pollinators to inflorescences, whereas increases in the number of inflorescences on the tree decrease visitation rates to inflorescences and flowers. This effect has been strongly correlated with urban environments in which trees with the largest floral displays are observed. Pollinating bees (and nectar robbers) visit few flowers per inflorescence and concentrate visits to a fraction of available flowers, generating an overdispersed distribution of the number of visits per inflorescence and per flower. This behaviour reflects preferential visits to young flowers (including flower buds) with a greater nectar supply. 相似文献
32.
Ana M. Anselmo Joaquim M. S. Cabral Júlio M. Novais 《Applied microbiology and biotechnology》1989,31(2):200-203
Summary Spores of Fusarium flocciferum were inserted in porous celite beads. The effects of bead size, adsorption time course, washing cycle and spore concentration on spore loading were investigated. Cell loadings up to 50% (dry weight/beads) were obtained. The degradation of phenol using adsorbed cells was studied in batch experiments. The immobilized cell system was shown to efficiently degrade high concentrations of the substrate (up to 2.0 g/l) and to remain active for more than 2 motths. The oxygen uptake rate of free and immobilized cells was determined at various concentrations of phenol. The kinetic constants K
s=85 mg/l, K
i=345 mg/l and SMI=170 mg/l were estimated from the experimental data by linearization of the Haldane function for the free cells. The uptake rates exhibited by the confined cells were lower (30%) than those obtained for free cells and no significant differences were found for phenol concentrations between 150 and 1200 mg/l. 相似文献
33.
Ludmilla Figueiredo Jochen Krauss Ingolf Steffan‐Dewenter Juliano Sarmento Cabral 《Ecography》2019,42(12):1973-1990
Extinction debt refers to delayed species extinctions expected as a consequence of ecosystem perturbation. Quantifying such extinctions and investigating long‐term consequences of perturbations has proven challenging, because perturbations are not isolated and occur across various spatial and temporal scales, from local habitat losses to global warming. Additionally, the relative importance of eco‐evolutionary processes varies across scales, because levels of ecological organization, i.e. individuals, (meta)populations and (meta)communities, respond hierarchically to perturbations. To summarize our current knowledge of the scales and mechanisms influencing extinction debts, we reviewed recent empirical, theoretical and methodological studies addressing either the spatio–temporal scales of extinction debts or the eco‐evolutionary mechanisms delaying extinctions. Extinction debts were detected across a range of ecosystems and taxonomic groups, with estimates ranging from 9 to 90% of current species richness. The duration over which debts have been sustained varies from 5 to 570 yr, and projections of the total period required to settle a debt can extend to 1000 yr. Reported causes of delayed extinctions are 1) life‐history traits that prolong individual survival, and 2) population and metapopulation dynamics that maintain populations under deteriorated conditions. Other potential factors that may extend survival time such as microevolutionary dynamics, or delayed extinctions of interaction partners, have rarely been analyzed. Therefore, we propose a roadmap for future research with three key avenues: 1) the microevolutionary dynamics of extinction processes, 2) the disjunctive loss of interacting species and 3) the impact of multiple regimes of perturbation on the payment of debts. For their ability to integrate processes occurring at different levels of ecological organization, we highlight mechanistic simulation models as tools to address these knowledge gaps and to deepen our understanding of extinction dynamics. 相似文献
34.
Cannabinoids, the active components of Cannabis sativa (marijuana) and their endogenous counterparts, exert their effects by binding to specific G-protein-coupled receptors that modulate adenylyl cyclase and ion channels. Recent research has shown that the CB1 cannabinoid receptor is also coupled to the generation of the lipid second messenger ceramide via two different pathways: sphingomyelin hydrolysis and ceramide synthesis de novo. Sustained ceramide accumulation in tumor cells mediates cannabinoid-induced apoptosis, as evidenced by in vitro and in vivo studies. This effect seems to be due to the impact of ceramide on key cell signalling systems such as the extracellular signal-regulated kinase cascade and the Akt pathway. These findings provide a new conceptual view on how cannabinoids act, and raise interesting physiological and therapeutic questions. 相似文献
35.
Ana Saurí Silvia Tamborero Arthur E. Johnson Ismael Mingarro 《Journal of molecular biology》2009,387(1):113-582
The targeting, insertion, and topology of membrane proteins have been extensively studied in both prokaryotes and eukaryotes. However, the mechanisms used by viral membrane proteins to generate the correct topology within cellular membranes are less well understood. Here, the effect of flanking charges and the hydrophobicity of the N-terminal hydrophobic segment on viral membrane protein topogenesis are examined systematically. Experimental data reveal that the classical topological determinants have only a minor effect on the overall topology of p9, a plant viral movement protein. Since only a few individual sequence alterations cause an inversion of p9 topology, its topological stability is robust. This result further indicates that the protein has multiple, and perhaps redundant, structural features that ensure that it always adopts the same topology. These critical topogenic sequences appear to be recognized and acted upon from the initial stages of protein biosynthesis, even before the ribosome ends protein translation. 相似文献
36.
Costa L Brissos V Lemos F Ribeiro FR Cabral JM 《Bioprocess and biosystems engineering》2008,31(4):323-327
The activity of various lipases was compared, in both free and immobilized forms, using the kinetics of the hydrolysis reaction of p-nitrophenyl butyrate, which was followed with in situ UV/Vis diode array spectrophotometry. Several enzymes were used to catalyze the reaction, namely Candida antarctica lipase B and Fusarium solani pisi cutinase wildtype and three single-mutation variants. The enzymes were tested in three different forms: free, immobilized as cross-linked aggregates and supported on zeolite NaY. A simple kinetic model was used to allow a quantitative comparison of the behavior of the different catalysts. It was concluded that although immobilization reduces the activity of the enzyme, the zeolite offers a much higher specific activity when compared to the cross-linked aggregates, thus supplying a heterogeneous catalyst with promising catalytic properties. 相似文献
37.
38.
39.
Juliana S. Maldarine Bruno D. A. Sanches Vitria A. Santos gata S. Cabral Maria L. D. Lima Carolina M. Bedolo Marília F. Calmon Paula Rahal Rejane M. Ges Patricia S. L. Vilamaior Sebastio R. Taboga 《Cell biology international》2020,44(6):1341-1352
The development and maintenance of prostate function depend on a fine balance between oestrogen and androgen levels. Finasteride inhibits 5α‐reductase, which is responsible for the conversion of testosterone into its most active form, dihydrotestosterone. Enzymes that metabolize these hormones have a highly relevant role in both the normal prostate metabolism and in the occurrence of pathological conditions. There are few studies on the impact of finasteride on male prostate development and fewer studies on the female prostate and possible intersexual differences. Therefore, we treated male and female gerbils from 7 to 14 days in postnatal life with a high dose of finasteride (500 μg/kg/day); the prostate complexes were then removed and submitted to immunohistochemistry, immunofluorescence and three‐dimensional reconstruction. In addition, hormonal serum dosages were administered. Treatment with finasteride resulted in an increased thickness of the periductal smooth musculature in the prostate of both male and female gerbils, such as well as a reduction in the thickness of developing prostate alveoli in both sexes. In addition, intersexual differences were observed as increased epithelial proliferation and decreases in the number of developing alveoli in females. Together, the data indicate that postnatal exposure to finasteride causes greater changes in the female gerbil prostate than in the male. 相似文献
40.
Mikovski Andréia Izabel da Silva Nayara Tayane Silva Lázara Aline Simões Machado Mariana de Souza Barbosa Lília Cristina Reis Aryane Campos de Matos Elyabe Monteiro Viccini Lyderson Facio da Silva Souza Claudinei Machado Marcelo Dias Otoni Wagner Campos de Carvalho Ilio Fealho Rocha Diego Ismael da Silva Maurecilne Lemes 《Plant Cell, Tissue and Organ Culture》2021,145(2):239-259
Plant Cell, Tissue and Organ Culture (PCTOC) - Somaclonal variation during in vitro culture is often an undesirable phenomenon but may also be a source of genetic variation useful for breeders. The... 相似文献