全文获取类型
收费全文 | 25674篇 |
免费 | 1110篇 |
国内免费 | 5篇 |
专业分类
26789篇 |
出版年
2024年 | 141篇 |
2023年 | 172篇 |
2022年 | 286篇 |
2021年 | 558篇 |
2020年 | 372篇 |
2019年 | 426篇 |
2018年 | 638篇 |
2017年 | 558篇 |
2016年 | 818篇 |
2015年 | 1309篇 |
2014年 | 1351篇 |
2013年 | 1583篇 |
2012年 | 2078篇 |
2011年 | 2094篇 |
2010年 | 1303篇 |
2009年 | 1048篇 |
2008年 | 1449篇 |
2007年 | 1440篇 |
2006年 | 1290篇 |
2005年 | 1089篇 |
2004年 | 1156篇 |
2003年 | 1026篇 |
2002年 | 1016篇 |
2001年 | 385篇 |
2000年 | 365篇 |
1999年 | 311篇 |
1998年 | 226篇 |
1997年 | 131篇 |
1996年 | 134篇 |
1995年 | 121篇 |
1994年 | 105篇 |
1993年 | 127篇 |
1992年 | 165篇 |
1991年 | 139篇 |
1990年 | 129篇 |
1989年 | 102篇 |
1988年 | 102篇 |
1987年 | 94篇 |
1986年 | 105篇 |
1985年 | 87篇 |
1984年 | 91篇 |
1983年 | 70篇 |
1982年 | 86篇 |
1981年 | 55篇 |
1980年 | 52篇 |
1979年 | 42篇 |
1978年 | 50篇 |
1976年 | 29篇 |
1974年 | 32篇 |
1973年 | 31篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
1.
Moran Jonathan A.; Merbach Marlis A.; Livingston Nigel J.; Clarke Charles M.; Booth Webber E. 《Annals of botany》2001,88(2):307-311
Old World pitcher plants (Nepenthes spp., Nepenthaceae) trapand digest invertebrate prey to derive nutrients, primarilynitrogen (N). In the majority of lowland Nepenthes species studiedto date, ants (Hymenoptera, Formicidae) are numerically thedominant prey taxon. Nepenthes albomarginata is unusual in showingan apparent bias towards the capture of termites (Isoptera).We tested the hypothesis that N. albomarginata derives N fromtermite capture, by comparison of foliar stable N isotope abundance( 相似文献
2.
Summary Phenotypic analyses of genetic combinations involving the gene extramacrochaetae (emc) reveal its participation in the differentiation of both sensory elements and wing veins. The study of near-amorphic alleles of emc in mitotitc recombination clones indicates that it also affects cell proliferation. These clones show abnormal sizes, shapes and spatial distribution. They differentiate extra sensory elements as well as extra veins. A gain of function mutation in the gene causes opposite phenotypes in both differentiation systems. The effects of the mutant on proliferation and patterning are consistent with the emc gene being involved in the transfer of information between neighbouring cells, which leads to the spatial expression of the achaetescute gene complex and genes involved in vein formation. 相似文献
3.
Victoria López-Rodas Antonio Flores-Moya Emilia Maneiro Nieves Perdigones Fernando Marva Marta E. García Eduardo Costas 《Evolutionary ecology》2007,21(4):535-547
Adaptation of Microcystis aeruginosa (Cyanobacteria) to resist the herbicide glyphosate was analysed by using an experimental model. Growth of wild-type, glyphosate-sensitive
(Gs) cells was inhibited when they were cultured with 120 ppm glyphosate, but after further incubation for several weeks, occasionally
the growth of rare cells resistant (Gr) to the herbicide was found. A fluctuation analysis was carried out to distinguish between resistant cells arising from rare
spontaneous mutations and resistant cells arising from other mechanisms of adaptation. Resistant cells arose by rare spontaneous
mutations prior to the addition of glyphosate, with a rate ranging from 3.1 × 10−7 to 3.6 × 10−7 mutants per cell per generation in two strains of M. aeruginosa; the frequency of the Gr allele ranged from 6.14 × 10−4 to 6.54 × 10−4. The Gr mutants are slightly elliptical in outline, whereas the Gs cells are spherical. Since Gr mutants have a diminished growth rate, they may be maintained in uncontaminated waters as the result of a balance between
new resistants arising from spontaneous mutation and resistants eliminated by natural selection. Thus, rare spontaneous pre-selective
mutations may allow the survival of M. aeruginosa in glyphosate-polluted waters via Gr clone selection. 相似文献
4.
Cabanillas-Beltrán H LLausás-Magaña E Romero R Espinoza A García-Gasca A Nishibuchi M Ishibashi M Gomez-Gil B 《FEMS microbiology letters》2006,265(1):76-80
During 2003 and during late September of 2004, more than 1230 cases of gastroenteritis were reported in the south of Sinaloa State, north-western Mexico. All cases were attributed to the consumption of raw or undercooked shrimp collected at the Huizache-Caimanero lagunary system. Vibrio parahaemolyticus was identified by standard biochemical methods, and many strains were positive for PCR amplifications of the tlh and tdh genes and negative for the trh gene. A representative strain belonged to the O3:K6 serogroup. This is the first outbreak of gastroenteritis caused by the pandemic strains of O3:K6 V. parahaemolyticus in México. 相似文献
5.
Non-antagonistic interactions between arthropods and leaves of insectivorous plants with adhesive traps so far have never been reported. The mites are common prey of such plants, but we have found a new subspecies of the mite Oribatula tibialis living on the leaves of Pinguicula longifolia. Because of its small size and the low glandular density of the host, the mite moves without being trapped by the mucilaginous droplets of the leaf surface. P. longifolia provides shelter and food for the mite, while the plant may also benefit because of its fungivorous and scavenging activities. This new interaction is another dramatic example of widespread miteplant associations. 相似文献
6.
Manuel Mari-Beffa José F. de Celis Antonio García-Bellido 《Development genes and evolution》1991,200(3):132-142
Summary The role of the achaete-scute complex and extramacrochaetae, Notch, Delta, Enhancer of split and Hairless genes in chaeta patterning in Drosophila tergites was studied in genetic mosaics and in mutant combinations. The mutant phenotypes of different alleles of each gene can be ordered in characteristic topographical seriations. These seriations are related to the pattern of proliferation of histoblasts and the time of singularization of sensory organ mother cells from surrounding epidermal cells. Genetic mosaics of lethal alleles show that these genes are fundamentally involved in this singularization and subsequent differentiation. The study of mutant combinations of alleles of these genes reveals specific relationships of epistasis and synergism between them. The results suggest that spatial and temporal variations in achaete-scute complex functional products in cells, modulated by the activity of other genes involved in signal transduction, define the patterned differentiation of sensory organs in tergites.
Offprint requests to: A. García-Bellido 相似文献
7.
Porcel ER López JL Ferrón MA Pérez JA Sánchez JL Chisti Y 《Bioprocess and biosystems engineering》2006,29(1):1-5
The production of biomass and lovastatin by spore-initiated submerged fermentations of Aspergillus terreus ATCC 20542 was shown to depend on the age of the spores used for inoculation. Cultures started from older spores produced significantly higher titers of lovastatin. For example, the lovastatin titer increased by 52% when the spore age at inoculation rose from 9 to 16 days. The lovastatin titer for a spore age of 16 days was 186.5±20.1 mg L−1. The time to sporulation on surface cultures was sensitive to the light exposure history of the fungus and the spore inoculation concentration levels. A light exposure level of 140 μE m−2 s−1 and a spore concentration of 1,320 spore cm−2 produced the greatest extent of sporulation within about 50 h of inoculation. Sporulation was slowed in the dark and with diluted inoculants. A rigorous analysis of the data of statistically designed experiments showed the above observations to be highly reproducible. 相似文献
8.
Jonathan M. Adams Wei Fang Ragan M. Callaway Don Cipollini Elizabeth Newell 《Biological invasions》2009,11(4):1005-1016
Acer platanoides (Norway maple) is a widespread native tree species in Europe. It has been introduced to North America where it has often
established dense stands in both secondary woodlands and relatively undisturbed mature woodlands. In Europe A. platanoides is also extending its original range, but generally seems to exist at much lower densities. One explanation for the ‘aggressiveness’
of invasive plants such as A. platanoides is that they have left behind pests and diseases which limit their population densities in their native lands (the enemy
release hypothesis or ERH). To assess the ERH for Norway maple, a large network of collaborators assessed leaf herbivory rates
in populations throughout Europe and North America. We found significantly lower total leaf herbivory (1.6% ± 0.19, n = 21 vs. 7.4% ± 1.94, n = 34) and lower fungal damage (1.0% ± 0.35, n = 13 vs. 3.7% ± 0.85, n = 34) in North America than in Europe over a 2 year period, which is consistent with the predictions of the Enemy Release
Hypothesis. Across years, the average total leaf herbivory was significantly correlated with average annual temperature of
the site (P < 0.05), although this was mostly due to sites in Europe (P < 0.001), and not sites in North America (P > 0.05). Furthermore, only populations in Europe showed very high levels of herbivory (e.g., nine sites had total leaf herbivory
ranging from 10.0 to 51.2% in at least 1 year) or leaf fungal damage (only one site in North America showed high levels of
fungal damage in 1 year), suggesting the possibility of more frequent episodic outbreaks in the native range. Leaf herbivory
and fungal damage are only two aspects of consumer pressure and we do not know whether the differences reported here are enough
to actually elicit release from top-down population control, but such large scale biogeographic differences in herbivory contribute
towards understanding exotic invasions.
Jonathan M. Adams and Wei Fang—equally contributed as first authors.
A list of the participating members of the Transatlantic Acer platanoides Invasion Network is given in the Appendix 3. 相似文献
9.
Dr. Francisco Gracia-Navarro David Porter Socorro García-Navarro Paul Licht 《Cell and tissue research》1989,256(3):623-630
Summary The colloidal gold immunocytochemical technique was used to determine the ultrastructural features of the glandular cells in the pituitaries of male frogs, Rana pipiens, both in vivo and after superfusion in vitro. Specific reactions to antisera against bullfrog gonadotropins, human prolactin, and synthetic 1–39 corticotropin allowed identification of the 3 corresponding types of glandular cells. No immunoreaction was obtained with antisera against human or ovine-growth hormone, human -thyrotropin hormone, and bovine S-100 protein. General morphological features of these immunocytochemically identified glandular cells were similar to those of equivalent cells previously described in other amphibian species. Non-glandular folliculo-stellate cells were distinctive. In freshly removed pituitaries, these folliculo-stellate cells contained lysosome-like structures, but did not show phagocytic vacuoles in the cytoplasm; they contained many mitochondria, and the Golgi complex and endoplasmic reticulum were relatively undeveloped. After 4 or 18 h of superfusion, some immunoreactive gonadotropic, prolactin, and corticotropic cells showed degeneration and destruction. In the same gland, folliculo-stellate cells retained a viable appearance, but showed phagocytic vacuoles containing secretory granule-like structures which were immunoreactive to gonadotropic, prolactin, and corticotropic antibodies. Some folliculo-stellate cells showed phagocytic vacuoles containing complete glandular cells. These results suggest that superfusion causes a destruction of some of the glandular cells, and that folliculo-stellate cells act as phagocytes when cellular debris or moribund cells are present in the intercellular space in the pituitary parenchyma.Supported by grant DCB 8710462 from the National Science Foundation, grant 2148-83 from the CAICYT (Spain) and the Junta de Andalucia (Spain) 相似文献
10.