全文获取类型
收费全文 | 2160篇 |
免费 | 153篇 |
专业分类
2313篇 |
出版年
2022年 | 15篇 |
2021年 | 36篇 |
2020年 | 15篇 |
2019年 | 27篇 |
2018年 | 43篇 |
2017年 | 32篇 |
2016年 | 52篇 |
2015年 | 89篇 |
2014年 | 94篇 |
2013年 | 119篇 |
2012年 | 128篇 |
2011年 | 129篇 |
2010年 | 96篇 |
2009年 | 73篇 |
2008年 | 102篇 |
2007年 | 95篇 |
2006年 | 93篇 |
2005年 | 74篇 |
2004年 | 95篇 |
2003年 | 80篇 |
2002年 | 80篇 |
2001年 | 38篇 |
2000年 | 48篇 |
1999年 | 44篇 |
1998年 | 21篇 |
1997年 | 20篇 |
1996年 | 14篇 |
1995年 | 17篇 |
1994年 | 14篇 |
1993年 | 25篇 |
1992年 | 23篇 |
1991年 | 22篇 |
1990年 | 16篇 |
1989年 | 17篇 |
1988年 | 25篇 |
1987年 | 19篇 |
1986年 | 13篇 |
1985年 | 23篇 |
1984年 | 18篇 |
1983年 | 19篇 |
1982年 | 14篇 |
1981年 | 16篇 |
1980年 | 13篇 |
1978年 | 16篇 |
1977年 | 13篇 |
1976年 | 20篇 |
1975年 | 13篇 |
1974年 | 21篇 |
1973年 | 14篇 |
1970年 | 17篇 |
排序方式: 共有2313条查询结果,搜索用时 15 毫秒
51.
Within eight annual and perennialMicroseridinae species studied, the duration of the mitotic cycle is positively correlated with the nuclear DNA content, cycle time (hrs) = 7.3 + 0.32 × pg DNA/nucleus. Within the generaAgoseris andMicroseris, the annuals have lower DNA contents and more rapid mitotic cycle times than do the perennials. This relationship is predicted by the nucleotypic theory ofBennett. Annual species ofPyrrhopappus have relatively high DNA contents and a proportionately longer mitotic cycle time, but contrary to that expected by the nucleotypic theory as originally proposed have the fastest growth rate and shortest generation time observed in theMicroseridinae. This rapid developmental rate is discussed, nucleotypically, however, by analyzing relationships between DNA content, mitotic cycle time, and cell size. 相似文献
52.
Two "new" phenotypes of the esterase D system, named EsD 4-1 and EsD 4-2, were observed in a father and his daughter, respectively. An additional allele EsD4 is postulated. 相似文献
53.
In Wistar rats the intraveneous injection of streptozotocin (65 mg/kg body weight) caused a permanent hyperglycemia. After 5 days there were lesions in the exocrine parenchyme of the pancreas and its nerve fibers. Pathological changes were found in cytoplasm, cell membrane, nucleus and all other cell organelles, too. The zymogen granules remaining after extensive degranulation may disintegrate in two different ways: 1. Shrinking of the granules and formation of a hale between granule membrane and core, the electronic density of which is decreased; indistinct demonstrability of the granule membrane and finally its decomposition. 2. Shrinking of the granules, decrease of the electron density and either homogeneous or mainly peripheral arrangement of the disintegrated material of the granules; irregular shape of the granules and splitting of their membranes. 相似文献
54.
Faunal composition of geometrid moths changes with altitude in an Andean montane rain forest 总被引:2,自引:0,他引:2
Aim The objective of this study was to describe and interpret the changes in faunal composition in the moth family Geometridae (Lepidoptera) along a small‐scale elevational gradient in a tropical montane rain forest. This gradient was compared with a large‐scale latitudinal gradient in Europe. Location Investigations were carried out in the province Zamora‐Chinchipe in southern Ecuador along a gradient ranging from 1040 to 2677 m above sea level at twenty‐two sites. Methods Moths were sampled with light‐traps in three field periods in 1999 and 2000 and subsequently sorted and determined to species or morphospecies. Results We analysed 13,938 specimens representing 1010 species of geometrid moths. The proportional contribution of subtaxa to the local geometrid fauna changes along the elevational gradient at all systematic levels considered. While proportions of species of the subfamilies Ennominae, Sterrhinae and Geometrinae significantly decrease, the proportion of Larentiinae increases with increasing altitude. Changes also occur within the subfamilies Ennominae and Larentiinae. The host–plant specialist ennomine tribes Cassymini, Macariini and Palyadini completely vanish, and the proportion of the tribe Boarmiini decreases at high altitudes. In contrast, the remaining tribes (mostly comprising polyphagous species) either do not show proportional changes (Azelinini, Nacophorini, Nephodiini, Ourapterygini) or even increase (Caberini, ‘Cratoptera group’). Within Larentiinae, the species proportion of the genus Eois decreases, whereas concomitantly the proportion of Eupithecia increases. There is a remarkable similarity between the altitudinal patterns in Ecuador and those found along the latitudinal gradient in Europe. Main conclusions Species of the subfamily Larentiinae seem to be particularly well‐adapted to harsh environmental conditions, towards both high altitudes and latitudes. They might disproportionately profit from lower predation at higher altitudes. Many changes in the faunal composition can be explained by expected host–plant requirements of the species involved. Our results show that diversity estimates based on taxon ratios which are assumed to be constant must be regarded with caution because such ratios can change rapidly along environmental gradients. 相似文献
55.
Fiedler K 《Oecologia》1990,83(2):284-287
Summary Fourth instar larvae of Polyommatus icarus maintain myrmecophilous associations with ants. In laboratory experiments, the attractiveness of larvae reared on foliage of the tree Robinia pseudacacia was significantly reduced compared with caterpillars fed with herbaceous Fabaceae. The ability to secrete carbohydraterich liquids from the dorsal nectary organ was also strongly reduced, while the function of the tentacle organs remained unaffected. The reductions in attractiveness and secretion abilities are caused by the food quality itself and not by secondary effects such as reduced larval size. The ecological significance of the results are discussed with respect to facultatively and obligately myrmecophilous lycacnids. 相似文献
56.
Caterpillars of the lycaenid butterfly Callophrys rubi accept a variety of hostplants. When fed inflorescences or leaves of Genista tinctoria (a natural hostplant) or Lupinus polyphyllus (a non-host), the larvae completely eliminate quinolizidine alkaloids ingested from their food in their frass. No alkaloids are stored. Infestation by the parasitoid wasp Distatrix sancus (Braconidae) did not affect alkaloid elimination. The presence of an effective anti-toxin system is discussed with reference to the evolution of hostplant relationships in the genus Callophrys. There is no evidence that in the secondarily myrmecoxenous larvae of C. rubi hostplantderived chemical defense takes the place of former myrmecophily. 相似文献
57.
Ana Gonzlez Moreno Eva Domínguez Konrad Mayer Nannan Xiao Peter Bock Antonio Heredia Notburga Gierlinger 《Plant physiology》2023,191(1):219
The cuticle is a protective extracellular matrix that covers the above-ground epidermis of land plants. Here, we studied the cuticle of tomato (Solanum lycopersicum L.) fruits in situ using confocal Raman microscopy. Microsections from cuticles isolated at different developmental stages were scanned to visualize cuticle components with a spatial resolution of 342 nm by univariate and multivariate data analysis. Three main components, cutin, polysaccharides, and aromatics, were identified, with the latter exhibiting the strongest Raman scattering intensity. Phenolic acids and flavonoids were differentiated within the cuticle, and three schematic cuticle models were identified during development. Phenolic acids were found across the entire cuticle at the earliest stage of development, i.e. during the formation of the procuticle layer. Based on a mixture analysis with reference component spectra, the phenolic acids were identified as mainly esterified p-coumaric acid together with free p-hydroxybenzoic acid. During the cell expansion period of growth, phenolic acids accumulated in an outermost layer of the cuticle and in the middle region of the pegs. In these stages of development, cellulose and pectin were detected next to the inner cuticle region, close to the epidermal cell where flavonoid impregnation started during ripening. In the first ripening stage, chalconaringenin was observed, while methoxylated chalcones were chosen by the algorithm to fit the mature cuticle spectra. The colocation of carbohydrates, esterified p-coumaric acid, and methoxylated chalconaringenin suggests that the latter two link polysaccharide and cutin domains. Elucidating the different distribution of aromatics within the cuticle, suggests important functions: (1) overall impregnation conferring mechanical and thermal functions (2) the outermost phenolic acid layer displaying UV-B protection of the plant tissue.Raman mapping and multivariate data analysis provide insights into the distribution of cutin, carbohydrates, and phenolics along cross sections of green and mature tomato fruit cuticles. 相似文献
58.
59.
60.
Theodor Wieland Gerhart Griss Konrad Haider Barbara Haccius 《Archives of microbiology》1960,35(4):415-427
Ohne ZusammenfassungTeil der Dissertation, Universität Frankfurt am Main 1958. 相似文献