全文获取类型
收费全文 | 1893篇 |
免费 | 188篇 |
国内免费 | 128篇 |
出版年
2024年 | 16篇 |
2023年 | 56篇 |
2022年 | 45篇 |
2021年 | 83篇 |
2020年 | 102篇 |
2019年 | 108篇 |
2018年 | 93篇 |
2017年 | 93篇 |
2016年 | 71篇 |
2015年 | 75篇 |
2014年 | 107篇 |
2013年 | 153篇 |
2012年 | 96篇 |
2011年 | 105篇 |
2010年 | 69篇 |
2009年 | 71篇 |
2008年 | 72篇 |
2007年 | 83篇 |
2006年 | 79篇 |
2005年 | 73篇 |
2004年 | 61篇 |
2003年 | 50篇 |
2002年 | 62篇 |
2001年 | 47篇 |
2000年 | 37篇 |
1999年 | 49篇 |
1998年 | 28篇 |
1997年 | 25篇 |
1996年 | 20篇 |
1995年 | 29篇 |
1994年 | 18篇 |
1993年 | 12篇 |
1992年 | 20篇 |
1991年 | 11篇 |
1990年 | 11篇 |
1989年 | 6篇 |
1988年 | 8篇 |
1987年 | 5篇 |
1986年 | 10篇 |
1985年 | 13篇 |
1984年 | 11篇 |
1983年 | 7篇 |
1982年 | 4篇 |
1981年 | 4篇 |
1980年 | 1篇 |
1979年 | 3篇 |
1978年 | 2篇 |
1976年 | 1篇 |
1975年 | 3篇 |
1974年 | 1篇 |
排序方式: 共有2209条查询结果,搜索用时 15 毫秒
1.
2.
Effects of the level of early productivity on the lifespan of ewes in contrasting flock environments
《Animal : an international journal of animal bioscience》2016,10(12):2034-2042
Selection for high levels of prolificacy has allowed substantial improvements in the production efficiency of New Zealand (NZ) sheep farms, but the consequences on ewe lifetime performance are mostly unknown. In this study, the relationship between the level of prolificacy early in ewes’ productive lives and their probability to survive later (i.e. stayability) was evaluated in two contrasting NZ flock environments. Records were obtained from 6605 ewes from four ram breeder flocks representing either a moderate (n=2) or a highly variable (n=2) nutritional environment. All ewes lambed for the first time at 2 years of age and were mated the following year. The number of lambs born during the first 2 years of productive life (NLB2–3) was used as a measure of early prolificacy. Effects of NLB2–3 on stayability to 4, 5, 6, 7 and 8 years old were analysed using logistic regression. Curvilinear effects (logit-transformed) were detected (P<0.05) until stayability to 6 years and to 8 years old in the highly variable and the moderate environment, respectively. The NLB2–3 that resulted in maximum expected stayability to various ages was 3.9 to 4.2, and 4.5 to 4.7 lambs in the highly variable and in the moderate flock environments, respectively. In addition, ewe stayability was reduced when the proportion of the litter that survived from birth to weaning (i.e. ewe rearing ability) was submaximal during the early productive life. High prolific ewes had a low rearing ability whatever the environment whereas the rearing ability of lowly prolific ewes was apparently more sensitive to the nutritional environment. The poor maternal performance of ewes with low levels of NLB2–3 led to a premature culling by breeders whereas the high early reproductive effort associated with high levels of NLB2–3 seemed to be at the cost of ewes’ survival, even in the moderate flock environment. In conclusion, the flock environment influenced the level of early prolificacy beyond which ewe longevity was reduced. It is suggested that further selection for high and early prolificacy in NZ flocks is likely to impair ewes’ lifetime productivity. 相似文献
3.
Five popular but iron-inefficient cultivars were crossed with three efficient genotypes and both parents and F1s were evaluated for iron-efficiency in potted calcareous and noncalcareous soil. The iron-efficient genotypes were dark green or green in both noncalcareous and calcareous soils whereas inefficient types were light green to yellow in calcareous soil. The chlorophyll and active iron (Fe2+) concentration of leaves was less in iron-efficient genotypes compared to efficient types in calcareous soil and reduction of both the parameters from noncalcareous to calcareous soil was considerably high in iron-inefficient lines. There was significant correlation between visual scores, chlorophyll and active iron content. There were no differences among F1s for iron chlorosis and they were all iron-inefficient. The frequency of iron-inefficient plants was higher than the efficient plants in all F2 populations. But most of the productive plants came from iron-efficient segregants indicating strong association between iron-efficiency and productivity. Based on the results selection for iron-efficiency in early generations and extensive evaluation for productivity in advanced generations is suggested for developing varieties for cultivation in calcareous soils. 相似文献
4.
A. Vuillemin D. Ariztegui G. Nobbe C. J. Schubert the Pasado Science Team 《Geomicrobiology journal》2014,31(4):285-298
Methanogenic populations were investigated in subsaline Laguna Potrok Aike sediments, southern Argentina. Microbial density and activity were assessed via cell count and in situ ATP detection for the last ~11K years. Methanogen phylogenetics highlighted species stratification throughout depth, whereas CO2 reduction was the major pathway leading to methane production. Organic substrates, characterized using pore water analysis, bulk organic fractions and saturated fatty acids, showed a clear link between sediment colonization and initial organic sources. Concentrations and δ13C compositions of methane and fatty acids provided final evidence of a microbial imprint on Holocene organic proxies in the most colonized intervals. 相似文献
5.
6.
SANDRINE LADEVÈZE 《Zoological Journal of the Linnean Society》2007,150(1):85-115
Metatherian petrosal bones were recovered from the early Late Palaeocene Itaboraí, Brazil, and are formally described. A cladistic analysis of the distribution of 56 petrosal and basicranial characters among extant and fossil metatherians was conducted, resulting in seven parsimonious trees. Relationships among metatherian ingroup taxa are congruent with current understanding of metatherian phylogeny. Metatheria is diagnosed by eight petrosal synapomorphies: stapedial artery absent in adults; reduced, intramural prootic canal; extrabullar internal carotid artery; inferior petrosal sinus between petrosal, basisphenoid, and basioccipital; cava supracochleare and epiptericum completely separated; reduction of the lateral flange; reduction of the anterior lamina; separation of the jugular foramen from the opening for the inferior petrosal sinus. The Palaeocene taxa Mayulestes , Pucadelphy s, and Andinodelphys from Tiupampa, and Petrosal Type II from Itaboraí are the sister groups of all other South American and Australian metatherians. This analysis confirms previous results showing the South American 'monito del monte' Dromiciops nested within the Australasian radiation. Within this australidelphian clade, Dromiciops is closely related to the dasyurids. The South American Caenolestes appears more closely related to the Australidelphia than to the South American didelphids. The Petrosal Types I, III, IV and V from Itaboraí are the stem taxa of the clade Australidelphia plus Caenolestes . The significant synapomorphies supporting this relationship are: enlargement of the fossa subarcuata that produces a bulbous ventral aspect of the mastoid and loss of post-temporal canal. Journal compilation © 2007 The Linnean Society of London, Zoological Journal of the Linnean Society , 2007, 150 , 85–115. No claim to original French government works. 相似文献
7.
JERZY DZIK REY Yu. IVANTSOV YURIY V. DEULIN 《Zoological Journal of the Linnean Society》2004,142(1):83-90
A mass occurrence of schooling, weakly sclerotized eocaridid shrimps in the Early Devonian pelagic environment of northern Russia suggests that the body-plan of the Eumalacostraca originated as an adaptation to swimming in the open sea. They probably replaced their ecological analogues, the archaeostracan phyllocarids Caryocaris , as a consequence of the rebuilding of the global marine environment during the later Ordovician. This may explain the rarity of the eumalacostracans in the Devonian and the difficulty with documenting their links with the archaeostracans. The new eocaridid shows similarities to the Carboniferous Anthracophausia in the general appearance and weak sclerotization of the carapace and appendages, but lacks its derived characters. The morphology of the pleural lobes, which have minute spines, is the only identified distinguishing character of the proposed new genus and species Archangeliphausia spinosa gen. et sp. nov. Its carapace was probably firmly connected dorsally with the thoracic segments. The only associated fossils of the assemblage represent a bizarre archaeostracan Pechoracaris aculicauda gen. et sp. nov., with a weakly sclerotized carapace lacking any hinge or rostral plates but with reduced furca, an extremely long caudal spine and pleopods transformed into spines. © 2004 The Linnean Society of London, Zoological Journal of the Linnean Society , 2004, 142 , 83–90. 相似文献
8.
9.
10.
Biological sensors are becoming more important to monitor the quality of the aquatic environment. In this paper the valve movement response of freshwater (Dreissena polymorpha) and marine (Mytilus edulis) mussels is presented as a tool in monitoring studies. Examples of various methods for data storage and data treatment are presented, elucidating easier operation and lower detection limits. Several applications are mentioned, including an early warning system based on this valve movement response of mussels. 相似文献