全文获取类型
收费全文 | 1735篇 |
免费 | 224篇 |
国内免费 | 171篇 |
出版年
2024年 | 4篇 |
2023年 | 26篇 |
2022年 | 37篇 |
2021年 | 48篇 |
2020年 | 83篇 |
2019年 | 66篇 |
2018年 | 56篇 |
2017年 | 77篇 |
2016年 | 77篇 |
2015年 | 76篇 |
2014年 | 96篇 |
2013年 | 140篇 |
2012年 | 91篇 |
2011年 | 100篇 |
2010年 | 71篇 |
2009年 | 96篇 |
2008年 | 89篇 |
2007年 | 86篇 |
2006年 | 105篇 |
2005年 | 94篇 |
2004年 | 64篇 |
2003年 | 56篇 |
2002年 | 48篇 |
2001年 | 48篇 |
2000年 | 54篇 |
1999年 | 41篇 |
1998年 | 25篇 |
1997年 | 21篇 |
1996年 | 23篇 |
1995年 | 29篇 |
1994年 | 21篇 |
1993年 | 18篇 |
1992年 | 26篇 |
1991年 | 18篇 |
1990年 | 12篇 |
1989年 | 17篇 |
1988年 | 6篇 |
1987年 | 8篇 |
1986年 | 12篇 |
1985年 | 13篇 |
1984年 | 9篇 |
1983年 | 6篇 |
1982年 | 7篇 |
1981年 | 6篇 |
1980年 | 10篇 |
1979年 | 3篇 |
1978年 | 5篇 |
1977年 | 4篇 |
1976年 | 1篇 |
1972年 | 1篇 |
排序方式: 共有2130条查询结果,搜索用时 31 毫秒
31.
Łukasz Wejnerowski Ewa Poniecka Jakub Buda Piotr Klimaszyk Agnieszka Piasecka Marcin Krzysztof Dziuba Gianmarco Mugnai Nozomu Takeuchi Krzysztof Zawierucha 《Journal of phycology》2023,59(5):939-949
Cryoconite, the dark sediment on the surface of glaciers, often aggregates into oval or irregular granules serving as biogeochemical factories. They reduce a glacier's albedo, act as biodiversity hotspots by supporting aerobic and anaerobic microbial communities, constitute one of the organic matter (OM) sources on glaciers, and are a feeder for micrometazoans. Although cryoconite granules have multiple roles on glaciers, their formation is poorly understood. Cyanobacteria are ubiquitous and abundant engineers of cryoconite hole ecosystems. This study tested whether cyanobacteria may be responsible for cryoconite granulation as a sole biotic element. Incubation of Greenlandic, Svalbard, and Scandinavian cyanobacteria in different nutrient availabilities and substrata for growth (distilled water alone and water with quartz powder, furnaced cryoconite without OM, or powdered rocks from glacial catchment) revealed that cyanobacteria bind mineral particles into granules. The structures formed in the experiment resembled those commonly observed in natural cryoconite holes: they contained numerous cyanobacterial filaments protruding from aggregated mineral particles. Moreover, all examined strains were confirmed to produce extracellular polymeric substances (EPS), which suggests that cryoconite granulation is most likely due to EPS secretion by gliding cyanobacteria. In the presence of water as the only substrate for growth, cyanobacteria formed mostly carpet-like mats. Our data empirically prove that EPS-producing oscillatorialean cyanobacteria isolated from the diverse community of cryoconite microorganisms can form granules from mineral substrate and that the presence of the mineral substrate increases the probability of the formation of these important and complex biogeochemical microstructures on glaciers. 相似文献
32.
33.
Midori Tuda 《Population Ecology》1996,38(2):133-140
The effects of spatial structure in terms of local capacity, or the maximum number of larvae surviving competition at resource patches, and temporal structure in terms of the period vulnerable to parasitoid attack in host populations on the persistence of host-parasitoid systems were quantitatively evaluated by laboratory experiments and well-parameterized model analyses. One of two bruchid beetles,Callosobruchus maculatus andC. phaseoli, were used as a host with Heterospilus prosopidis used as the parasitoid.C. maculatus, in which few larvae survive competition to become adults in each bean, andC. phaseoli, in which many larvae become adults in each bean, along with two kinds of beans, the mung and the azuki, were combined to construct four (2×2) resource-herbivorous host-parasitoid systems that differed in local capacity and vulnerable period. The mung-C. maculatus system with the parasitoid was the most persistent, i.e., took the longest time for extinction of either the host or parasitoid to occur. Since this resource-herbivorous host combination exhibited the lowest local capacity and the shortest vulnerable period, these two conditions possibly promoted the persistence of the system. A model incorporating the host population structure supported the observed persistence. Furthermore, the possible contribution of the timing of density-dependent competition of the host on the host-parasitoid persistence is predicted. 相似文献
34.
Colonization history of north European field voles (Microtus agrestis) revealed by mitochondrial DNA
The genetic structure of field vole (Microtus agrestis) populations from northern Europe was examined by restriction fragment length polymorphisms of mitochondrial DNA (mtDNA) in 150 individuals from 67 localities. A total of 83 haplotypes was observed, most of which were rare and highly localized geographically. Overall nucleotide diversity was high (134%), but showed a tendency to decrease with higher latitude. Two major mtDNA lineages differing by 2% in nucleotide sequence were identified. A southern mtDNA lineage was observed in field voles from Britain, Denmark and southern and central Sweden, whereas voles from Finland and northern Sweden belonged to a northern lineage. The strict phylogeographic pattern suggests that the present population generic structure in field voles reflects glacial history: the two groups are derived from different glacial refugia, and recolonized Fennoscandia from two directions. A 150–200-km-wide secondary contact zone between the two mtDNA groups was found in northern Sweden. Distinct phylogeographic substructuring was observed within both major mtDNA groups. 相似文献
35.
Short-term variations in the relative catch in each of two or three interacting pheromone traps for the pea moth,Cydia nigricana (F.), were investigated for traps aligned along the wind. The proportional catch in each trap varied widely, although the mean values accorded with previous estimates. Over consecutive short intervals during a single trapping period the proportion caught in the centre trap of a three-trap line was constant. The proportion caught in the upwind trap of two-and three-trap lines showed trends in time. These trends differed between trapping periods, but two lines of traps operated simultaneously gave similar results to each other. It is suggested that these results, which are predicted by a model based on various components of moth orientation behaviour, are caused by changes in systematic behavioural processes, not random effects. Possible mechanisms are discussed.
Variations à court terme des captures deCydia nigricana dans des pièges à phéromones en interaction
Résumé Des données antérieures concernant les interactions entre des pièges à phéromone alignés le long du vent ont été utilisées pour déduire les caractéristiques du comportement d'orientation deC. nigricana. Ces données ont été introduites dans un modèle de simulation quantitative qui prédisait que quand les captures totales sur une ligne ont été regroupées sur une période globale de piégage, la proportion capturée dans chaque piège devrait aussi avoir des valeurs moyennes semblables, mais varie plus largement que précédemment indiqué. Les simulations concernant des intervalles consécutifs beaucoup plus courts pendant la même période de piégage ont suggéré une forme spécifique de cette variation.Cette note signale des variations à court terme dans la proportion capturée au piège qui confirment ces prédictions. Nous montrons que sur des intervalles consécutifs brefs pendant une simple période de piégage, la proportion capturée dans le piège central d'une ligne de 3 pièges est contstante, bien que sa valeur change suivant les périodes de piégage. La proportion capturée dans le piège face au vent de 2 ou 3 lignes de pièges suit cette tendance dans le temps, ce qui est généralement bien représenté avec des courbes simples. Ces tendances changent suivant les périodes de piégage, mais des lignes de pièges fonctionnant simultanément fournissaient des résultats similaires. On suggère que ces résultats sont dus à des changements dans des processus comportementaux systématiques et non à des effects aléatoires. Les mécanismes possibles sont discutés.相似文献
36.
The estimation of latent and infectious periods 总被引:1,自引:0,他引:1
37.
38.
J. P. Noe 《Journal of nematology》1991,23(4):468-476
Florunner peanut and three soybean cultivars, Centennial, Gasoy 17, and Wright, were inoculated with 48-hour age cohorts of Meloidogyne arenari race 1 second-stage juveniles and placed in a growth chamber set to simulate early season (low temperature) and midseason (high temperature) conditions. Percentages of the initial inoculum penetrating roots 4 and 8 days after inoculation were 2-3 times higher in soybean cultivars than in peanut; 25% on susceptible soybean and 9% on peanut. Penetration and early development of M. arenaria were greater in the higher temperature environment. Penetration percentages were expressed as a function of cumulative degree-days by regression models. Development of M. arenaria 10, 20, and 30 days after inoculation was more rapid on peanut than on soybean. The resistant soybean cultivar Wright had slower development rates than did the other two soybean cultivars. Nematode growth and development were dependent on temperature. In greenhouse experiments, production of eggs by M. arenaria was more than 10 times greater on peanut than on susceptible soybean. The reproductive factor for Wright soybean was less than one, but plant growth parameters indicated that this cultivar was intolerant of M. arenavia. 相似文献
39.
J. P. R. Ochieng'-Odero 《Entomologia Experimentalis et Applicata》1990,54(1):21-27
Adult reproductive performance is linked to the period of feeding done by the final instar larva after attainment of a larval critical weight (LCW). The highest weight attained by a final instar larva is referred to as the larval maximum weight (LMW) and is the onset of the pre-pupal period. The relationships between LCW, pupal weight (PW) and adult weight (AW) are described as functions of the LMW. In the leafroller Cnephasia jactatana (Walker) (Lepidoptera: Tortricidae) LCW was dependent on larval size and was approximately 75% of the mean LMW. LCW was about 29 mg and 36 mg for male and female larvae of 1.18 mm and 1.20 mm head-capsule width, respectively. Over three successive generations of laboratory rearing, PW was approximately 30% and 25% lower than the LMW for males and females, respectively. AW was consistently about 50% and 40% lower than the PW for males and females, respectively. The decrease in weight from LMW to PW was named as the constant DP and found to be 0.3 for males and 0.25 for females. The total decrease from LMW to AW was the constant DA and was 0.6 for males and 0.5 for females. The duration of the latent feeding period was positively correlated to PW and AW. LCW may be used to derive quality indices that describe and predict pupal and adult performance.
Résumé Les performances reproductives sont liées à la période d'alimentation du dernier stade larvaire après l'obtention d'un poids critique (LCW). Le poids maximal atteint au dernier stade larvaire est défini comme le poids larvaire maximal (LMW), il correspond au début de la période prénymphale. Les relations entre poids de la chrysalide (PW) et poids de l'audulte AW) sont présentées comme des fonctions de LMW). Chez Cnephasia jactatana Walker (Lep. Tortricidae), LCW dépend de la taille de la chenille et correspond à environ 75% de la valeur moyenne de LMW. LCW est respectivement de 29 mg et 36 mg pour les chenilles mâles et femelles dont les capsules céphaliques ont 1,18 et 1,20 mm. Pour 3 générations successives, PW est environ 30% et 25% inférieur à LMW des mâles et des femelles. La régression du poids de LMW à PW est désignée comme la constante DP et vaut 0,3 pour les mâles et 0,25 pour les femelles. La régression de LMW à AW est désignée comme la régression DM et vaut 0,6 pour les mâles et 0,5 pour les femelles. La durée de la période d'alimentation latente est liée positivement à PW et AW. LCW peut être utilisé comme indice dérivé décrivant et prédisant les potentialités nymphales et imaginales.相似文献
40.
Summary Studies on the survival ofPseudomonas solanacearum, the causal agent of bacterial wilt of tomato, under laboratory conditions showed that soil amendments had little effect
on the population of the pathogen. When the host plant was grown in amended soil there was a positive influence on growth
of the pathogen. The population level of the pathogen incorporated into the soil was reduced to one-half within a period of
46 days.
The pathogen survived in the rhizosphere of non-host plants belonging to the families Acanthaceae and Leguminosae even in
the absence of the natural hosts, but its incidence in the rhizosphere of plants belonging to Graminae and Cyperaceae was
comparatively low indicating possibilities of reducing the inoculum potential of the pathogen in tomato fields by allowing
such plants to grow during fallow periods.
Part of the thesis of the Senior Author presented to the Kerala Agricultural University, Trichur, Kerala State, India. 相似文献