全文获取类型
收费全文 | 4440篇 |
免费 | 410篇 |
国内免费 | 1874篇 |
专业分类
6724篇 |
出版年
2024年 | 54篇 |
2023年 | 125篇 |
2022年 | 188篇 |
2021年 | 240篇 |
2020年 | 202篇 |
2019年 | 220篇 |
2018年 | 255篇 |
2017年 | 196篇 |
2016年 | 228篇 |
2015年 | 193篇 |
2014年 | 261篇 |
2013年 | 333篇 |
2012年 | 214篇 |
2011年 | 244篇 |
2010年 | 208篇 |
2009年 | 241篇 |
2008年 | 285篇 |
2007年 | 250篇 |
2006年 | 274篇 |
2005年 | 273篇 |
2004年 | 236篇 |
2003年 | 218篇 |
2002年 | 190篇 |
2001年 | 148篇 |
2000年 | 177篇 |
1999年 | 133篇 |
1998年 | 113篇 |
1997年 | 113篇 |
1996年 | 99篇 |
1995年 | 89篇 |
1994年 | 81篇 |
1993年 | 67篇 |
1992年 | 88篇 |
1991年 | 76篇 |
1990年 | 53篇 |
1989年 | 47篇 |
1988年 | 42篇 |
1987年 | 33篇 |
1986年 | 48篇 |
1985年 | 24篇 |
1984年 | 32篇 |
1983年 | 16篇 |
1982年 | 25篇 |
1981年 | 20篇 |
1980年 | 15篇 |
1979年 | 6篇 |
1978年 | 12篇 |
1977年 | 9篇 |
1976年 | 10篇 |
1972年 | 5篇 |
排序方式: 共有6724条查询结果,搜索用时 15 毫秒
991.
采用动态密闭气室法(IRGA)对农牧交错区10种植物群落最大生物量时期的土壤呼吸日动态进行了测定,并将该方法得到的土壤日呼吸速率与碱液吸收法(AA)进行了比较。结果表明:1)10个群落土壤呼吸的昼夜变化比较明显,均为单峰型曲线,主要受土壤温度的驱动,但同时也受到当日降水情况和云量、风速等气象因子的较大影响。因此,这些群落土壤呼吸日动态的一致性较差,规律性并不明显。2)用碱液吸收法和动态密闭气室法测定的10个群落的土壤呼吸速率变化范围分别为394~894 mg C·m-2·d-1和313~2043 mg C·m-2·d-1,其中碱液吸收法测定结果平均为动态气室法的67.5%,明显低于动态密闭气室法。3)两种测定方法具有很好的相关性,R2为0.873 9。本研究中发现,在土壤呼吸速率低的情况下,两种方法的测定结果十分接近甚至碱液吸收法测定结果稍大于动态密闭气室法,而在土壤呼吸速率较高的情况下,动态密闭气室法测定结果则显著高于碱液吸收法。上述结果与国内外同类研究的结果高度一致,从而为校正以往采用碱液吸收法在该区域的测定结果提供了可靠依据。 相似文献
992.
Cyclotides, a family of disulfide-rich mini-proteins, show a wide range of biological activities, making them interesting
targets for pharmaceutical and agrochemical applications, but little is known about their natural function and the events
that trigger their expression. An investigation of nutritional variations and irradiation during a batch process involving
plant cell cultures has been performed, using the native African medical herb, Oldenlandia affinis, as a model plant. The results demonstrated the biosynthesis of kalata B1, the main cyclotide in O. affinis, in a combined growth/nongrowth-associated pattern. The highest concentration, 0.37 mg g−1 dry weight, was accumulated in irradiated cells at 35 μmol m−2 s−1. Furthermore, 12 novel cyclotides were identified and the expression of various cyclotides compared in irradiated vs non-irradiated
cultures. The results indicate that cyclotide expression varies greatly depending on physiological conditions and environmental
stress. Kalata B1 is the most abundant cyclotide in plant suspension cultures, which underlies its importance as a natural
defense molecule. The identification of novel cyclotides in suspension cultures, compared to whole plants, indicates that
there may be more novel cyclotides to be discovered and that the genetic network regulating cyclotide expression is a very
sensitive system, ready to adapt to the current environmental growth condition. 相似文献
993.
景观格局对洞庭湖区鼠害的影响 总被引:1,自引:1,他引:1
景观空间格局与生态学过程的关系是景观生态学研究中的一个核心问题.不论物种保护还是生物灾害的防治,均与生物生存环境的景观格局息息相关.选择洞庭湖区为研究区域,分析了典型样区的景观格局特征,并对耕地和房舍中的鼠类捕获率与反映景观格局特征的景观指数进行相关分析.结果表明:耕地中鼠类的捕获率与景观聚集度、连接度以及斑块形状显著相关;房舍中鼠害的捕获率与建设用地的斑块形状显著相关.从景观格局角度提出了降低耕地和房舍中鼠害捕获率的方法. 相似文献
994.
景观格局-土壤侵蚀研究中景观指数的意义解释及局限性 总被引:14,自引:0,他引:14
景观格局分析是景观生态学研究的重要组成部分。景观指数是景观格局分析的有力工具。近年来,景观格局与土壤侵蚀关系的相关研究增多,常规景观格局指数得到应用。但针对土壤侵蚀过程的景观指数意义解释不足,景观指数在刻画景观格局-土壤侵蚀过程关系存在局限。选择了连接性、多样性、边界/斑块密度、形状4个方面的12个常用景观指数,对这些指数在景观格局-土壤侵蚀过程关系研究中的意义进行阐述,对指数应用的局限性及其原因进行了分析。景观数据属性、景观指数本身性质和土壤侵蚀过程的复杂性使得常规景观格局指数在景观格局-土壤侵蚀关系研究中存在不足。这3方面的影响使得常规景观格局指数与土壤侵蚀表征变量之间不存在确定的关系,从而难以通过景观指数来表征景观土壤侵蚀特征。缺乏土壤侵蚀过程基础是常规景观指数在土壤侵蚀研究应用中存在局限的主要原因。因此,构建基于土壤侵蚀过程的景观指数是景观格局-土壤侵蚀关系研究的需要和新的发展方向。 相似文献
995.
岷江上游景观格局及生态水文特征分析 总被引:18,自引:0,他引:18
基于1994年岷江上游TM遥感影像分类,结合6个不同集水区1992、1993、1995年植被生长季降雨、径流及同期NOAA/AVHRR的N DVI数据,构建了植被保水指数作为表征植被生态水文功能分析的指标。并用此对岷江上游6个不同集水区景观格局与生态水文特征进行分析。结果表明:不同集水区植被组成及景观结构有显著差异;不同集水区植被保蓄降雨能力即保水指数有明显差异;不同集水区景观结构指数与保水指数之间具有很高相关性,其中边界密度、多样性指数与保水指数呈负相关,聚集度指数与保水指数呈正相关。保水指数的构建对植被建设具有一定的科学意义 相似文献
996.
Martin Schütz Anita C. Risch Gérald Achermann Conny Thiel-Egenter Deborah S. Page-Dumroese Martin F. Jurgensen Peter J. Edwards 《Ecosystems》2006,9(4):624-633
We examined the role of red deer (Cervus elaphus L.) in translocating phosphorus (P) from their preferred grazing sites (short-grass vegetation on subalpine grasslands) to
their wider home range in a subalpine grassland ecosystem in the Central European Alps. Phosphorus was used because it is
the limiting nutrient in these grasslands. When we compared P removal of aboveground biomass due to grazing with P input due
to the deposit of feces on a grid of 268 cells (20 m × 20 m) covering the entire grassland, we detected distinct spatial patterns:
the proportion of heavily grazed short-grass vegetation increased with increasing soil-P pool, suggesting that red deer preferably
grazed on grid cells with a higher soil-P pool. Biomass consumption related to increased proportion of short-grass vegetation,
and therefore P removal, increased with increasing soil-P pool. However, within the two vegetation types (short-grass and
tall-grass), consumption was independent from soil-P pool. In addition, P input rates from defecation increased with increasing
soil-P pool, resulting in a constant mean net P loss of 0.083 kg ha−1 y−1 (0.03%–0.07% of soil-P pool) independent of both soil-P pool and vegetation type. Thus, there was no P translocation between
grid cells with different soil-P pools or between short-grass and tall-grass vegetation. Based on these results, it is likely
that the net rate of P loss is too small to explain the observed changes in vegetation composition from tall-herb/meadow communities
to short-grass and from tall-grass to short-grass on the grassland since 1917. Instead, we suggest that the grazing patterns
of red deer directly induced succession from tall-herb/meadow communities to short-grass vegetation. Yet, it is also possible
that long-term net soil-P losses indirectly drive plant succession from short-grass to tall-grass vegetation, because nutrient
depletion could reduce grazing pressure in short-grass vegetation and enable the characteristic tall-grass species Carex sempervirens Vill. to establish. 相似文献
997.
Rodríguez-González I Marín C Vargas F Córdova O Barrera M Gutiérrez-Sánchez R Alunda JM Sánchez-Moreno M 《Experimental parasitology》2006,112(1):44-51
Eight Leishmania promastigotes were isolated from different geographical areas: three (LP1, LP2, and LP3) from the provincial department La Libertad and the fourth (LP4) from the department of Cajamarca (northern Peru); another three (LM1, LM2, and LM3) in the province of Campeche (Mexico); and the last (LS1) from a clinical case of a dog in Madrid (Spain). The isolates were characterized by carbohydrate cell-surface residues using agglutinations with four purified lectins, by isoenzyme analysis using different isoenzymes, by analysis of kinetoplast DNA (kDNA) restriction fragment length polymorphism using four different restriction endonucleases and by the final metabolite patterns after in vitro culture. These isolates were compared with four reference strains and typified as: Leishmania (Leishmania) donovani, two strains of L. (L.) infantum, and one species of L. (Viania) peruviana. According to our results and the statistical study, the Peruvian isolates represent three different strains: one would be L. (V.) peruviana, another the strain isolated in Cajamarca (LP4) and the third would include the three strains from the department of La Libertad (LP1, LP2, and LP3), these latter three isolates being phylogenetically closer to the reference strain L. (L.) donovani. Meanwhile, the three isolates from Mexico form a group with close phylogenetic relationships to each other. The isolate from Spain belongs to the species L. (L.) infantum. Thus, a close correlation was drawn between the identity of each strain and its geographical origin. 相似文献
998.
Cetacean respiration usually happen in bouts. The most widely applied quantitative method used to analyze the structure of these bouts is the loge-survivorship analysis, based on the assumption that the respiratory intervals are distributed as negative exponentials. However, for the data collected from three captive Yangtze finless porpoises (Neophocaena phocaenoides asiaeorientalis), we failed to obtain a convergent result with the application of loge-survivorship analysis. However, the two-Gaussian model, which was recently proposed to analyze the feeding behavior of cows, was successfully fitted to the data. According to the fitting results, the overall respiratory pattern of the captive Yangtze finless porpoises can be described as a dive with a mean duration of around 30–40 s, followed by two or three ventilations with a mean interval of approximately 9 s. The average intra-bout intervals during both active and inactive periods are constant at 7.7–9.9 s for all individuals. However, when shifting from active to inactive states, the adult male and female decrease their mean numbers of respirations per bout and average length of inter-bout respiratory intervals, while the estimates of both parameters increase for the juvenile female. It was pointed out that the two-Gaussian model might be more adequate for cetacean respiratory-bout structure analyses than the loge-survivorship technique. 相似文献
999.
Andrew S. Pullin 《Entomologia Experimentalis et Applicata》1986,41(3):237-242
The variation in response to photoperiod and temperature of different populations of the peacock butterfly, Inachis io (L.) (Lepidoptera: Nymphalidae), was investigated to test the extent to which species can adjust their response to the environment, and therefore maximise their reproductive potential. The photoperiodic (adult) diapause induction response varies between populations, and appears to be finely tuned to the local conditions. There is however variation within populations and the response can be adjusted in a population by selective breeding. The developmental rate is not significantly different between three latitudinally distinct populations, over the range of temperatures tested, and pupal weights are similar at given temperatures. However, pupal weights increase with decreasing development temperature. The implications of these findings are discussed with reference to modelling life history strategies.
Résumé Inachis io (L.), observable dans une grande partie de l'Europe, y présente des différences quant au cycle biologique, au voltinisme et à la durée du développement. Trois populations provenant de la zone de transition entre monovoltinisme et bivoltinisme ont été échantillonnées pour examiner les différences de réponses à la photopériode et à la température, et pour évaluer l'étendue des possibilités d'adaptation de cette espèce à l'environnement et ainsi optimaliser son potentiel reproductif. L'induction photopériodique de la diapause est de type jour long pour toutes les souches examinées, mais la photopériode critique 50 (CPh50) varie suivant les populations et paraît étroitement ajustée aux conditions locales. Il y a cependant assez de variabilité à l'intérieur des populations pour que le seuil puisse être rapidement abaissé dans chaque population par des expériences d'élevage sélectif. Par contre, la vitesse de développement ne varie pas significativement entre les populations pour la gamme de température: 15–27°C. Les poids de chrysalides ne diffèrent pas suivant les populations, bien qu'ils augmentent quand la température de dévelppement diminue. On peut penser que des modèles, prédisant que la diminution du nombre de générations pendant une saison sera accompagnée d'une prolongation de la durée de développement et d'une augmentation de la taille, et que ceci est d'origine génétique et non le résultat seul du refroidissement de l'environnement, ne tiendront pas compte de l'absence de variation entre populations dans la relation entre température et développement.相似文献
1000.