全文获取类型
收费全文 | 18166篇 |
免费 | 1797篇 |
国内免费 | 2567篇 |
出版年
2024年 | 89篇 |
2023年 | 427篇 |
2022年 | 479篇 |
2021年 | 646篇 |
2020年 | 726篇 |
2019年 | 859篇 |
2018年 | 717篇 |
2017年 | 850篇 |
2016年 | 804篇 |
2015年 | 779篇 |
2014年 | 978篇 |
2013年 | 1436篇 |
2012年 | 793篇 |
2011年 | 998篇 |
2010年 | 824篇 |
2009年 | 1059篇 |
2008年 | 1143篇 |
2007年 | 1054篇 |
2006年 | 958篇 |
2005年 | 774篇 |
2004年 | 713篇 |
2003年 | 623篇 |
2002年 | 513篇 |
2001年 | 466篇 |
2000年 | 463篇 |
1999年 | 410篇 |
1998年 | 320篇 |
1997年 | 275篇 |
1996年 | 243篇 |
1995年 | 226篇 |
1994年 | 219篇 |
1993年 | 191篇 |
1992年 | 176篇 |
1991年 | 172篇 |
1990年 | 138篇 |
1989年 | 121篇 |
1988年 | 99篇 |
1987年 | 98篇 |
1986年 | 106篇 |
1985年 | 67篇 |
1984年 | 79篇 |
1983年 | 64篇 |
1982年 | 95篇 |
1981年 | 53篇 |
1980年 | 67篇 |
1979年 | 37篇 |
1978年 | 35篇 |
1977年 | 15篇 |
1976年 | 14篇 |
1974年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
321.
Mechanisms of food selection in Daphnia 总被引:1,自引:0,他引:1
A conceptual behavioural and mechanistic Holling-type model of food selection in Daphnia pulicaria is derived from SEM observations with animals feeding on mixtures of spherical-cylindrical diatoms, oblongate green algae, and filamentous cyanobacteria, as well as ultrafine particles. The algae used were Stephanodiscus hantzschii (<- 6 µm length), Monoraphidium setiforme ( 20 µm), and Oscillatoria aghardii (strands, >- 80 µm). Cell (strand) selection can occur at any or all of three stages: (i) interception from the feeding currents, (ii) collection and channeling to the food groove, and (iii) compaction and transport to the mouth. During each stage, given equal initial cell densities, elongate cells are more likely to escape collection than spherical cells and are more likely to be rejected. In addition, filaments require increased handling time at stages (ii) and (iii) and promote entanglement with limb 5 and the postabdominal claw. Food is collected primarily with the aid of limbs 3 (and 4), but limbs 1 and 2 also intervene. Neither the leaky sieve hypothesis alone nor any other single-process hypothesis explains the observations on examined in corpore positions, morphology, and derived movements of the feeding limbs. Attachment and mucus appear to be important for the ingestion of bacteria and ultrafine particles.The model is consistent with many experimental results of differential feeding by Daphnia pulicaria on mixtures of variously shaped algae and other observations on Daphnia feeding behaviour. The paradigm of invariate, nonselective feeding by Daphnia is rejected. 相似文献
322.
Jill E. Maddison Wendy E. J. Watson Peter R. Dodd Graham A. R. Johnston 《Journal of neurochemistry》1991,56(6):1881-1888
Excitatory amino acid receptor binding parameters were investigated in a spontaneous dog model of chronic hepatic encephalopathy. L-[3H]Glutamate, (+)-[3H]-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-im ine maleate ([3H]MK-801), [3H]kainate, and alpha-[3H]-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid ([3H]AMPA) binding experiments were performed using crude cerebrocortical synaptosomal membrane preparations from dogs with congenital portosystemic encephalopathy (PSE) and control dogs. There was no change in the affinity or density of L-[3H]-glutamate or [3H]MK-801 binding sites in dogs with congenital PSE compared with control dogs. However, in the PSE dogs there was a significant reduction in the density of [3H]kainate binding sites compared with control dogs and abolition of the low-affinity [3H]AMPA binding site. The relative binding capacity of PSE synaptosomal membranes for [3H]kainate and [3H]AMPA was expressed as the ratio Bmax/KD. There was a significant inverse correlation between the Bmax/KD ratio for [3H]AMPA binding and the worst grade of encephalopathy experienced by each dog. These results suggest that there is a significant perturbation of cerebrocortical non-N-methyl-D-aspartate receptor binding in dogs with congenital PSE which may have relevance to the pathogenesis of hepatic encephalopathy. 相似文献
323.
李凤民;苗云;张振万 《武汉植物学研究》1991,9(1):75-80
本文提出了一种植物生态学数学模型参数估计的新方法——线性规划法,并结合最小二乘法对模型参数的估计进行了实例分析。认为某种程度上前者更优于后者。是一种值得进一步研究的方法。 相似文献
324.
325.
一、云南植胶区的自然地理条件云南橡胶种植区位于云南高原南部,北纬20°09′—24°59′,东经93°31′—105°08′,海拔100—1000m之间的热带季风地区。包括文山、红河、思茅、西双版纳、临沧和德宏地区 相似文献
326.
应用地理信息系统模拟森林景观动态的研究 总被引:17,自引:1,他引:16
根据地理信息系统的结构,通过数据文件的转换,把它与森林动态模型有机地结合起来,可实现对森林景观动态的模拟和预测。这种模拟方法的数据输入灵活,运行速度快,尤其是它可以获得一些以各种图表的形式输出的模拟结果。本文用文字和框图详细描述了地理信息系统的组成与结构,及森林动态模型的选择与运行方式,并以长白山森林景观为例,叙述了这种模拟方法的整个过程。 相似文献
327.
328.
本文从实际问题出发,结合已有的描述群落生态的数学模型,提出了一组描述马尾松毛虫(Dendrolimus pnnctalus,walker)、条毒蛾(Lymantria dissoluta,Swinhoe)、天敌和食料之间动态关系的数学模型: w(k+1=(a_1x(k+1)/(1+a_2x(k+1)/_z(k))+a_3w(k)/(1+a_4w(k)/y(k)) x(k+1)=b_1w(k)/(1+b_2w(k)/y(k))+b_3x(k)/(1+b_4x(k)/z(k))+ y(k+1)=c_1z(k)/[1+c_sz(k)+c_3y(k)][1+c_4w(k)+(k+1)] z(k+1)=d_1z(k)/[1+d_2z(k)][1+d_3x(k)+d_4w(k)] 对于这个模型的线性化形式,详细讨论了控制松毛虫的暴发所需的条件及其生态学机制。 相似文献
329.
An age-structured population dynamics model is presented that incorporates pheromone-trapping and food-trapping as control methods for an insect pest. The model yields the following results. Low rates of pest survivorship allow lower trapping rates for control. Species with long developmental periods are easier to control than those with shorter developmental periods (other factors being equal) due to lower net survival. The rates of pheromone trapping alone for effective control are usually very high. The combination of pheromone and food trapping allows control with much lower trapping rates than either method alone. Even small amounts of immigration of adult pests into the control area renders pheromone control ineffective, whereas food traps suppress both the immigrants and the resident population. Food- (or odor-) baited traps which attract both males and females are only somewhat more efficient than those which attract females alone. The existence of density-dependent population regulation assists the control program substantially, but this assistance declines as food trapping becomes a more important part of the control program. Larval competition strongly affects the required trapping rates for eradication; species in which all larvae exert strong competition are much easier to control than those in whic the younger larvae contribute little to the total competitive depression. 相似文献
330.
We explore the relationship between transition probabilities in the Leslie model and those derived from experimental cumulative distributions. The nature of the two kinds of probabilities are discussed, and a formula derived for converting from one to the other. A numerical example is given to illustrate the differences. 相似文献