全文获取类型
收费全文 | 8772篇 |
免费 | 927篇 |
国内免费 | 5篇 |
出版年
2022年 | 83篇 |
2021年 | 140篇 |
2020年 | 69篇 |
2019年 | 86篇 |
2018年 | 122篇 |
2017年 | 93篇 |
2016年 | 167篇 |
2015年 | 297篇 |
2014年 | 285篇 |
2013年 | 404篇 |
2012年 | 453篇 |
2011年 | 523篇 |
2010年 | 332篇 |
2009年 | 285篇 |
2008年 | 436篇 |
2007年 | 397篇 |
2006年 | 363篇 |
2005年 | 403篇 |
2004年 | 329篇 |
2003年 | 340篇 |
2002年 | 310篇 |
2001年 | 196篇 |
2000年 | 167篇 |
1999年 | 182篇 |
1998年 | 131篇 |
1997年 | 88篇 |
1996年 | 94篇 |
1995年 | 77篇 |
1994年 | 97篇 |
1993年 | 79篇 |
1992年 | 132篇 |
1991年 | 119篇 |
1990年 | 148篇 |
1989年 | 122篇 |
1988年 | 127篇 |
1987年 | 107篇 |
1986年 | 89篇 |
1985年 | 100篇 |
1984年 | 99篇 |
1983年 | 76篇 |
1982年 | 77篇 |
1981年 | 86篇 |
1980年 | 66篇 |
1979年 | 83篇 |
1978年 | 64篇 |
1976年 | 60篇 |
1975年 | 65篇 |
1974年 | 65篇 |
1973年 | 71篇 |
1972年 | 65篇 |
排序方式: 共有9704条查询结果,搜索用时 281 毫秒
1.
2.
Stewart A. Hill Stephen E. Scheckler James F. Basinger 《American journal of botany》1997,84(1):85-103
A new fern-like fossil plant is described from the lower Upper Devonian of southern Ellesmere Island, Canadian Arctic Archipelago. The plant occurs in an Archaeopteris-dominated flora preserved in the Nordstrand Point Formation (Mid-Late Frasnian) near Bird Fiord. The plant has a pinnate vegetative system with three branch orders and laminate sphenopteroid pinnules. Primary pinnae usually diverge from the main axis in distichous pairs (quadriseriate), but can depart singly (biseriate). Each primary pinna bears a basal catadromic aphlebia. Anatomically, the plant exhibits a mesarch, bipolar protostele that is ribbon- to clepsydropsoid-shaped in the main axis. Primary pinna traces are also initially bipolar and crescent-shaped, but may become four-ribbed before dividing into a pair of bipolar traces. The morphology and anatomy of this plant are nongymnospermous and are most similar to Zygopteridales (particularly Rhacophytaceae and Zygopteridaceae). The Frasnian age of Ellesmeris shows that laminated foliage had evolved in some zygopterid ferns much earlier than previously recognized. The Sphenopteris-like pinnules of Ellesmeris indicate the need for caution when attributing such a convergent foliar design to other plant groups, such as the Devonian gymnosperms. 相似文献
3.
4.
5.
Charles L. Nunn Peter H. Thrall Kelly Stewart Alexander H. Harcourt 《Evolutionary ecology》2008,22(4):519-543
Emerging infectious diseases threaten a wide diversity of animals, and important questions remain concerning disease emergence
in socially structured populations. We developed a spatially explicit simulation model to investigate whether—and under what
conditions—disease-related mortality can impact rates of pathogen spread in populations of polygynous groups. Specifically,
we investigated whether pathogen-mediated dispersal (PMD) can occur when females disperse after the resident male dies from
disease, thus carrying infections to new groups. We also examined the effects of incubation period and virulence, host mortality
and rates of background dispersal, and we used the model to investigate the spread of the virus responsible for Ebola hemorrhagic
fever, which currently is devastating African ape populations. Output was analyzed using regression trees, which enable exploration
of hierarchical and non-linear relationships. Analyses revealed that the incidence of disease in single-male (polygynous)
groups was significantly greater for those groups containing an average of more than six females, while the total number of
infected hosts in the population was most sensitive to the number of females per group. Thus, as expected, PMD occurs in polygynous
groups and its effects increase as harem size (the number of females) increases. Simulation output further indicated that
population-level effects of Ebola are likely to differ among multi-male–multi-female chimpanzees and polygynous gorillas,
with larger overall numbers of chimpanzees infected, but more gorilla groups becoming infected due to increased dispersal
when the resident male dies. Collectively, our results highlight the importance of social system on the spread of disease
in wild mammals. 相似文献
6.
The potential physiological impact of morphological changes in the active dendritic spines, which are believed to be associated
with altered synaptic efficacy, was investigated in a computer simulation study using the NEURON package [1]. A compartmental
model of a simplified neuron was built, which included 30 complex spines (neck, head, and active zone) and accommodating AMPA-type
synaptic inputs with alpha-function conductances. Hodgkin-Huxley type excitable membranes were inserted into the spine heads.
It was shown that arranging spines in dense clusters, as opposed to a uniformly random spine distribution, has a negligible
effect on the synaptic signal transfer (other model conditions, including synaptic input and spine density, remained unchanged).
However, if a proportion (e.g., 3–20%) of the spines partly fuse with their neighbors forming branched spines, this could
increase dramatically the cell response to the unchanged synaptic input. Results of this pilot study provide the basis for
a more detailed investigation of the relationship between the spine arrangement and synaptic function, considering dual-component
synaptic currents and mechanisms controlling ion fluxes in the dendritic compartments. 相似文献
7.
8.
9.
Catherine E. Rogers Karl R. Mattox Kenneth D. Stewart 《American journal of botany》1980,67(5):774-783
Chlorokybus atmophyticus has a sarcinoid growth habit and produces scale-covered zoospores. Flagella are laterally inserted and attached internally to a multilayered structure characteristic of the Charophyceae. There are two kinds of pyrenoid in each cell, a feature previously observed in only one scaly green flagellate. C. atmophyticus demonstrates that the sarcinoid growth habit arose independently at least twice in the green algae and cannot be used to define taxonomic groups unless combined with other criteria. It is further concluded that C. atmophyticus should be classified in a separate family Chlorokybaceae and a separate order Chloroky bales. 相似文献
10.
Does maternal condition or predation risk influence small mammal population dynamics? 总被引:2,自引:0,他引:2
There is strong debate over whether the intrinsic traits of individuals or the extrinsic environment exert the greater influence on small mammal population dynamics. We test the roles of maternal effects (an intrinsic factor) and predation risk (an extrinsic factor) in the population dynamics of wild strain house mice using a 2-factor enclosure experiment. Pre-release supplemental feeding with a high-fat diet created female treatment founders that were 6–10% heavier than controls, a condition that we predicted would be passed on as a maternal effect. Predation risk was enhanced using regular application of predator (red fox Vulpes vulpes ) scats. Founder populations of six females and six males released into eight, 15×15 m enclosures showed near exponential population growth over 17 weeks (maximum 3 generations). But there were no responses to either treatment in terms of survival, inherited body weights, fecundity or population size. We suggest that elevated maternal condition may have only minor and transient intergenerational effects with little long-term consequence. We also suggest that the general significance of predator scats as a cue to predation risk to alter prey behaviour may have been overestimated. Hence our results question the role of either factor in causing long-term responses that influence condition to affect population processes. 相似文献