全文获取类型
收费全文 | 1561篇 |
免费 | 102篇 |
专业分类
林业 | 60篇 |
农学 | 19篇 |
基础科学 | 17篇 |
187篇 | |
综合类 | 315篇 |
农作物 | 41篇 |
水产渔业 | 40篇 |
畜牧兽医 | 874篇 |
园艺 | 32篇 |
植物保护 | 78篇 |
出版年
2021年 | 12篇 |
2020年 | 28篇 |
2019年 | 20篇 |
2018年 | 21篇 |
2017年 | 23篇 |
2016年 | 42篇 |
2015年 | 19篇 |
2014年 | 24篇 |
2013年 | 56篇 |
2012年 | 54篇 |
2011年 | 66篇 |
2010年 | 36篇 |
2009年 | 29篇 |
2008年 | 63篇 |
2007年 | 54篇 |
2006年 | 47篇 |
2005年 | 45篇 |
2004年 | 51篇 |
2003年 | 53篇 |
2002年 | 46篇 |
2001年 | 49篇 |
2000年 | 43篇 |
1999年 | 34篇 |
1998年 | 24篇 |
1997年 | 23篇 |
1996年 | 18篇 |
1993年 | 18篇 |
1992年 | 22篇 |
1991年 | 24篇 |
1990年 | 28篇 |
1989年 | 46篇 |
1988年 | 37篇 |
1987年 | 37篇 |
1986年 | 37篇 |
1985年 | 19篇 |
1984年 | 18篇 |
1983年 | 22篇 |
1982年 | 15篇 |
1980年 | 11篇 |
1979年 | 19篇 |
1978年 | 16篇 |
1977年 | 15篇 |
1976年 | 17篇 |
1974年 | 17篇 |
1973年 | 19篇 |
1972年 | 23篇 |
1971年 | 11篇 |
1970年 | 18篇 |
1969年 | 12篇 |
1968年 | 12篇 |
排序方式: 共有1663条查询结果,搜索用时 171 毫秒
101.
This study was designed to examine whether or not specific tree species (Picea glauca, Picea mariana, Pinus banksiana, Populus tremuloides), their post-fire stand age, or their position in a successional pathway had any significant effect on the functional diversity of associated soil microbial communities in a typical mixed boreal forest ecosystem (Duck Mountain Provincial Forest, Manitoba, Canada). Multivariate analyses designed to identify significant biotic and/or abiotic variables associated with patterns of organic substrate utilization (assessed using the BIOLOG™ System) revealed the overall similarity in substrate utilization by the soil microbial communities. The five clusters identified differed mainly by their substrate-utilization value rather than by specific substrate utilization. Variability in community functional diversity was not strongly associated to tree species or post-fire stand age; however, redundancy analysis indicated a stronger association between substrate utilization and successional pathway and soil pH. For example, microbial communities associated with the relatively high pH soils of the P. tremuloides-P. glauca successional pathway, exhibited a greater degree of substrate utilization than those associated with the P. banksiana-P. mariana successional pathway and more acidic soils. Differences in functional diversity specific to tree species were not observed and this may have reflected the mixed nature of the forest stands and of their heterogeneous forest floor. In a densely treed, mixed boreal forest ecosystem, great overlap in tree and understory species occur making it difficult to assign a definitive microbial community to any particular tree species. The presence of P. tremuloides in all stand types and post fire stand ages has probably contributed to the large amount of overlap in utilization profiles among soil samples. 相似文献
102.
103.
Steve Cherry Gary C. White Kim A. Keating Mark A. Haroldson Charles C. Schwartz 《Journal of Agricultural, Biological & Environmental Statistics》2007,12(2):195-215
Current management of the grizzly bear (Ursus arctos) population in Yellowstone National Park and surrounding areas requires annual estimation of the number of adult female bears
with cubs-of-the-year. We examined the performance of nine estimators of population size via simulation. Data were simulated
using two methods for different combinations of population size, sample size, and coefficient of variation of individual sighting
probabilities. We show that the coefficient of variation does not, by itself, adequately describe the effects of capture heterogeneity,
because two different distributions of capture probabilities can have the same coefficient of variation. All estimators produced
biased estimates of population size with bias decreasing as effort in creased. Based on the simulation results we recommend
the Chao estimator for model M
h
be used to estimate the number of the female bears with cubs of the year; however, the estimator of Chao and Shen may also
be useful depending on the goals of the research. 相似文献
104.
105.
106.
This paper offers projections of potential effects of climate change on rusts of wheat and how we should factor in a changing
climate when planning for the future management of these diseases. Even though the rusts of wheat have been extensively studied
internationally, there is a paucity of information on the likely effects of a changing climate on the rusts and their influence
on wheat production. Due to the lack of published empirical research we relied on the few published studies of other plant
diseases, our own unpublished work and relevant information from the vast literature on rusts of wheat to prepare this overview.
Three broad areas of potential risks from a changing climate were described: increased loss from wheat rusts, new rust pathotypes
evolving faster and the reduced effectiveness of rust resistances. Increased biomass of wheat crops grown in the presence
of elevated CO2 concentrations and higher temperatures will increase the leaf area available for attack by the pathogen leading to increased
inoculum production. If changed weather conditions were to accelerate the life cycle of a pathogen, the increased inoculum
can lead to severe rust epidemics in many environments. Likewise should the effects of climate change result in more conducive
conditions for rust development there will also be a corresponding increase in the rate of evolution of new pathotypes which
could increase the rate of appearance of new virulences. The effectiveness of some rust resistance genes is influenced by
temperature and crop development stage. Climate change may directly or indirectly influence the effectiveness of some resistance
genes but this can not be ascertained due to a complete lack of knowledge. Since disease resistance breeding is a long term
strategy it is important to determine if any of the important genes may become less effective due to climate change. Studies
must be made to acquire new information on the rust disease triangle to increase the adaptive capacity of wheat under climate
change. Leadership within the Borlaug Global Rust Initiative (BGRI) is needed to broker research on rust evolution and the
durability of resistance under climate change. 相似文献
107.
108.
Evoked cortical potentials and the numsber of flashes perceived were compared when subjects were presented with short trains of flashes under conditions where each presented flash could not be counted individually, but the train of flashes appeared to be flickering (1 to 14 flashes at 33.3 flashes per second). The rate at which each successive perceived flash was added appeared to correspond with the rate at which the successive components of the evoked response pattern were added. The temporal nature of this pattern was similar for both single flashes and trains of flashes. The results suggest that the onset of stimulation triggers a process whlich has a marked effect on both the cortical and perceptual response to subsequent stimulation. 相似文献
109.
110.
The objective precision of operant conditioning methodology validates the power of hypnosis to induce alterations in time perception. Personal tempo was systematically modified by instructions to trained hypnotic subjects, with significant behavioral effects observed on a variety of response rate measures. 相似文献