首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   8313篇
  免费   647篇
  国内免费   2篇
  8962篇
  2023年   60篇
  2022年   91篇
  2021年   194篇
  2020年   157篇
  2019年   172篇
  2018年   214篇
  2017年   212篇
  2016年   312篇
  2015年   480篇
  2014年   475篇
  2013年   626篇
  2012年   749篇
  2011年   617篇
  2010年   443篇
  2009年   393篇
  2008年   481篇
  2007年   512篇
  2006年   441篇
  2005年   396篇
  2004年   377篇
  2003年   329篇
  2002年   333篇
  2001年   91篇
  2000年   66篇
  1999年   71篇
  1998年   79篇
  1997年   65篇
  1996年   51篇
  1995年   59篇
  1994年   51篇
  1993年   42篇
  1992年   38篇
  1991年   41篇
  1990年   20篇
  1989年   26篇
  1988年   31篇
  1987年   14篇
  1986年   19篇
  1985年   12篇
  1984年   14篇
  1983年   16篇
  1982年   16篇
  1981年   15篇
  1980年   6篇
  1979年   8篇
  1978年   10篇
  1977年   3篇
  1975年   4篇
  1974年   8篇
  1962年   3篇
排序方式: 共有8962条查询结果,搜索用时 0 毫秒
101.
102.
103.
104.
Populations on the limits of species’ distribution can show different behavioral adaptations to strong ecological pressure than in the central part of the range. We investigated space use patterns of alpine mountain hare (Lepus timidus) at two areas on the southern edge of the species’ range. We monitored 34 hares between 2005 and 2008, estimating home range size, overlap, and site fidelity, and compared our results with space use in Scottish and North-European populations. Home ranges of mountain hares did not differ between two study areas with different habitat types. Subadult animals used larger ranges than adults and both age groups reduced home range size in autumn, a period that might be critical for hares due to changes in diet and/or high energy expenditure during the previous breeding season. Home ranges in these alpine populations were smaller than in Scandinavian populations but within the range of populations in different habitat types in Scotland. Seasonal home ranges overlapped considerably, but differed among the sexes: male–female overlap was higher than same sex (male–male and female–female) spatial overlap. Seasonal shifts of home ranges were small, and site fidelity remained high over the seasons, suggesting that resource distribution remained constant throughout the year and that the knowledge of an intensively frequented area is an important element of habitat quality. We concluded that habitat structure and availability of mates interact in affecting mountain hare space use in alpine habitats.  相似文献   
105.
The effect of partial or total dietary substitution of fishmeal (FM) by vegetal protein sources on growth and feed efficiency was carried out in on-growing gilthead sea bream (mean initial weight 131 g). The Control diet (FM 100) contained FM as the primary protein source, while in Diets FM 25 and FM 0 the FM protein was replaced at 75% and 100%, respectively, by a vegetable protein mixture consisting of wheat gluten, soybean meal, rapeseed meal and crystalline amino acids. Diets FM 25 and FM 0 also contained krill meal at 47 g/kg in order to improve palatability. At the end of the trial (after 158 d), fish survival was above 90%. Final weight and the specific growth rate were statistically lower in fish fed the Control diet (361 g and 0.64%/d), compared with 390–396 g and 0.69–0.70%/d after feeding vegetal diets. No significant differences were found regarding feed intake and feed conversion ratio. The digestibility of protein and amino acids (determined with chromium oxide as indicator) was similar in all diets. The blood parameters were not significantly affected by treatments. The activity of trypsin and pepsin was significantly reduced after feeding Diet FM 0. In the distal intestine, the villi length in fish fed Diet FM 25 was significantly longer and the intestine of the fish fed the FM 100 diet showed a smaller number of goblet cells. In conclusion, a total FM substitution by a vegetal mix supplemented with synthetic amino acids in on-growing sea bream is feasible.  相似文献   
106.
A substantial part of current research efforts on desertification are devoted to establish monitoring systems to evaluate the status of natural resources and the onset of desertification processes. Methodologies based on ground-collected soil and plant indicators are being increasingly used for this aim because they are affordable yet do not compromise accuracy. Despite their inherent value, these methods have limitations regarding the extent of the area that can be monitored using them. Such limitations can be overcome combining field-based approaches with remote sensing data, which allow the establishment of monitoring programs over large areas. In this article we tested the relationship between a field methodology based on indicators of ecosystem functioning, the landscape function analysis (LFA), and a vegetation index (NDVI) obtained from satellite images of the ASTER sensor using data gathered in Stipa tenacissima steppes from central Spain. LFA uses soil surface indicators to assess the condition of a given ecosystem by producing three numerical indices (stability, infiltration and nutrient cycling) reflecting the status of basic soil functions. We found a significant positive linear relationship between the NDVI, the three LFA indices and some key structural attributes of vegetation related to the cover of perennial plants. Our results indicate that NDVI can be used as a surrogate of ecosystem functioning in semi-arid Mediterranean steppes, and thus can be a helpful index to monitor the functional status of large areas in these ecosystems, and the possible onset of desertification processes.  相似文献   
107.

Introduction

Physical activity and physical inactivity patterns can affect health status. In the elderly people, their study is relevant given the importance that they have on the morbidity and mortality.

Objective

To present preliminary data on activity and inactivity patterns of a sub-sample of older adults from the IMPACT65+ Study.

Material and methods

The sample included the first 84 participants (57% women) over 65 years (age 70.7±4.7). Time spent in activity and inactivity patterns was obtained from an Intelligent Device for Energy Expenditure and Activity monitor over a continuous period of 24 hours. The patterns analysed were: standing, lying down, sitting or reclining, and the transition between them. The physical activity patterns analysed were; walking, step up or step down, running, and jumping.

Results

Time spent in inactivity patterns like reclining, lying down, and sitting was 16.1±1.9 hours (67% day), while the amount of time spent in activity patterns was 2.4±1.9 hours (10% day). Differences were observed between men and women in the amount of hours sitting (9.7±3 men vs. 7.5±2.7 women) and standing (4.5±1.4 men vs. 5.6±2.7 women). These differences were greater in the older participants.

Conclusions

Preliminary results show that older adults spend a great part of day in inactivity patterns like sitting, and that gender is the only factor analysed that affects the time spent in the activity patterns analysed.  相似文献   
108.
109.
Summary Explants from petioles, folioles or hypocotyls ofOnonis natrix have been used for calli initiation. Hypocotyls inoculated on MS medium supplemented with 2% sucrose and 0.5 mg.1–1 2,4-D / 1 mg.1–1 Kin showed to be the best primary explant. Cell suspension cultures were established in MS basal medium supplemented with 2% sucrose, 0.5 mg.1–1 NAA or 2,4-D and 1 mg.1–1 Kin. Different subculturing periods, inoculum density, hormonal supplementation and sucrose concentration were assayed in order to obtain the best culture growth conditions. The optimal conditions were achieved with cultures initiated with 40 g.1–1 of initial inoculum, growing in MS basal medium supplemented with 4% sucrose, 0.5 mg.1–1 NAA and 1 mg.1–1 Kin subcultured every twelve days. Under these experimental conditions, the cultures showed a doubling time of 36.3 hours.  相似文献   
110.
Ramírez-Sanz  L.  Casado  M.A.  de Miguel  J.M.  Castro  I.  Costa  M.  Pineda  F.D. 《Plant Ecology》2000,149(1):63-70
In the rural Mediterranean landscape, mosaics of patches of sclerophyllous scrubland and semi-natural grasslands are frequent. The plant communities of these patches, which are physiognomically easy to recognise, are very heterogeneous. The objective of this paper is to determine whether the patches of scrub-grassland represent an integrated response unit of the vegetation with regard to the physical environment (climatic, geographical and edaphic factors) and human use, or whether, on the contrary, this is an independent response. In order to do this a total of 50 sampling sites where scrubland and grassland patches were in contact were studied along a 370 km E-W mesoclimatic gradient from central Spain to Portugal (Iberian Peninsula). Two distinct zones, the east and the west halves of the study area, were identified according to the plant communities. Within each of these zones, each type of patch responded to the previously mentioned factors, differentially and independently. This determined a general lack of inter-community association or floristic correlation between scrub and grassland plant communities in contact. The spatial association is a random process probably related to the particular human management realised on every scrubland and grassland patch. The scrub–grassland pattern, which is characteristic of the Mediterranean landscape, does not represent an integrated response unit of the vegetation to a given environment, but rather the sum of the independent responses to the environment of the two patches in contact.Nomenclature for taxa: T. G. Tutin et al. 1694–1980. Flora Europaea Cambridge University Press, Cambridge.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号