全文获取类型
收费全文 | 501篇 |
免费 | 2篇 |
出版年
2015年 | 2篇 |
2014年 | 3篇 |
2013年 | 11篇 |
2012年 | 10篇 |
2011年 | 20篇 |
2010年 | 17篇 |
2009年 | 28篇 |
2008年 | 23篇 |
2007年 | 28篇 |
2006年 | 19篇 |
2005年 | 21篇 |
2004年 | 7篇 |
2003年 | 9篇 |
2002年 | 4篇 |
2001年 | 6篇 |
2000年 | 9篇 |
1999年 | 6篇 |
1998年 | 14篇 |
1997年 | 20篇 |
1996年 | 17篇 |
1995年 | 16篇 |
1994年 | 13篇 |
1993年 | 10篇 |
1992年 | 9篇 |
1991年 | 11篇 |
1990年 | 8篇 |
1989年 | 13篇 |
1988年 | 8篇 |
1987年 | 8篇 |
1986年 | 10篇 |
1985年 | 10篇 |
1984年 | 2篇 |
1983年 | 11篇 |
1982年 | 6篇 |
1981年 | 5篇 |
1980年 | 8篇 |
1979年 | 8篇 |
1978年 | 5篇 |
1977年 | 2篇 |
1976年 | 9篇 |
1975年 | 4篇 |
1974年 | 6篇 |
1973年 | 6篇 |
1972年 | 7篇 |
1971年 | 8篇 |
1970年 | 4篇 |
1969年 | 4篇 |
1968年 | 2篇 |
1963年 | 2篇 |
1933年 | 1篇 |
排序方式: 共有503条查询结果,搜索用时 15 毫秒
391.
CHEN SHAO QIANQIAN ZHANG KHALED A. AL-RASHEID ALAN WARREN WEIBO SONG 《The Journal of eukaryotic microbiology》2010,57(1):33-39
ABSTRACT. The cortical development of Diophryopsis hystrix , a small marine euplotid ciliate, was observed during binary division. The pattern of morphogenesis is generally similar to that of other Diophrys- like species, with the exception of the marginal cirrus, which develops in a unique manner in D. hystrix being formed from the posterior end of the dorsal kinety anlage, in a similar manner to the formation of the caudal cirri. In the small subunit (SS) rRNA gene sequence tree, Diophryopsis represents a distinct clade separate from Diophrys . Furthermore, our ontogenetic and SSrRNA gene sequence data both support the suggestion that the Diophrys complex is sister to Uronychia , and should be considered as a distinct subfamily within the family Uronychiidae, i.e. Diophryinae Jankowski, 1979, comprising Diophrys , Diophryopsis , and Paradiophrys . Diagnoses of the subfamilies Diophryinae and Uronychiinae are provided. 相似文献
392.
JONATHAN P. BENSTEAD WYATT F. CROSS JAMES G. MARCH WILLIAM H. McDOWELL ALONSO RAMÍREZ ALAN P. COVICH 《Freshwater Biology》2010,55(10):2047-2061
1. Excretion of nitrogen (N) and phosphorus (P) is a direct and potentially important role for aquatic consumers in nutrient cycling that has recently garnered increased attention. The ecosystem‐level significance of excreted nutrients depends on a suite of abiotic and biotic factors, however, and few studies have coupled measurements of excretion with consideration of its likely importance for whole‐system nutrient fluxes. 2. We measured rates and ratios of N and P excretion by shrimps (Xiphocaris elongata and Atya spp.) in two tropical streams that differed strongly in shrimp biomass because a waterfall excluded predatory fish from one site. We also made measurements of shrimp and basal resource carbon (C), N and P content and estimated shrimp densities and ecosystem‐level N and P excretion and uptake. Finally, we used a 3‐year record of discharge and NH4‐N concentration in the high‐biomass stream to estimate temporal variation in the distance required for excretion to turn over the ambient NH4‐N pool. 3. Per cent C, N, and P body content of Xiphocaris was significantly higher than that of Atya. Only per cent P body content showed significant negative relationships with body mass. C:N of Atya increased significantly with body mass and was higher than that of Xiphocaris. N : P of Xiphocaris was significantly higher than that of Atya. 4. Excretion rates ranged from 0.16–3.80 μmol NH4‐N shrimp?1 h?1, 0.23–5.76 μmol total dissolved nitrogen (TDN) shrimp?1 h?1 and 0.002–0.186 μmol total dissolved phosphorus (TDP) shrimp?1 h?1. Body size was generally a strong predictor of excretion rates in both taxa, differing between Xiphocaris and Atya for TDP but not NH4‐N and TDN. Excretion rates showed statistically significant but weak relationships with body content stoichiometry. 5. Large between‐stream differences in shrimp biomass drove differences in total excretion by the two shrimp communities (22.3 versus 0.20 μmol NH4‐N m?2 h?1, 37.5 versus 0.26 μmol TDN m?2 h?1 and 1.1 versus 0.015 μmol TDP m?2 h?1), equivalent to 21% and 0.5% of NH4‐N uptake and 5% and <0.1% of P uptake measured in the high‐ and low‐biomass stream, respectively. Distances required for excretion to turn over the ambient NH4‐N pool varied more than a hundredfold over the 3‐year record in the high‐shrimp stream, driven by variability in discharge and NH4‐N concentration. 6. Our results underscore the importance of both biotic and abiotic factors in controlling consumer excretion and its significance for nutrient cycling in aquatic ecosystems. Differences in community‐level excretion rates were related to spatial patterns in shrimp biomass dictated by geomorphology and the presence of predators. Abiotic factors also had important effects through temporal patterns in discharge and nutrient concentrations. Future excretion studies that focus on nutrient cycling should consider both biotic and abiotic factors in assessing the significance of consumer excretion in aquatic ecosystems. 相似文献
393.
SUMMARY. 1. The effects of nutrient addition upon algal periphyton standing crop and benthic community metabolism were investigated in Lake Lacawac, Pennsylvania. Nutrient-diffusing flower pot substrates were filled with combinations of P (0.015 mol Na2HPO4), N (0.15molNaNO3) and carbon as glucose (0.015mol (LG) or 0.15mol (HG)) or 0.58 mol NaHCO3 (C). Fifty-two pots representing ten treatments were placed in the lake on 5 May 1985 and sampled after 32-38 days. 2. Maximum chlorophyll-a values of 23.82μgcm?2 and 29.72μgcm?2 were obtained on NPC and NPHG pots respectively, compared to 0.82μgcm?2 on control pots. Algal growth was not enhanced unless N and either organic or inorganic carbon were supplied. Ash Free Dry Mass was especially high on NPHG pots. 3. Gross production, net production and community respiration were assessed with light-dark chambers affixed to the pots using microwinkler measurements of dissolved oxygen. Highest production rates were obtained on NC and NPC pots. Pots with NPLG and NPHG also showed significant increases in gross production compared to control levels (P < 0.01). Specific production (=gross production/chlorophyll-a) was significantly related to standing crop as chlorophylls (P < 0.001), alkalinity in the chambers (P < 0.00l) and light (P < 0. 05), but not to N or P supplied by the pots. 4. The periphyton community was dominated by Chlorophyta, which averaged 86.1% of total algal biovolume. Enrichment with NC and NPC significantly enhanced the growth of three Chlorophyceae (Stigeoclonium, Scenedesmus and an unidentified coccoid) and two Cryptophyceae (Cryptomonas. Rhodomonas) (all P < 0.05). In contrast, the filamentous green alga Mougeotia dominated other treatments, but declined significantly with NPC enrichment (P < 0.05). Addition of NPHG stimulated an 8-fold increase in total biovolume and significant increases in growth of Cryptomonas, Rhodomonas and Euglena (all P < 0.05). 相似文献
394.
Scales of observation of riparian forests and distributions of suspended detritus in a prairie river
1. Detrital inputs from riparian forests can provide the main source of energy to aquatic consumers in stream communities. However, the supply of coarse organic detritus to stream communities is difficult to predict. Patchy riparian inputs and connectivity between reaches have complicated studies and disrupted patterns of the distribution of suspended coarse particulate organic matter within streams and rivers.
2. In this study we emphasize the importance of spatial and temporal scales in determining potential distribution of instream leaf litter. Although large pulses of detritus are transported by streams during storm flows, the main supply of benthic leaf litter used by shredders and of suspended particulate organic matter used by filter feeders is transported during prolonged periods of baseflow. The local, fine-scale distribution of this organic matter is determined by the location and continuity of leaf litter sources (riparian vegetation) and specific features of channel roughness (such as woody debris, roots and rocks).
3. Viewing riparian vegetation at several scales results in variable conclusions regarding the amount of potential source area of leaf detritus. The percentage of suspended whole leaves at sites in the Little Washita River, Oklahoma, U.S.A. was best explained by the percentage of riparian forest cover in 500 m and 1000 m reaches upstream of the sites, as viewed by remote sensing imagery. The amount of leaf fragments was best explained by distance downstream along the longitudinal gradient. Ash-free dry mass of suspended coarse particulate matter did not correlate with any measures of riparian cover.
4. Our results suggest that leaves originate over longer reach lengths than those generally considered as source areas. Scale is an important consideration in studies of riparian patterns and related instream processes because of the need to integrate point dynamics as well as upstream influences. 相似文献
2. In this study we emphasize the importance of spatial and temporal scales in determining potential distribution of instream leaf litter. Although large pulses of detritus are transported by streams during storm flows, the main supply of benthic leaf litter used by shredders and of suspended particulate organic matter used by filter feeders is transported during prolonged periods of baseflow. The local, fine-scale distribution of this organic matter is determined by the location and continuity of leaf litter sources (riparian vegetation) and specific features of channel roughness (such as woody debris, roots and rocks).
3. Viewing riparian vegetation at several scales results in variable conclusions regarding the amount of potential source area of leaf detritus. The percentage of suspended whole leaves at sites in the Little Washita River, Oklahoma, U.S.A. was best explained by the percentage of riparian forest cover in 500 m and 1000 m reaches upstream of the sites, as viewed by remote sensing imagery. The amount of leaf fragments was best explained by distance downstream along the longitudinal gradient. Ash-free dry mass of suspended coarse particulate matter did not correlate with any measures of riparian cover.
4. Our results suggest that leaves originate over longer reach lengths than those generally considered as source areas. Scale is an important consideration in studies of riparian patterns and related instream processes because of the need to integrate point dynamics as well as upstream influences. 相似文献
395.
FREDERIC BARY ALAN C. GANGE MARK CRANE KAREN J. HAGLEY 《Journal of Applied Ecology》2005,42(1):171-180
396.
397.
Lack of Effect of Staphylococcal Toxins on Ciliated Protozoa 总被引:1,自引:0,他引:1
ALAN W. BERNHEIMER 《The Journal of eukaryotic microbiology》1963,10(2):166-166
SYNOPSIS. Highly purified α and β toxins of staphylococci, reported by others to be lethal for paramecia and entamoebae, proved harmless to Paramecium caudatum and Tetrahymena pyriformis. 相似文献
398.
399.
400.
THE DISTRIBUTION OF APHID INFESTATION IN RELATION TO LEAF AGE 总被引:1,自引:0,他引:1
Infestations of apterous Aphis fabae Scop, on potted sugar beets have been followed in detail for several weeks. The plants were somewhat stunted and their crowns presented an unusually complete series of leaf ages. Records were kept of the changing number and size of the leaves and of their stage of growth. Parallel records were kept of the changing population of aphids on every leaf, and the figures are analysed in various ways to show how suitability for the aphids varied through the life cycle of the leaves.
The leaves were very suitable when young, became unsuitable as they matured, became suitable again just after maturity and then unsuitable again as they senesced. But among leaves at any given stage, those which were growing or senescing rapidly were more suitable than those changing slowly, unless the rate of senescence was very high. The differences of population density on different-aged leaves were due largely to the preferences exercised by the apterous adults. The added effect of differences in the fecundity of these mothers while feeding on different leaves was not excluded, but could not be assessed. It is concluded that the physiological development of the plant as a whole determines, through the growth and senescence among its total complement of leaves, the progress and pattern of its aphid infestation. 相似文献
The leaves were very suitable when young, became unsuitable as they matured, became suitable again just after maturity and then unsuitable again as they senesced. But among leaves at any given stage, those which were growing or senescing rapidly were more suitable than those changing slowly, unless the rate of senescence was very high. The differences of population density on different-aged leaves were due largely to the preferences exercised by the apterous adults. The added effect of differences in the fecundity of these mothers while feeding on different leaves was not excluded, but could not be assessed. It is concluded that the physiological development of the plant as a whole determines, through the growth and senescence among its total complement of leaves, the progress and pattern of its aphid infestation. 相似文献