全文获取类型
收费全文 | 1820篇 |
免费 | 13篇 |
国内免费 | 2篇 |
出版年
2013年 | 42篇 |
2012年 | 28篇 |
2011年 | 41篇 |
2010年 | 86篇 |
2009年 | 101篇 |
2008年 | 67篇 |
2007年 | 105篇 |
2006年 | 70篇 |
2005年 | 65篇 |
2004年 | 29篇 |
2003年 | 23篇 |
2001年 | 16篇 |
2000年 | 24篇 |
1999年 | 13篇 |
1998年 | 38篇 |
1997年 | 32篇 |
1996年 | 35篇 |
1995年 | 38篇 |
1994年 | 31篇 |
1993年 | 36篇 |
1992年 | 37篇 |
1991年 | 33篇 |
1990年 | 25篇 |
1989年 | 41篇 |
1988年 | 36篇 |
1987年 | 22篇 |
1986年 | 27篇 |
1985年 | 26篇 |
1984年 | 30篇 |
1983年 | 23篇 |
1982年 | 31篇 |
1981年 | 30篇 |
1980年 | 41篇 |
1979年 | 32篇 |
1978年 | 27篇 |
1977年 | 28篇 |
1976年 | 23篇 |
1975年 | 29篇 |
1974年 | 38篇 |
1973年 | 37篇 |
1972年 | 45篇 |
1971年 | 64篇 |
1970年 | 13篇 |
1969年 | 9篇 |
1968年 | 10篇 |
1967年 | 9篇 |
1966年 | 14篇 |
1965年 | 13篇 |
1959年 | 13篇 |
1958年 | 9篇 |
排序方式: 共有1835条查询结果,搜索用时 15 毫秒
81.
The RCE: a Riparian, Channel, and Environmental Inventory for small streams in the agricultural landscape 总被引:14,自引:0,他引:14
1. The Riparian, Channel and Environmental (RCE) Inventory has been developed to assess the physical and biological condition of small streams in the lowland, agricultural landscape. It consists of sixteen characteristics which define the structure of the riparian zone, stream channel morphology, and the biological condition in both habitats. 2. The inventory is based on the view that in landscapes where non-point source pollution and agriculture dominate, the environmental condition of small streams can be assessed by an appraisal of the physical condition of the riparian zone and stream channel. It is assumed that disturbance of this physical structure is a major cause for reduction of stream biological structure and function. This assumption is supported by a case study using fifteen Italian stream locations in which the RCE was found to be positively correlated to the benthic macroinvertebrate community as measured by the Extended Biotic Index (r = 0.80, P < 0.001) and the Shannon Diversity Index (r = 0.73; P < 0.001). 3. The inventory is designed for quick use to cover a large number of streams in a short period of time. When used it generates a numerical score which can be used to compare the physical and biological condition between different streams within a region. The numerical score is divided into five, colour-coded classes to facilitate use in streammonitoring programmes and to allow comparison with biological indices. 相似文献
82.
LANG JUDITH C.; LASKER HOWARD R.; GLADFELTER ELIZABETH H.; HALLOCK PAMELA; JAAP WALTER C.; LOSADA FREDDY J.; MULLER ROBERT G. 《Integrative and comparative biology》1992,32(6):696-706
Western Atlantic coral reefs were differentially affected bya mass bleaching (discoloration) event in 1987. We periodicallyassessed the "appearance" of zooxanthellate organisms betweenDecember 1987 and June 1988 at nine conspicuously affected sitesin the Bahamas, Florida, St. Croix, and Venezuela, using a standardizedpoint-count technique. Three to four months after the localinitiation of the event, the "bleached" state was still presentin one to three of the most abundant reef coral taxa and ina few of the less common species (n = 5 sites). "Recovery" occurredsomewhat faster at shallower depths, at least in the Bahamasand Florida. Scleractinian corals which were "prolonged bleachers"had foliaceous or massive, rather than branching, morphologies."Bleached" points disappeared from the point counts after $6to $8 months. Long-term field data on spatial and temporal variability inthe dynamics of zooxanthellate organisms would help us to understandthe ecological consequences of bleaching. More generally, weneed to distinguish anthropogenic changes in the structure andfunctioning of reef ecosystems from those which occur naturally.Point-count techniques are well suited for collaborative studiesinvolving rapid quantification of coloration states and healthin reef corals. 相似文献
83.
ROBERT L. CONNER JOSEPHINE R. LANDREY NANCY CZARKOWSKI 《The Journal of eukaryotic microbiology》1982,29(1):105-109
The size and fatty acid composition of Tetrahymena pyriformis W cells were influenced by the provision of a nutritional supplement of ergosterol, cholesterol, or tetrahymanol, but not of 20-isocholesterol. Ergosterol and cholesterol addition led to a reduction in cellular volume, an increase in glycerophospholipid saturated fatty acid content, and an increase in palmitoleic acid and its metabolic products when compared to unsupplemented controls. Tetrahymanol supplementation resulted in an increase in cellular volume, a decrease in saturated fatty acid content, and a reduction in palmitoleic acid and derivatives. 20-Isocholesterol was accumulated by the cells; however, this compound had no effect on any of the parameters followed in this investigation and had only a small depressant effect on tetrahymanol biosynthesis. Ergosterol and cholesterol had the same impact on the ciliates, even though the ergosterol-supplemented cells contained approximately three times as much free sterol as did cholesterol-grown cells. The amount of the free cholesterol and metabolic products in supplemented cultures was similar to the amount of tetrahymanol present in control cultures. This observation suggests that the cells recognize qualitative differences among the various polycyclic alcohols rather than responding to the amount of sterol present. Increased cellular levels of tetrahymanol led to a response unlike that of the true sterols, which again suggests that the high degree of specificity depends on the structure of the added polycyclic alcohol. The changes in fatty acid composition may be required to maintain proper interaction of the polar lipids and the polycyclic alcohols to give an appropriate degree of membrane fluidity. 相似文献
84.
The influence of animals on patterns of seed dispersal has not been well explored, despite the importance of seed dispersal in plant population biology. Furthermore, the exploitation of animals as dispersal agents has been considered one reason for the replacement of gymnosperms by angiosperms in the Cretaceous. We studied seed dispersal in a cycad, Macrozamia riedlei, and showed that in the absence of animals, ripe seeds drop directly beneath the parent not more than 40 cm from the base. Several animals eat the fleshy outer integument of the seeds, and possums (Trichosurus vulpecula), transport seeds up to 24 m from the parent. The postdispersal positioning of seeds gave a log density-log distance relationship of y = 7.20 – 2.85x (r2= 0.96, N = 17). Effective local dispersal may have contributed to the persistence of cycads in the world flora for the last 200 million years. 相似文献
85.
DOUGLAS A. EAGLES ROBERT A. GREGG DONALD M. SPOON 《The Journal of eukaryotic microbiology》1980,27(4):426-431
SYNOPSIS. The surface membrane potentials of suctorian genus Heliophrya were studied with intracellular electrodes. Resting membrane potentials averaged -32 mV, and spontaneous depolarizing potentials occurring at apparently random intervals and having a variety of waveforms were routinely observed. Such spontaneous potentials were correlated in time with visually monitored contractile vacuole activity. Individual contractile vacuoles had unique, although somewhat variable, electrical signatures. In the presence of an intracellular electrode all vacuoles contracted independently, but at approximately the same frequency. The amplitude of the electrical potentials increased when the membrane was hyperpolarized and decreased when it was depolarized. The sign of such potentials reversed at between -10 mV and the zero membrane potential. A 20% decrease in the membrane resistance was measured at the peak of the spontaneous depolarizing potentials. 相似文献
86.
Flowering phenology and seed set characteristics of five species of Banksia were studied in relation to the nectarivorous birds which feed at their inflorescences. Within the Banksia woodland at the study site near Perth, the flowering seasons of the Banksia species were sequential and only slightly overlapping, providing a year-round nectar source. Although honeyeaters visited alt five species, seed set was very low in each case. Caging experiments indicated that, in B. attenuata at least, alternative pollinators may play a more important role in pollination than do nectar-feeding birds. It is suggested that non-avian pollinators, predatory insects, and characteristics of the breeding system may also have been important in the evolution of the observed flowering phenology and patterns of seed set. 相似文献
87.
Abstract. A brief high temperature treatment (45 C) promoted anthocyanin synthesis in 2-d-old dark-grown red cabbage seedlings. The increased effectiveness of a temperature/red light treatment as opposed to the reverse sequence suggests that an elevated temperature 'induces' some component which facilitates the phytochrome response. 相似文献
88.
ELIZABETH S. SWANSON MARIA TOIVIO-KINNUCAN† ROBERT HEATH‡ WILLIAM P. CUNNINGHAM 《Plant, cell & environment》1982,5(5):375-383
Abstract. Modifications in plasma membrane structure and permeability were observed in Chlorella sorokiniana following exposure to 0.2 gm−3 (140 p.p.m.) O3 for 30 min. Sixty-eight per cent of the cells were plasmolysed after 15 min O3 exposure with disruption of organelles similar to that previously described in higher plants. Freeze-fracture exposed large areas of plasma membrane in 90% of the control cells and those exposed to O3 for short periods. After 20 min O3 90% of the cells cross-fracture, which indicates a change in molecular interactions in the membrane exposed to O3 The earliest observed ultraslructural alteration is an aggregation of particles on the plasma membrane P face, statistically significant after 10 min O3 Changes in 86 Rb influx occur during a similar time. After more extended exposure to O3 the plasma membrane P face shows regions of lipid phase transition to the crystalline state. 相似文献
89.
Several events are associated with fertilization in oocytes. Two such events are an increase in cytoplasmic Ca2+ concentration and the resumption of meiosis. Oocytes of the marine annelid, Pectinaria gouldii , are in metaphase I arrest when they are spawned. In this report we investigate the relationship between Ca2+ and resumption of meiosis in this species. Meiosis in unfertilized oocytes could be re-initiated with the divalent cation ionophore, A23187. Oocytes in Ca2+ free sea water, however, did not resume meiosis in the presence of the ionophore. Furthermore, it was observed that Ca2+ must be present for at least 15 min following ionophore treatment for meiosis to resume. These results suggest that extracellular Ca2+ is required for the re-initiation of meiosis in this species. 相似文献
90.
The Effects of Salinity upon Galactosyl-Glycerol Content and Concentration of the Marine Red Alga Porphyra purpurea (Roth) C.Ag. 总被引:2,自引:0,他引:2
REED ROBERT H.; COLLINS JULIAN C.; RUSSELL GEORGE 《Journal of experimental botany》1980,31(6):1539-1554
Changes in the major alcohol-soluble, low molecular weight carbohydratesof P. purpurea, O--D-galactopyranosyl-(1-2)-glycerol (floridoside)and O--D-galactopyranosyl-(1-1)-glycerol (isofloridoside),have been examined in response to salinity variation. Floridosideis shown to vary in absolute amount, increasing in hypersalineand decreasing in hyposaline media. Isofloridosidecontent per cell does not change in a similar manner. Responsesare almost identical under light or dark conditions, floridosidechanges being complete within 24 h. Decreasing the externalwater potential using ionic and non-ionic solutes has the sameeffect upon galactosyl-glycerol content. The amount of floridosidesynthesized, and degraded under hypersaline and hyposaline conditionsrespectively is shown to be insufficient to restore cell volumeto its original value. It is therefore suggested that the primaryfunction of floridoside increases in concentratedsea-waters is that of a compatible solute, serving to protectthe cell during periods when the external salt content is increaseddramatically. 相似文献