全文获取类型
收费全文 | 357篇 |
免费 | 41篇 |
专业分类
398篇 |
出版年
2022年 | 3篇 |
2021年 | 3篇 |
2020年 | 4篇 |
2019年 | 4篇 |
2018年 | 3篇 |
2017年 | 5篇 |
2016年 | 8篇 |
2015年 | 11篇 |
2014年 | 10篇 |
2013年 | 13篇 |
2012年 | 15篇 |
2011年 | 10篇 |
2010年 | 14篇 |
2009年 | 11篇 |
2008年 | 14篇 |
2007年 | 15篇 |
2006年 | 14篇 |
2005年 | 10篇 |
2004年 | 15篇 |
2003年 | 29篇 |
2002年 | 12篇 |
2001年 | 4篇 |
2000年 | 7篇 |
1999年 | 4篇 |
1997年 | 4篇 |
1995年 | 3篇 |
1994年 | 5篇 |
1992年 | 6篇 |
1991年 | 12篇 |
1990年 | 7篇 |
1989年 | 9篇 |
1988年 | 5篇 |
1987年 | 8篇 |
1986年 | 4篇 |
1984年 | 7篇 |
1983年 | 5篇 |
1982年 | 5篇 |
1981年 | 9篇 |
1980年 | 9篇 |
1979年 | 7篇 |
1978年 | 5篇 |
1977年 | 4篇 |
1976年 | 4篇 |
1974年 | 8篇 |
1973年 | 2篇 |
1970年 | 2篇 |
1969年 | 5篇 |
1968年 | 3篇 |
1967年 | 2篇 |
1966年 | 4篇 |
排序方式: 共有398条查询结果,搜索用时 0 毫秒
1.
Changes in the Physical State of Membrane Lipids during Senescence of Rose Petals 总被引:5,自引:3,他引:2 下载免费PDF全文
Changes in the physical state of microsomal membrane lipids during senescence of rose flower petals (Rosa hyb. L. cv Mercedes) were measured by x-ray diffraction analysis. During senescence of cut flowers held at 22°C, lipid in the ordered, gel phase appeared in the otherwise disordered, liquid-crystalline phase lipids of the membranes. This was due to an increase in the phase transition temperature of the lipids. The proportion of gel phase in the membrane lipids of 2-day-old flowers was estimated as about 20% at 22°C. Ethylene may be responsible, at least in part, for the increase in lipid transition temperature during senescence since aminooxyacetic acid and silver thiosulfate inhibited the rise in transition temperature. When flowers were stored at 3°C for 10 to 17 days and then transferrd to 22°C, gel phase lipid appeared in membranes earlier than in freshly cut flowers. This advanced senescence was the result of aging at 3°C, indicated by increases in membrane lipid transition temperature and ethylene production rate during the time at 3°C. It is concluded that changes in the physical state of membrane lipids are an integral part of senescence of rose petals, that they are caused, at least in part, by ethylene action and that they are responsible, at least in part, for the increase in membrane permeability which precedes flower death. 相似文献
2.
3.
Summary We tested the Enemy Impact Hypothesis, which predicts that communities of one tropic level are organized by the tropic level above. In the case of gallforming insect communities, the hypothesis predicts that gall morphology will diverge, minimizing the number of parasitoids shared among species. We used the monophyletic group of gallforming cecidomyiids (Asphondylia spp.) on creosote bush (Larrea tridentata) to test this hypothesis, predicting that species with thicker gall walls should exclude species of parasitoids with shorter ovipositors and have lower levels of parasitism. Of 17 parasitoid species reared from Asphondylia galls on creosote bush, 9 accounted for over 98% of parasitism. Seven of these 9 species had ovipositors long enough to penetrate 10 of 13 gall morphs measured. There was no significant relationship between gall wall thickness and number of associated parasitoid species (r
2=0.01, P>0.05, n=13). There was no relationship between gall wall thickness and types of parasitoid species colonizing galls: parasitoids with the shortest ovipositors colonized all types of gall morphs and were dominant members of the parasitoid assemblages in galls with the thickest walls. Ultimately, there were no significant differences in percent parasitism among Asphondylia species, regardless of gall wall thickness. We found no difference in numbers of associated parasitoids or percent parasitism in galls with different textures (e.g. hairy versus smooth), different locations on the plant or different phenologies. Our results suggest that enemy impact has not influenced the diversity of this gall community. Gall wall thickness, phenology, location on the plant and surface structure do not appear to influence the distribution of parasitoid species. Other explanations are offered to account for diversity in gall morphology among these species. 相似文献
4.
Marijo G. Kent Robert N. Shoffner Allan Hunter Keith O. Elliston Wesley Schroder Elizabeth Tolley Stephen S. Wachtel 《Human genetics》1988,79(4):321-328
Summary An inherited genetic disorder causes XY embryos of the horse to develop as mares. On the basis of our study of 38 such mares, we have identified four grades or classes of XY sex reversal according to this scheme: class I, nearly normal female, of which some are fertile; class II, female with gonadal dysgenesis, normal mullerian development; calss III, intersex mare with gonadal dysgenesis, abnormal mullerian development, enlarged clitoris; class IV, virilized intersex characterized by high levels of testosterone. In general, class I and calss II mares were typed H-Y antigen-negative whereas class III and class IV mares were typed H-Y antigen-positive. 相似文献
5.
Gwendolyn L. Waring 《Ecological Research》1988,3(3):205-216
The effects of watering and fertilizer treatments on the vigor and biochemistry of the willow,Salix lasiolepis, and subsequent colonization and survivorship of its gallforming herbivore,Euura lasiolepsis, were investigated in two field experiments. Some plants received low (LW), intermediate (MW) or high (HW) levels of water
as treatments, while others received no (OF), low (LF) or high (HF) fertilizer levels. In the watering experiment, plant protein
concentrations decreased, while growth rate and number of galls per plant increased with increased water treatments. Plant
growth proved to be the best correlate of sawfly attack. Sawfly survivorship increased slightly with greater watering, and
phenol concentrations showed no pattern among treatments. In the fertilization experiment, leaf protein increased with fertilization,
although shoot length, number of galls and survivorship ofE. lasiolepis survivorship were greatest in intermediate treatment plants. In both experiments, plant growth, rather than protein or phenol
levels, was the best predictor of sawfly attack and survivorship.
In a natural experiment with galls on wild plants, galled tissue had significantly greater protein concentrations and lower
phenol concentrations than did ungalled tissue. We suggest that gallformers modify host plant biochemistry within willow galls,
which may explain why the chemical parameters of ambient plant quality we tested were less predictive than plant growth. 相似文献
6.
This article examines "wire configuration coding" as used to estimate relative residential AC magnetic field exposure in epidemiological studies--and the need to alter such coding for time or locations other than those in which the code was developed. Effects of different secondary wire practices are particularly examined. 相似文献
7.
Extrafibrillar proteoglycans osmotically regulate the molecular packing of collagen in cartilage 总被引:2,自引:0,他引:2
The molecular packing density of collagen and hence the intrafibrillar water content appears to be regulated in cartilage by the osmotic pressure gradient existing between the extrafibrillar and the intrafibrillar compartments. 相似文献
8.
Special cutaneous receptor organs of fish. IV. Ampullary organs of the nonelectric catfish, Kryptopterus 总被引:1,自引:0,他引:1
Ampullary organs of the transparent catfish, Kryptopterus bicirrhus, are present in large numbers on the head and in a regular pattern of lines on the body and fins. The organs lie in the epidermis, and have a pore that opens to the surface. Flattened cells form a roof and walls. On the floor of the organ there are a “sensory hillock,” composed of spherical receptor cells and columnar supporting cells, and a “secretory hillock” composed of columnar secretory cells. The receptor cells are nonciliated and have only afferent innervation. The organ cavity is filled with jelly. The organs are compared with ampullary organs of the weakly electric fish Eigenmannia, ampullae of Lorenzini of Raja, and small pit organs of Amiurus. Structural characteristics of the ampullary organs of Kryptopterus make them especially suitable for electrophysiological studies. 相似文献
9.
Fetal cells in maternal blood: recovery by charge flow separation 总被引:11,自引:0,他引:11
S. S. Wachtel David Sammons Michael Manley Gwendolyn Wachtel Garland Twitty Joseph Utermohlen Owen P. Phillips Lee P. Shulman Douglas J. Taron U. R. Müller Peter Koeppen Teresa M. Ruffalo Karen Addis Richard Porreco Joyce Murata-Collins Natalie B. Parker Loris McGavran 《Human genetics》1996,98(2):162-166
Fetal blood cells can be recovered from the maternal circulation by charge flow separation (CFS), a method that obviates the
risks associated with amniocentesis and chorionic villus sampling. By CFS, we processed blood samples from 13 women carrying
male fetuses, 2 carrying fetuses with trisomy 21, and 1 who had delivered a stillborn infant with trisomy 18. On average more
than 2000 fetal nucleated red blood cells were recovered per 20-ml sample of maternal blood. Recovery of fetal cells was confirmed
by fluorescence in situ hybridization with probes for chromosomes Y, 18 and 21. After culturing of CFS-processed cells, amplification
by the polymerase chain reaction revealed Y-chromosomal DNA in clones from four of six women bearing male fetuses, but not
in clones from three women bearing female fetuses.
Received: 8 January 1996 / Revised: 22 March 1996 相似文献
10.
Patch clamp techniques were used to record whole cell and single channel Na+ currents from NB41A3 neuroblastoma cells grown in culture. Cells were grown for two weeks in control medium or medium supplemented with 30 mm
d-glucose of 30 mm l-fucose.Cells exposed to glucose or l-fucose had smaller whole cell Na+ currents than cells grown in unsupplemented medium, consistent with earlier studies (Yorek, Stefani & Wachtel, 1994). Whole cell macroscopic currents showed no change in activation or inactivation kinetics. Single channel current properties and opening probability were also unchanged.The number of [3H]saxitoxin binding sites, and therefore the total number of Na+ channels, was not reduced in cells grown in glucose or l-fucose (Yorek et al., 1994). Therefore, we conclude that some of the channels must have been rendered nonfunctional by the conditioning media. The finding that single channel properties are not altered suggests that channels become nonfunctional in an all-or-none manner.This work was supported by Merit Review Awards to M.A. Yorek and R.E. Wachtel from the Department of Veterans Affairs and by National Institutes of Health grant DK45453 to M.A. Yorek. 相似文献