全文获取类型
收费全文 | 2012篇 |
免费 | 81篇 |
国内免费 | 113篇 |
出版年
2023年 | 24篇 |
2022年 | 21篇 |
2021年 | 20篇 |
2020年 | 32篇 |
2019年 | 25篇 |
2018年 | 31篇 |
2017年 | 28篇 |
2016年 | 24篇 |
2015年 | 37篇 |
2014年 | 51篇 |
2013年 | 77篇 |
2012年 | 45篇 |
2011年 | 30篇 |
2010年 | 42篇 |
2009年 | 58篇 |
2008年 | 85篇 |
2007年 | 83篇 |
2006年 | 94篇 |
2005年 | 81篇 |
2004年 | 80篇 |
2003年 | 104篇 |
2002年 | 82篇 |
2001年 | 67篇 |
2000年 | 65篇 |
1999年 | 60篇 |
1998年 | 51篇 |
1997年 | 62篇 |
1996年 | 67篇 |
1995年 | 59篇 |
1994年 | 55篇 |
1993年 | 58篇 |
1992年 | 35篇 |
1991年 | 48篇 |
1990年 | 49篇 |
1989年 | 43篇 |
1988年 | 39篇 |
1987年 | 34篇 |
1986年 | 31篇 |
1985年 | 49篇 |
1984年 | 35篇 |
1983年 | 21篇 |
1982年 | 29篇 |
1981年 | 31篇 |
1980年 | 27篇 |
1979年 | 4篇 |
1978年 | 6篇 |
1977年 | 5篇 |
1976年 | 7篇 |
1974年 | 5篇 |
1972年 | 3篇 |
排序方式: 共有2206条查询结果,搜索用时 15 毫秒
971.
A brief account is given of the discovery of abscisic acid (ABA) in roots and root caps of higher plants as well as the techniques by which ABA may be demonstrated in these tissues. The remainder of the review is concerned with examining the rôle of ABA in the regulation of root growth. In this regard, it is well established that when ABA is supplied to roots their elongation is usually inhibited, although at low external concentrations a stimulation of growth may also be found. Fewer observations have been directed at exploring the connection between root growth and the level of naturally occurring, endogenous ABA. Nevertheless, the evidence here also suggests that ABA is an inhibitory regulator of root growth. Moreover, ABA appears to be involved in the differential growth that arises in response to a gravitational stimulus. Recent reports that deny a rôle for ABA in root gravitropism are considered inconclusive. The response of roots to osmotic stress and the changes in ABA levels which ensue, are summarised; so are the interrelations between ABA and other hormones, particularly auxin (e.g. indoleacetic acid); both are considered in the context of the root growth and development. Quantitative changes in auxin and ABA levels may together provide the root with a flexible means of regulating its growth. 相似文献
972.
Summary To determine whether sweet potato (Ipomoea batatas (L.) Poir.) takes up organic matter through the roots from the medium, the concentrations of natural14C (14C) in plant organic matter, atmospheric CO2 and compost applied to media were examined under soil and sand culture conditions. In these experiments, three kinds of composts of different 14C were used. CO2 derived from the mineralization of compost was continuously pumped out from the pots and its direct uptake by the leaves was prevented.14C of plant parts harvested after the 43 days experimental period were affected by the 14C of the compost in the treatments where the compost of rice straw was applied, and which suggested that a significant amount of plant carbon was derived from the compost. 相似文献
973.
Excised Na+-starved barley roots were suspended in solutions of Na+ in combination with NO
3
-
, Cl-, and SO
4
2-
, and effects of the added phytohormone, abscisic acid (ABA), to the medium were determined. Abscisic acid increased the rate of Na+ (22Na+) accumulation and the amount of Na+ deposited in the vacuoles. These stimulating effects of ABA were modified by anions following the sequence NO
3
-
>Cl->SO
4
2-
. Testing whether the magnitude of the pH gradient across the plasmalemma of the cells of the root cortex affects rates of Na+ accumulation and their dependence upon ABA, we observed that, in the pH range from 4 to 8, the ABA-induced stimulation was strongest at pH 5.8, and least at pH 4. Changes in pH during the experiment caused changes in the rates of Na+ accumulation in agreement with experiments performed at constant pH values. Simultaneously with ABA-enhanced accumulation, loss of Na+ occurred. Loss of Na+ was strongest at pH 4 and was affected by anions, being greatest with SO
4
2-
and following the sequence SO
4
2-
>Cl->NO
3
-
. On the basis of the finding that initial acceleration of uptake as well as loss of Na+ depended on the pH of the medium we suggest that, in barley roots, ABA stimulates an exchange of Na+ for H+ at the plasmalemma of the cortical cells. The results indicate that ABA-stimulated expulsion of Na+, in combination with ABA-stimulated sequestration in the vacuoles, constitutes one of the mechanisms which enable barley plants to tolerate higher than normal levels of Na+.Abbreviations ABA
abscisic acid
- FW
fresh weight 相似文献
974.
Summary This fine structural study ofUlothrix flacca (Dillw.) ThuretRoscoffensis variety (Berger-Perrot), a marineUlothrix, describes in detail the flagellar apparatus configuration of gametesin situ in the gametangia and in motile zygotes. The gametes's flagellar apparatus shows two basal bodies overlapping at their proximal end at a 30° angle, in an 11/5 o'clock configuration or with a counterclockwise absolute orientation. The basal bodies are interconnected by a non-striated band or capping plate. They are wrapped in their proximal part by an electron-dense sheath and obtured by a bilobed terminal cap. A cruciate microtubular root system having a 4-2-4-2 alternation pattern is present. A striated microtubule associated component (S.M.A.C.) or system I fibres accompany the two membered root R2. The system II fibres or rhizoplasts along with striated bands associated to the microtubular roots, were not observed and are presumed to be absent.In the motile zygotes, the basal bodies are paired in a cruciate pattern. During the fusion process, two basal bodies, one of each pair, slide in a face to face position with a slight displacement into the 11/5 o'clock direction; the other two make a 30° counterclockwise rotation, thus making a 60° angle between the two basal bodies of each pair instead of 30° in the gamete.After comparison with the flagellar apparatus of other green alga gametes, it is concluded that the taxonomic affinities ofUlothrix flacca var.Roscoffensis, lie with theUlvophyceae sensuStewart andMattox 1978.Abbreviations
CP
capping plate
-
ER
endoplasmic reticulum
-
G
Golgi body
-
LG
lipid globule
-
M
mitochondria
-
MS
presumed mating structure
-
N
nucleus
-
R
2,R
4
microtubular roots
-
SH
sheath
-
SMAC
striated microtubule associated component
-
TC
terminal cap
-
V
vacuole
-
Ve
vesicles in the anterior papilla
-
1, 2, 1, 2
basal bodies numerotation 相似文献
975.
The occurrence of greenbug, Schizaphis graminum (Rondani ), biotypes is a constant threat to the development and stability of pest-resistant cereal crop varieties. Of the five biotypes (A, B, C, D, E) reported, only biotype B can be differentiated in life from the others by the physical characteristics of color. The other biotypes are differentiated on the basis of their ability to kill certain hosts or to withstand selected insecticides. The purpose of this research was to differentiate biotypes of apterous viviparous female greenbugs by multivariate techniques. Appendages from individuals of biotypes B, C, and E were measured. Some of the characters including the length of first and fourth flagellar segments, profemur, mesofemur, mesotibia, metafemur and metatibia differentiated the biotypes in multiple comparisons in univariate analyses. Using discriminant functions based on within-group covariance matrices, all greenbugs were classified into their correct group. The shortest Mahalanobis distance was between biotypes B and C. This suggests that biotype B is more closely related to biotype C than it is to biotype E. The coefficients of variation in the body measurements of biotype E were higher as compared to those of the other biotypes, indicating that a new biotype may evolve from biotype E in the future. 相似文献
976.
Summary Flagellar scales from the green flagellateTetraselmis striata (Prasinophyceae) were isolated, purified by isopycnic cesium chloride-gradient and zonal sucrose gradient centrifugation and their structure and biochemical composition investigated. Three types of flagellar scales were purified to more than 90% purity, a fourth type up to 75% purity. In addition to the previously known types of flagellar scales (pentagonal scales, rod-shaped scales, hair-scales), a novel scale type (i.e., the knotted scales) was discovered. New information about the asymmetric structure of the rod-shaped scales is presented and consequently they are renamed man scales. Flagellar scales consist mainly of carbohydrate (50–70%), significant amounts of protein (11% of dry weight) were found only in pentagonal scales. The main sugars (90%) of the pentagonal and man scales are the unusual 2-keto-sugar acids 3-deoxy-5-O-methyl-2-octulosonic acid (5 OMeKDO), 3-deoxy-2-heptulosaric acid (DHA), and 3-deoxy-2-octulosonic acid (KDO), the knotted scales contain as major sugars galactose and arabinose in addition to KDO and 5 OMeKDO but lack DHA. 13 major polypeptides were identified in flagellar scales by one-dimensional SDS-PAGE, 11 of these are of high molecular mass (>116 kDa). While the majority of polypeptides was found associated with pentagonal scales, at least 4 polypeptides were tentatively assigned to the hair-scales and knotted scales.Abbreviations CSF
crude scale fraction
- PS
pentagonal scales
- MS
man scales
- HS
hair-scales
- KS
knotted scales
- SDS-PAGE
sodium dodecyl sulfate polyacrylamide gel electrophoresis
- DHA
3-deoxylyxo-2-heptulosaric acid
- 5 OMeKDO
3-deoxy-5-O-methyl-manno-2-octulosonic acid
- KDO
3-deoxy-manno-2-octulosonic acid
- GA
Golgi apparatus 相似文献
977.
Four tronchuda (Brassica oleracea var. tronchuda Bailey) cultivars were tested for their ability to regenerate in vitro on Murashige & Skoog (MS) medium supplemented with 3 different combinations of -naphthaleneacetic acid (NAA) and 6-benzylaminopurine (BAP). Explants were either axillary bud-free whole cotyledons or hypocotyls from 7-day-old darkgrown seedlings. The ability to regenerate varied by cultivars, explants and the concentration of growth regulators. Hypocotyl explants of all 4 cultivars, and cotyledon explants of 2 cultivars, developed plantlets within 4 weeks. Hypocotyl explants produced more shoots than cotyledons. Cotyledon explants produced more roots than hypocotyls. Best shoot regeneration was on MS medium supplemented with 2 mgl-1 BAP and 0.1 mgl-1 NAA. Portuguesa produced the most shoots. Some regenerants varied in leaf shape and phyllotaxy.Abbreviations BAP
benzylaminopurine
- NAA
napththaleneacetic acid
- IBA
indolebutyric acid 相似文献
978.
渗透胁迫对小麦幼苗根系呼吸的影响 总被引:1,自引:0,他引:1
用PEG—6000调节培养液的渗透势,研究了渗透胁迫对小麦幼苗根系呼吸作用的影响。在-0.5 MPa的溶液中根总呼吸强度显著降低,不同苗龄根的反应差异明显;随胁迫加强呼吸强度随之降低;根系ATP含量减少。在胁迫初期呼吸废物对呼吸强度的降低无补偿作用,而在后期(72 h后)则可提高呼吸强度。 中度水分胁迫下,HMP支路活性上升,EMP-TCAO途径活性降低;抗氰呼吸活性增大,而对氰敏感的系统活性减低;细胞色素氧化酶活性显著低于对照。 相似文献
979.
980.
A symbiotic, heterocystous, N2-fixing blue-green alga, isolated from the coralloid roots of a xerophytic plant,Cycas revoluta, grew best in liquid medium supplemented with 4 mM NO
3
–
. Morphologically, the isolated alga was identical to that of the natural endophyte but the cell size had decreased markedly. The alga was heterotrophic. Intact coralloid roots had nearly 4 to 5 times more nitrogenase activity compared with natural- and laboratory-grown agla but nitrate reductase was inducible in both the forms. Plasmid(s) were found in both algal forms. 相似文献