全文获取类型
收费全文 | 655篇 |
免费 | 80篇 |
专业分类
735篇 |
出版年
2021年 | 9篇 |
2020年 | 11篇 |
2019年 | 7篇 |
2018年 | 17篇 |
2017年 | 9篇 |
2016年 | 13篇 |
2015年 | 27篇 |
2014年 | 33篇 |
2013年 | 20篇 |
2012年 | 29篇 |
2011年 | 26篇 |
2010年 | 33篇 |
2009年 | 24篇 |
2008年 | 22篇 |
2007年 | 24篇 |
2006年 | 21篇 |
2005年 | 16篇 |
2004年 | 20篇 |
2003年 | 18篇 |
2002年 | 7篇 |
2001年 | 18篇 |
2000年 | 21篇 |
1999年 | 26篇 |
1998年 | 15篇 |
1997年 | 9篇 |
1996年 | 13篇 |
1995年 | 11篇 |
1994年 | 14篇 |
1992年 | 11篇 |
1991年 | 11篇 |
1990年 | 10篇 |
1989年 | 8篇 |
1988年 | 12篇 |
1987年 | 9篇 |
1986年 | 9篇 |
1985年 | 6篇 |
1984年 | 9篇 |
1981年 | 7篇 |
1980年 | 5篇 |
1979年 | 19篇 |
1978年 | 5篇 |
1977年 | 8篇 |
1976年 | 9篇 |
1975年 | 4篇 |
1974年 | 10篇 |
1973年 | 7篇 |
1971年 | 5篇 |
1970年 | 10篇 |
1969年 | 5篇 |
1965年 | 4篇 |
排序方式: 共有735条查询结果,搜索用时 15 毫秒
1.
Internode length in Pisum. I. The effect of the Le/le gene difference on endogenous gibberellin-like substances 总被引:2,自引:0,他引:2
The allelopathic potential of the dry fruits of Washingtonia filifera (L. Linden) H. Wendl. was investigated. Leachates from fruits inhibited the germination of lettuce, wheat, red cabbage and cucumber seeds. The inhibitory effect was partly neutralized by kinetin (20 mg 1−1 ) and gibberellic acid (50 mg 1−1 ). The effect of kinetin was more pronounced at 25°C than at 20°C. Substances inhibiting germination were localized in the pericarp of the fruit and were resistant to high temperature. 相似文献
2.
G V Segre M Rosenblatt B L Reiner J E Mahaffey J T Potts 《The Journal of biological chemistry》1979,254(15):6980-6986
3.
Jennifer Beckensteiner David M. Kaplan Warren M. Potts Carmen V. Santos Michael R. O’Farrell 《PloS one》2016,11(2)
Excessive truncation of a population’s size structure is often identified as an important deleterious effect of exploitation, yet the effect on population persistence of size-structure truncation caused by exploitation is often not quantified due to data limitations. In this study, we estimate changes in eggs per recruit (EPR) using annual length-frequency samples over a 9 year period to assess persistence of the two most important recreational fishes in southern Angola: west coast dusky kob (Argyrosomus coronus) and leerfish (Lichia amia). Using a length- and age-structured model, we improve on an existing method to fit this type of model to length-frequency data and estimate EPR. The objectives of the methodological changes are to add flexibility and robustness to the approach for assessing population status in data-limited situations. Results indicate that dusky kob presents very low levels of EPR (5%-10% of the per recruit reproductive capacity in the absence of fishing) in 2013, whereas large inter-annual variability in leerfish estimates suggest caution must be applied when drawing conclusions about its exploitation status. Using simulated length frequency data with known parameter values, we demonstrate that recruitment decline due to overexploitation leads to overestimation of EPR values. Considering the low levels of EPR estimated for the study species, recruitment limitation is not impossible and true EPR values may be even lower than our estimates. It is, therefore, likely that management action, such as the creation of Marine Protected Areas, is needed to reconstitute the west coast dusky kob population. 相似文献
4.
Kevin B. Potts 《Journal of Mammalian Evolution》2010,17(3):221-222
5.
W. N. Tibbits B. M. Potts M. H. Savva 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1991,83(1):126-135
Summary The inheritance of freezing resistance in interspecific F1 hybrid families of Eucalyptus encompassing 27 different species combinations and a range of levels of hardiness was examined. Freezing resistance was assessed by determining the temperatures required to cause either 30% (T30), 40% (T40), or 50% (T50) leakage of electrolytes from excised leaf discs subjected to artificial freezing. Highly significant variation in freezing resistance occurred between species; the maximum difference between parents in any specific combination was over 9°C (E. gunnii x E. globulus). Freezing resistance was inherited in a predominantly additive manner in interspecific hybrids, although there was a tendency towards partial dominance toward the more sensitive species in some combinations (e.g., E. nitens x E. Globulus, E. nitens x E. camaldulensis, E. gunnii x E. globulus). The full expression of this genetic variation appeared to increase with hardiness and in some cases appeared to vary with ontogeny. Estimates of individual narrow-sense heritability of freezing resistance for pure E. nitens families were h
2
= 0.66±0.44 and 0.46±0.44. Across all species combinations examined, the heritability of F1 family means estimated from midparent regression was h
2
= 0.76±0.06 and h
2
= 0.89±0.06 for T40 and T50 values, respectively. The advantage of using selected parents for interspecific hybridization is demonstrated and the implications of these results for breeding for freezing resistance in Eucalyptus are discussed. 相似文献
6.
At the end of the spring 1987 growing season, the mycoparasite Sporidesmium sclerotivorum was applied at 0, 0.2, 2 or 20 kg ha‐1 to lettuce plants infected with Sclerotinia minor. Disease incidence was monitored in the same plots for five subsequent crops (three fall and two spring crops) without additional application of either pathogen or mycoparasite. Logistic growth curves were fitted to the data to describe disease progression over time for each inoculum level within each of the five crops. Within each crop, increasing the quantity of mycoparasite inoculum resulted in positive horizontal displacement of the curve with respect to time. As quantities of inoculum of S. sclerotivorum increased, inflection points of the disease progress curves increased at a decreasing rate. Thus, additional mycoparasite inoculum resulted in ever‐smaller increases in inflection point, and after a certain threshold level of mycoparasite inoculum (< 0.2 kg ha‐1), increases in inflection point did not result in meaningful increases in harvestable lettuce. Maximum rates of disease increase were not different among the treatments within each crop, but were different between crops. Maximum rates of disease increase averaged 3.4, 3.4, 2.1, 3.6 and 1.5% day‐1 for the fall 1987, spring 1988, fall 1988, spring 1989, and fall 1989, respectively. At all inoculum levels, the fall epidemics began later after planting than the spring epidemics. 相似文献
7.
Cloning of nifHD from Nostoc commune UTEX 584 and of a flanking region homologous to part of the Azotobacter vinelandii nifU gene. 总被引:3,自引:2,他引:1 下载免费PDF全文
The heterocystous cyanobacterium Nostoc commune UTEX 584 contains two nifH-like sequences (nifH1 and nifH2) in addition to nifHD. A region of DNA 1 kilobase upstream from the 5' end of nifH showed considerable sequence similarity to part of the published nifU sequences of Azotobacter vinelandii and Klebsiella pneumoniae. 相似文献
8.
Protein synthesis and proteolysis in immobilized cells of the cyanobacterium Nostoc commune UTEX 584 exposed to matric water stress. 总被引:7,自引:4,他引:3 下载免费PDF全文
M Potts 《Journal of bacteriology》1985,164(3):1025-1031
Cells of the cyanobacterium Nostoc commune UTEX 584 in exponential growth were subjected to acute water stress by immobilizing them on solid supports and drying them at a matric water potential (psi m) of -99.5 MPa. Cells which had been grown in the presence of Na235SO4 before immobilization and rapid drying continued to incorporate 35S into protein for 90 min. This incorporation was inhibited by chloramphenicol. No unique proteins appeared to be synthesized during this time. Upon further drying, the level of incorporation of 35S in protein began to decrease. In contrast, there was an apparent increase in the level of certain phycobiliprotein subunits in solubilized protein extracts of these cells. Extensive proteolysis was detected after prolonged desiccation (17 days) of the cells in the light, although they still remained intact. Phycobilisomes became dissociated in both light- and dark-stored desiccated material. 相似文献
9.
The effects of mispair and nonpair correction in hybrid DNA on base ratios (G + C content) and total amounts of DNA 总被引:1,自引:0,他引:1
Base ratios and total DNA amounts can vary substantially between and within
higher taxa and genera, and even within species. Gene conversion is one of
several mechanisms that could cause such changes. For base substitutions,
disparity in conversion direction is accompanied by an equivalent disparity
in base ratio at the heterozygous site. Disparity in the direction of gene
conversion at meiosis is common and can be extreme. For transitions (which
give purine [R]/pyrimidine [Y] mispairs) and for transversions giving
unlike R/R and Y/Y mispairs in hybrid DNA, this disparity could give slow
but systematic changes in G + C percentage. For transversions giving like
R/R and Y/Y mispairs, it could change AT/TA and CG/GC ratios. From the
extent of correction direction disparity, one can deduce properties of
repair enzymes, such as the ability (1) to excise preferentially the purine
from one mispair and the pyrimidine from the other for two different R/Y
mispairs from a single heterozygous site and (2) to excise one base
preferentially from unlike R/R or Y/Y mispairs. Frame-shifts usually show
strong disparity in conversion direction, with preferential cutting of the
nonlooped or the looped-out strand of the nonpair in heterozygous h-DNA.
The opposite directions of disparity for frame-shifts and their intragenic
suppressors as Ascobolus suggest that repair enzymes have a strong,
systematic bias as to which strand is cut. The conversion spectra of
mutations induced with different mutagens suggest that the nonlooped strand
is preferentially cut, so that base additions generally convert to mutant
and deletions generally convert to wild-type forms. Especially in
nonfunctional or noncoding DNA, this could cause a general increase in DNA
amounts. Conversion disparity, selection, mutation, and other processes
interact, affecting rates of change in base ratios and total DNA.
相似文献
10.
Thomas V. Potts Tanaji Mitra Terrance O'Keefe Joseph J. Zambon Robert J. Genco 《Archives of microbiology》1986,145(2):136-141
In order to assess the relationships among strains of the genera Actinobacillus and Haemophilus, DNAs from 50 strains of these genera were isolated and purified. The guanine plus cytosine (G+C) content of DNAs from strains of Haemophilus segnis and Haemophilus para-influenzae were determined by thermal denaturation. DNA-DNA homologies were measured using labelled probes from one strain representing Haemophilus segnis (strain ATCC 10977), and two strains representing Haemophilus parainfluenzae (strains ATCC 9796 and ATCC 7901). Strains isolated as H. segnis had a G+C content of 39.0 to 42.9% and were 49–92% homologous with the ATCC 10977 DNA probe. All of the strains freshly isolated as H. parainfluenzae were 70–81% homologous with the ATCC 9796 DNA probe and had a G+C content of 34.9 to 38.3%. Strain ATCC 7901 was 11% homologous with the ATCC 9796 DNA probe, had a G+C content of 42.4%, and was 65–78% homologous to DNA from strains identified as Haemophilus aphrophilus and Haemophilus paraphrophilus. From these results we conclude that strain ATCC 7901 is a mislabelled strain of H. paraphrophilus. The results of multiple DNA-DNA hybridizations indicated that separate species designations were appropriate for H. segnis, H. parainfluenzae, Actinobacillus actinomycetemcomitans (Haemophilus actinomycetemcomitans), and H. aphrophilus. H. aphrophilus and H. paraphrophilus were closely related organisms and did not fulfill the generally accepted criteria for designation as separate species. 相似文献