全文获取类型
收费全文 | 10733篇 |
免费 | 973篇 |
国内免费 | 789篇 |
专业分类
12495篇 |
出版年
2024年 | 25篇 |
2023年 | 178篇 |
2022年 | 169篇 |
2021年 | 284篇 |
2020年 | 383篇 |
2019年 | 432篇 |
2018年 | 327篇 |
2017年 | 346篇 |
2016年 | 351篇 |
2015年 | 343篇 |
2014年 | 395篇 |
2013年 | 722篇 |
2012年 | 435篇 |
2011年 | 417篇 |
2010年 | 385篇 |
2009年 | 515篇 |
2008年 | 482篇 |
2007年 | 568篇 |
2006年 | 499篇 |
2005年 | 483篇 |
2004年 | 443篇 |
2003年 | 396篇 |
2002年 | 357篇 |
2001年 | 362篇 |
2000年 | 297篇 |
1999年 | 265篇 |
1998年 | 287篇 |
1997年 | 260篇 |
1996年 | 234篇 |
1995年 | 222篇 |
1994年 | 164篇 |
1993年 | 186篇 |
1992年 | 154篇 |
1991年 | 155篇 |
1990年 | 116篇 |
1989年 | 91篇 |
1988年 | 84篇 |
1987年 | 84篇 |
1986年 | 79篇 |
1985年 | 105篇 |
1984年 | 100篇 |
1983年 | 49篇 |
1982年 | 53篇 |
1981年 | 53篇 |
1980年 | 39篇 |
1979年 | 24篇 |
1978年 | 21篇 |
1977年 | 13篇 |
1976年 | 27篇 |
1975年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
Effects of several population densities ofMeloidogyne incognita on the sweet potato cultivars Centennial (susceptible) and Jasper (moderately resistant) were studied. Field plots were infested with initial levels (Pi) of 0, 10, 100, 1,000, 5,000, and 10,000 eggs and juveniles/500 cm³ soil in 1980 and 0, 100, 1,000, 2,000, 3,000, 4,000, and 5,000 in 1981. M. incognita population development trends were similar on both cultivars; however, at high Pi, more eggs and juveniles were recovered from Centennial than from Jasper. The highest Pi did not result in the highest mid-season (Pm) counts. Pi was negatively correlated with the number of marketable roots and root weight but positively correlated with total cracked roots, percentage of cracked roots, and cracking severity. Jasper tolerated higher Pi with greater yields and better root quality than Centennial. Cracking of fleshy roots occurred with both cultivars at low Pi. 相似文献
82.
Floret specialization, seed production and gender in Artemisia vulgaris L. (Asteraceae, Anthemideae)
P. J. GARNOCK-JONES 《Botanical journal of the Linnean Society. Linnean Society of London》1986,92(4):285-302
Flowering and fruiting behaviour of female and hermaphrodite florets is described and assessed in samples from three populations from Denmark, England and Sweden. Between 25 and 50% of the florets in capitula are female, and flowering gender varies little among plants in each population. Fruiting gender of individuals, G (femaleness), varies from 0 to 0.85, because of variation in fruit set and fruit abortion. Variation in fruiting gender was correlated with plant size parameters in two populations, but not in the third. The data suggest that post-anthesis regulation of maternal investment may be operating. Florets of A. vulgaris are either totally specialized for pollen receipt (female florets) or largely specialized for pollen donation (hermaphrodite florets), and show adaptations for avoiding interference with each other in these functions. Movement of capitula from a pendent position at flowering to an erect position at fruiting optimizes positions for dissemination of pollen and of seeds respectively. 相似文献
83.
The parasitoidEphedrus cerasicola Stary oviposited in the 4 nymphal instars and in newly moulted apterous adults ofMyzus persicae (Sulzer). Development and reproduction of unparasitized and parasitized aphids at 21°C were compared. Unparasitized aphids developed to adults in 6.5 days and started to reproduce after 7 days. Longevity varied between 7 and 42 days. Net reproductive rate (R0) was 40.7. In contrast to older nymphs, aphids parasitized in the 1 st instar almost never reached the adult stage before mummification. Aphids parasitized in 2nd, 3rd and 4th instar and as newly moulted adults produced respectively 0.07 %, 2 %, 23 % and 32 % of offspring produced by unparasitized aphids. Corresponding reproductive periods were 1, 1.4, 3 and 4 days. Host age at parasitization had a slight effect on the parasitoid's developmental rate and had no effect on egg or pupal survival, or on the sex ratio of the emerging parasitoids. 相似文献
84.
Polymorphism at the Hor 1 locus of barley (Hordeum vulgare L.) 总被引:1,自引:0,他引:1
The Hor 1 locus of barley encodes a group of seed storage polypeptides called C hordein. Two-dimensional electrophoretic analysis of C-hordein fractions from six cultivars with different alleles at the Hor 1 locus showed extensive polymorphism. A total of 34 major polypeptides was mapped, with between 4 and 18 present in each cultivar. There was less variation among the same cultivars in the numbers (6 to 10) of restriction fragments of genomic DNA which hybridized to a cDNA clone related to C hordein. The total number of restriction fragments was also lower (22), and most pairs of cultivars had more restriction fragments than polypeptides in common. A total number of about 20–30 C-hordein genes per haploid genome was estimated. The results indicate that cultivars differ mainly in the extent of gene and polypeptide divergence, rather than in the degree of gene reiteration. They are consistent with the proposed origin of the multiple structural genes at the Hor 1 locus by the duplication and divergence of a single ancestral gene.NACB was supported by a grant from the Home Grown Cereals Authority. 相似文献
85.
PAUL M. BRAKEFIELD 《Biological journal of the Linnean Society. Linnean Society of London》1985,24(2):189-206
Seasonal selection acting on the melanic polymorphism in the two-spot ladybird Adalia bipunctata was investigated in The Netherlands. An increase in melanic frequency over the spring-summer reproductive period was quantified. The selective advantage gained by melanics averaged 9%, but significant heterogeneity occurred between populations. Adult hibernation behaviour is described. The beetles when outdoors show a highly clumped distribution both between and within trees. The distribution of the morph classes between aggregations is random. Survivorship in a hibernating cohort (initial n= 1898) on a grid of 70 lime trees near Utrecht was monitored by making three counts over the winter of 1981–1982. Intense selection favouring each melanic morph occurred during December and January. The relative fitness of non-melanics was 0.55 (melanics =1). The discovery of dead beetles in late January (about 5% of total losses) and the absence of spatially density-dependent mortality were consistent with a climatic stress rather than selective predation. The period of selection was associated with very cold temperatures averaging up to 4°C below normal and an overall mortality of nearly 75%. There was no change in morph frequency, near normal temperatures and a lower mortality from February to early April. Examination of groups of nearby trees in late January strongly suggested that similar differential mortality had occurred except on some willows. This difference was probably due to the more protected hibernation sites available on these trees. Samples of hibernating cohorts at three other sites showed no evidence of differential mortality. Laboratory experiments with hibernating beetles found no difference in survivorship or rate of weight loss between starved non-melanics and melanics in temperature regimes with and without periods of adult activity. It is concluded that the intense winter selection on the study limes is probably exceptional. Examination of changes in morph frequency through the annual cycle suggests that at some sites the selection favouring melanics during reproduction is counterbalanced by selection against melanics in late summer or early autumn. The results are discussed in relation to mathematical models of cyclical selection and to other field studies including that of Timoféeff-Ressovsky (1940), who found large decreases in melanic frequency during hibernation in Berlin. 相似文献
86.
Pythium pythioides (Roze & Cornu) Ramsbottom is recorded from Algeria. Since its original discovery in 1869 this species had not been found back. A brief history is given and morphological and reproductive characters are discussed. 相似文献
87.
G. Charmet G. Branlard 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1985,71(2):193-200
Summary Sixty single seed descent (SSD) lines and about 25 anther-derived doubled-haploid (DH) lines were obtained from two triticale crosses. The frequency distributions of 10 quantitative agronomic traits were compared using parametric and non-parametric tests. A multivariate discriminant analysis was subsequently carried out. Gliadin patterns obtained from each line by polyacrylamide gel electrophoresis were used to calculate intra- and inter-population diversities from relative dissimilarity indices. It was found that DH and SSD lines show significant differences in frequency distributions of 1000 grain weight in both crosses, of heading date for one cross, and of lodging susceptibility for the other cross. The results of intra- and inter-population gliadin diversity indicate that although the SSD method theoretically provides more opportunity for recombination to occur than the DH method, it did not produce a greater range of recombinants. Since there is no significant difference between SSD- and DH-line distributions for grain yield, anther culture appears to be an efficient method for producing high yielding homozygous lines from F1 hybrids of triticale in a relatively short time. 相似文献
88.
Kees Nieuwenhuijsen Ad J. J. C. Lammers Karel J. de Neef A. Koos Slob 《International journal of primatology》1985,6(1):77-99
Reproductive physiology was studied in female stumptail macaques. Initially the monkeys were housed indoors (individually
and in small groups) and later as one large (92 individuals) social group in an outdoor cage. Most data were collected during
the 4-year outdoor period. Plasma progesterone determination in blood samples taken at weekly intervals allowed estimation
of ovulation and conception dates. The age at first ovulation (X =3.73 years) was positively correlated with body weight at 3 years of age. The average age at first birth was 4.90 years. Gestation
lengths averaged 176.6 days. Following a live birth ovulations returned after a mean interval of 11 months but following an
abortion or still birth this interval was 1 month. Usually a number of ovulatory cycles (X =2.37) preceded a conception. Interbirth intervals (IBIs) in the outdoor cage (X =619.4 days) were significantly longer than IBIs during the indoor period (X =523.1), because indoors the infants were weaned at the age of 7 months, while outdoors weaning occurred more naturally. IBIs
following abortions or still births (X =291.9 days) were significantly shorter than IBIs following live births. Age at first ovulation, age at first birth, IBIs,
and infant production rates were not correlated with dominance rank. Ovarian cycle lengths (X =30.2 days, mode = 28 days) were comparable to previously reported data from laboratory-housed stumptails. No systematic seasonal
fluctuations were found in the onset of sexual maturity, in ovarian cycle lengths, in copulation frequencies, and in distribution
of births. 相似文献
89.
90.
Abstract Chenopodium album L. plants, grown under controlled environmental conditions on different levels of soil nitrate, produced seeds with proportionately different NO?3 contents. Regardless of the endogenous NO?3 content, few seeds germinated in water or upon treatment with KNO3. Ethylene promoted germination, and the extent of germination was positively correlated with the endogenous seed NO?3 content. Combined application of ethylene and KNO3 in the dark had a synergistic effect on NO?3 -deficient seed. The synergism between ethylene and KNO3 was attributable to the NO?3 moiety of the nitrate salt. Ethylene and light showed moderate synergism in seeds with low or high endogenous nitrate. Addition of nitrate, however, masked the interaction between ethylene and light. Gibberellic acid4+7 (GA4+7) or red light, each alone or combined with KNO3, had little effect on germination. When applied together in the dark, ethylene and GA4+7 synergistically enhanced the germination of NO?3-deficient seed. The combined effects of the two hormones on this seed were further enhanced by the addition of KNO3. There was no synergism between ethylene and GA4+7 in NO?3-rich seed. These interactions among GA4+7, ethylene and KNO3 were not affected by light. The results confirm and further elaborate our earlier finding that the sensitivity of C. album seeds to ethylene may depend on nitrate availability. 相似文献