全文获取类型
收费全文 | 6334篇 |
免费 | 405篇 |
国内免费 | 316篇 |
出版年
2024年 | 4篇 |
2023年 | 40篇 |
2022年 | 82篇 |
2021年 | 85篇 |
2020年 | 106篇 |
2019年 | 123篇 |
2018年 | 121篇 |
2017年 | 139篇 |
2016年 | 175篇 |
2015年 | 181篇 |
2014年 | 296篇 |
2013年 | 279篇 |
2012年 | 270篇 |
2011年 | 343篇 |
2010年 | 251篇 |
2009年 | 350篇 |
2008年 | 377篇 |
2007年 | 406篇 |
2006年 | 367篇 |
2005年 | 307篇 |
2004年 | 266篇 |
2003年 | 230篇 |
2002年 | 161篇 |
2001年 | 145篇 |
2000年 | 155篇 |
1999年 | 185篇 |
1998年 | 176篇 |
1997年 | 116篇 |
1996年 | 141篇 |
1995年 | 117篇 |
1994年 | 106篇 |
1993年 | 101篇 |
1992年 | 82篇 |
1991年 | 93篇 |
1990年 | 75篇 |
1989年 | 85篇 |
1988年 | 67篇 |
1987年 | 79篇 |
1986年 | 62篇 |
1985年 | 51篇 |
1984年 | 43篇 |
1983年 | 30篇 |
1982年 | 32篇 |
1981年 | 40篇 |
1980年 | 34篇 |
1979年 | 28篇 |
1978年 | 9篇 |
1977年 | 11篇 |
1976年 | 11篇 |
1952年 | 3篇 |
排序方式: 共有7055条查询结果,搜索用时 46 毫秒
131.
The composition of the seed bank was compared with that of the above-ground vegetation in two British salt marshes. Vegetation and seed banks were sampled in autumn and spring. There was a relatively low correlation between seed bank and vegetation in most perennial-dominated zones. Percentage annual plant cover was significantly positively correlated to percentage similarity value between vegetation and seed bank. Cluster analysis indicated that the zones dominated by annuals and by Suaeda vera on the ungrazed Nod Marsh in Norfolk formed groups between seed bank and vegetation, whereas on the Llanrhidian Marsh in Wales the seed bank of four of the six zones formed a group. 相似文献
132.
Summary Regenerable embryogenic cell suspensions initiated from immature embryo-derived friable, fast growing, embryogenic calli of GK Ságvári winter wheat (Triticum aestivum L.) served as sources of protoplasts, which were cultured in different liquid or agarose-solidified media. Protocallus formation was best on KM8p (Kao and Michayluk 1975) and GM (Li and Murai 1990) media, and protocallus growth on MS (Murashige and Skoog 1962) callus growing medium. Green shoot/plant regeneration occurred on MS regenerating medium, and rooting on MS or N6M (Mórocz et al. 1990) hormone-free media. Protocalli maintained their morphogenic capacity over 4 months, and with multiple subcultures on half-strength MS regenerating medium, the total number of regenerants could be increased. Approximately 1000 shoots/plants were regenerated and over 500 plants were transplanted in the greenhouse. The majority of them had an abnormal chromosome number and low viability, however, one plant grew to maturity and set seed.Abbreviations BAP
6-benzylaminopurine
- 2,4-D
2,4-dichlorophenoxyacetic acid
- ECS
embryogenic cell suspension
- GA3
gibberellic acid
- GM
General medium
- IAA
indole-3-acetic acid
- IBA
indole-3-butyric acid
- MS
Murashige and Skoog medium
- NAA
1-naphthaleneacetic acid
- RECS
regenerable embryogenic cell suspension 相似文献
133.
To avoid seed predation, plants may invest in protective seed tissues. Often related to seed size, allocation in seeds' physical defenses can also be influenced by dispersers. We explore the relationships between seed traits (seed mass and hardness) and seed removal in 22 Myrtaceae species of the Brazilian Atlantic Forest, a dominant and diverse fleshy-fruited taxon dispersed by birds, rodents, and other mammals. Our goal is to understand how seed traits influence seed removal rates, and whether dispersers can affect tissue allocation in the seed coat. Seeds were exposed to field removal experiments. In the laboratory, total seed mass and seed coat mass were obtained. To evaluate the influence of seed traits on removal, we performed Kruskal–Wallis and Simple Linear Regression tests. We assessed seed coat and seed mass covariation through standardized major-axis allometric regressions. Harder seeds were larger than softer ones. Seed traits affect removal rates, as tougher and heavier seeds had lower removal. Seed mass significantly predicts seed coat proportion in seven of the 14 species tested. Bird-dispersed species tend to exhibit lower proportions of seed coat as seed mass increases, whereas rodent-dispersed species apparently present the opposite trend, with seed coat proportion increasing with seed mass. Such difference may be caused by the contrasting seed predation pressure represented by birds and rodents. Energy allocation for defense, expressed in seed coat proportion, is greater in large seeds, as these are mostly dispersed by rodents whose propensity to cache and disperse seeds is greater for large and well-protected seeds. 相似文献
134.
To study the roles of m5C in the differentiation of rice calli derived from protoplasts (protoclones), the m5C level of the total DNA was analyzed using the32P post-labeling method. The level of m5C in regenerable and nonregenerable protoclones was similar, as was in calli and leaves of plants regenerated from the same
protoclones. Treatment with 0.5 mM 5-azacytidine caused significant reduction of the level of m5C and of the regeneration frequency of callus. Significantly increased m5C levels were observed during prolonged culture. 相似文献
135.
Pollen, phytolith, and plant macrofossil analyses were carried out on the cultural layers of Helsinki Old Town excavated by the Helsinki City Museum and dated to the 15–17th centuries. Cultural mineral soil layers, a former ditch, a well, one waste pit, one waste heap and three house areas were investigated. The results show that-within certain limits-pollen in the mineral soil reflects the cultural changes of the local settlement from rural to urban. Among the main anthropogenic indicators, the high NAP (non-arboreal pollen) frequencies, the low pollen concentration values, and especially the high frequencies of Cichoriaceae (Liguliflorae) pollen should be emphasized. The macrofossil data were used to determine the species list of the herb pollen and to make a more detailed reconstruction of the local vegetation at Helsinki Old Town. 相似文献
136.
南苜蓿组织和原生质体培养及转化试验 总被引:2,自引:0,他引:2
主要探讨南苜宿子叶和下胚轴外植体,子叶原生岳体培养中的器官发生及遗传转化。南苜蓿子叶和下胚细外植体培养在附加1AA0.5-1mg/L和细胞分裂素(BA或ZT)0.5-2mg/L的MS培养基上,在有当照的条件下诱导形成不定芽,进而再生成完整植株。子叶原生质体培养在附加2,4-D0.5mg/L和KT0.2mg/L的B5液体培养基中,细胞分裂频率可达30-41%,原生质全来源的愈伤组织在MSB培养基(M 相似文献
137.
美味猕猴桃子叶愈伤组织的原生质体培养和再生植株 总被引:12,自引:0,他引:12
从B_5和NN-69培养基(含1mg/L 2,4-D)上分别选出美味猕猴桃子叶愈伤组织系A_(11)B_2和A_(16)N_1。在B_5原生质体培养基中,A_(11)B_2的原生质体再生细胞形成小细胞团;在NN-69原生质体培养基中,A_(16)N_1的原生质体再生细胞能持续分裂形成愈伤组织。经过分步诱导再生,获得A_(16)N_1原生质体再生植株。 相似文献
138.
Phenolic compounds were present in greater amounts in non‐infected petioles of genotypes of Hevea brasiliensis that are resistant to Phytophthora leaf disease than in genotypes that are susceptible. Phenolic compounds extracted from petioles of either susceptible (PB86) or resistant (RRIC100) genotypes, before or after infection with Phytophthora meadii, had anti‐fungal properties. Artificially infected petioles of PB86 had phenolic acids, triterpenoids or flavonoids, whereas healthy petioles contained only triterpenoids or flavonoids. However, healthy or infected petioles of RRIC100 contained only trace amounts of the above compounds and of vanillin (3‐methoxy‐4‐hydroxybenzaldehyde). Vanillin and umbelliferone (7‐hydroxycoumarin) were shown to suppress zoospore germination of P. meadii on glass slides and to inhibit its growth in pea broth and V‐8 juice agar. Vanillin was slightly more active than umbelliferone. Resistance of RRIC100 to Phytophthora was suspected as being related to the polymerisation of phenolic compounds to form lignin, which may suppress further spread of the pathogen's mycelium into healthy tissues. Formation of lignin from phenolic aldehydes as a barrier to disease spread may be a critical factor in resistance. 相似文献
139.
Tillers and seedlings ofHordeum jubatum L. from three sites with contrasting salinity regimes in central Saskatchewan, Canada were reciprocally transplanted in order to examine the tolerance of populations of this species to salinity and related habitat factors. Survival, growth and fecundity of the three populations were controlled more by transplant site characteristics than by genetic differences, i.e. differences among populations at a site tended to be smaller than differences among sites. Survival, growth and reproduction of all three populations were best at the non-saline site. The population originating at the non-saline site showed the poorest growth in the two saline habitats, but still had substantial salt tolerance. Fecundity was greatest when the populations were grow at their site of origin. 相似文献
140.
Dominique de Moulins 《Vegetation History and Archaeobotany》1996,5(1-2):153-156
Results of experimental retrieval of plant remains (both charred modern and fossil) from three sediment types (clay, gravel
and sandy loam) carried out by environmental archaeologists from the Ancient Monuments Laboratory and the Central Archaeology
Service, English Heritage, are reported. For each sediment type, six different treatments were carried out using three flotation
machines of the Siraf type, two other machines in use in two archaeological units and one experimental machine. The results
show that there is invariably considerable discrepancy between the amount of plant material present and that recovered, that
the Serif-type machine is among the best currently available and that pretreatment of particular mineral soils may improve
recovery of plant macrofossil material. 相似文献