首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   3110篇
  免费   214篇
  国内免费   155篇
  2023年   44篇
  2022年   53篇
  2021年   94篇
  2020年   78篇
  2019年   91篇
  2018年   78篇
  2017年   90篇
  2016年   99篇
  2015年   107篇
  2014年   122篇
  2013年   175篇
  2012年   116篇
  2011年   142篇
  2010年   77篇
  2009年   109篇
  2008年   128篇
  2007年   136篇
  2006年   126篇
  2005年   111篇
  2004年   119篇
  2003年   99篇
  2002年   81篇
  2001年   80篇
  2000年   64篇
  1999年   67篇
  1998年   55篇
  1997年   56篇
  1996年   70篇
  1995年   68篇
  1994年   67篇
  1993年   54篇
  1992年   49篇
  1991年   38篇
  1990年   35篇
  1989年   30篇
  1988年   30篇
  1987年   34篇
  1986年   46篇
  1985年   47篇
  1984年   34篇
  1982年   28篇
  1981年   23篇
  1980年   33篇
  1979年   30篇
  1978年   26篇
  1977年   22篇
  1976年   25篇
  1973年   18篇
  1972年   19篇
  1971年   14篇
排序方式: 共有3479条查询结果,搜索用时 437 毫秒
991.
棉花花芽分化及部分内源激素变化规律的研究   总被引:14,自引:2,他引:12  
棉花(Gossypium hirsutum)的腋芽原基,有的将来发育成叶枝;有的将来发育成果枝。这2种不同命运的腋芽,在其刚分化的初期就表现出了不同的解剖学特征。将来发育为叶枝的腋芽,其生长锥呈圆锥形或扁圆球形,体积较小,原套层数为1-2层;而将来发育为果枝的腋芽,其生长锥为圆柱形,顶端表面平坦,体积较大,原套层数为2-3层。从子叶展平后到肉眼可见花芽(现蕾),连续测茎尖的内源ABA及IAA的含量  相似文献   
992.
无融合生殖油菜AMR—1花托离体培养的研究   总被引:1,自引:0,他引:1  
林娟 《西北植物学报》2000,20(3):370-375
报道了不同激素浓度对无融合生殖没菜花托器官分化效果的研究,结果显示:(1)以MS为基本培养基,以带有子房和花柄的花托为外植体离体培养,花托、花柄切口部位直接芽诱导的最佳激素配比为4.0mg/L6-BA+0.01mg/L NAA,频率为58.82%,花托、花柄部位先形成愈伤组织,继而分化出丛生芽的最佳激素配比为5.0mg/L 6-BA+0.5mg/L NAA,频率为84.00%;(2)腋芽增殖的最佳  相似文献   
993.
多胺对月季切花衰老过程中生理生化和瓶插寿命的影响   总被引:10,自引:0,他引:10  
精胺处理可保持月季切花瓶插前期花瓣还原糖和蛋白质的较高水平、减缓花瓣和叶片细胞膜稼性的增加,减少共 积累以及降低乙烯释放速率,这与精胺处理后月季切花瓶插寿命提高相一致;但亚精胺处理对季切花瓶插寿命和改善观赏品质无明显影响。  相似文献   
994.
外源激素对风信子再生花芽发育的控制   总被引:10,自引:0,他引:10  
外源激素在诱导再生花芽花器官发生和控制它们的数目中的关键作用被进一步证实。首先通过2mg/L 6-BA和0.1mg/L 2,4-D的激素组合诱导风信子Hyacinthus orientalis L.cv.White pearl)花芽从花被外植体发生,然后保持这种激素浓度,成功地控制了100多片花被片的连续发生(自然情况下一个风信子花芽仅有6片花被片)。改变激素浓度(2mg/L 6-BA和0~0.0  相似文献   
995.
We studied the interrelation between nectary structure (13 parameters), nectar characteristics (yield, chemical composition), and flower size of 11 Labiatae species in a Mediterranean shrub community near Athens, Greece. We also explored whether the above attributes are affected by the Mediterranean summer drought constraints. Our findings show that among all nectary parameters studied, nectary size and stomatal opening are the most important in (positively) shaping nectar secretion, nectary size being the most meaningful. Nectary structure is correlated to quantity of the nectar secreted, not its quality. Wide flowers bear wide nectaries with large stomatal openings, whereas deep flowers are not related to any nectary size. Corolla size (both length and width) and nectary stomatal opening decrease with flowering time. This applies also to nectary size, nectar volume and sugar content of the perennials (9 species). All above cases of time dependence show that there is a constraint effect of Mediterranean climate on floral and nectary structure, reflected also as a decrease in nectar secretion. Nectary structure in Labiatae is largely shaped by both phylogenetic and climate constraints. On the other hand, although nectar is largely influenced by nectary structure, it is to a large extent ecologically biased, implying that, in addition to phylogeny, there are many other ecological parameters interfering in its secretion such as time within the season, life history, and light requirements.  相似文献   
996.
比克氏棉渐渗到陆地棉的花色基因的遗传分析   总被引:1,自引:0,他引:1  
比克氏棉的红花大基斑性状渐渗到陆地棉,并选育出了稳定的红花大基斑陆地棉纯系(HBRL)。报道了这些红花(R~(bic)_3)大基斑(R~(bic)_2)陆地棉纯系的遗传连锁实验和连锁分析结果。大基斑(R~(bic)_2)对小基斑(R_2)和无基斑(r_2)为显性,R~(bic)_2与R_2等位,R~(bic)_2、R_2和r_2构成了复等位基因系列。R~(bic)_2与Lc_1连锁,R.C%=37.31±2.44.R~(bic)_2与R~(bic)_3紧密连锁,并且R~(bic)_3对Y_1和y_1具有显性上位作用。建议从比克氏棉渐渗到陆地棉的大基斑的基因符号定为R~(bic)_2,红花瓣的基因符号定为R~(bic)_3。  相似文献   
997.
通过往孕鼠体内连续注入BrdU和小鼠胚胎细胞在含BrdU培养基中培养,证明小鼠胚胎细胞的NOR活性明显地被BrdU所抑制;当洗去BrdU后,NOR活性可逐步恢复,怀孕早中期胚胎NOR活性被抑制引起小鼠胚胎器官发育的异常:胎儿流产和出生后幼鼠死亡的比例明显增加;出生的幼鼠眼球晶状体上皮细胞异常地出现1-3个直径平均为0.3μm的不透明颗粒。经组织切片蛋白质特经学显色表明,这些颗粒全为蛋白质颗粒。随着  相似文献   
998.
简要介绍花境的形式和作用,通过对厦门地区调查分析,总结花境在厦门园林中的应用发展水平,并对如何改善花境植物品种、类型较为单调的问题提出可行性建议。  相似文献   
999.
The floral morphogenesis and androecium developmental sequence of Anemone rivularis Buch.-Ham. ex DC. var.flore-rninore Maxim. were observed under a scanning electron microscope (SEM) and by means of histological methods in order to expand our knowledge of the morphogenesis and development of the floral organs of the Ranunculaceae. The initiation of the floral elements is a centripetal spiral and the direction of the spiral is clockwise or anti-clockwise. However, the development of the androecium is highly unusual: in a longitudinal series of four stamens, the second stamen develops first from the inner to outer, then the third one, the fourth one and the first one in turn. The microsporogenesisand ant her maturation follows the same developmental sequence. The tepals are different from the bracts and the stamens in both shape and size in the early developmental stage, but there is no difference between the stamens and carpels in the early developmental stage. Therefore, we established a spatio-temporal process of the floral morphogenesis ofA. rivularis var.flore-rninore and offer another meaning of the floral diversity patterns attributed to the level of the genus.  相似文献   
1000.
Summary 150–190 photoreceptor cells form a basic structural component of the pineal organ of Ambystoma tigrinum. Most of the outer and inner segments of these cells project into the lumen horizontally. Only 10 percent of the total number of photoreceptor cells are located within the pineal roof which is composed of a single cell layer. The photoreceptor cells are connected with nerve cells by synapses displaying characteristic ribbons. Different types of synaptic contacts, i.e. simple, tangential, dyad, triad and invaginated, are found. They are embedded in extended neuropil zones. A particular type of synapse indicates the presence of interneurons. The basal processes of some photoreceptor cells leave the pineal organ and make synaptic contacts with nervous elements located within the area of the subcommissural organ. Employing the method of Karnovsky and Roots (1964) for histochemical demonstration of acetylcholinesterase (AChE) approximately 70 neurons (intrapineal neurons) can be discerned in the pineal organ of Ambystoma tigrinum. In analogy to the distribution of photoreceptor cells only few nerve cells are observed in the roof portion of the pineal organ. Evidently, two different types of AChE-positive intrapineal neurons are present. About 40–50 AChE-positive neurons (extrapineal neurons) are scattered in the area of the subcommissural organ. In this area two types of nerve cells can be distinguished: 1) neurons which send pinealofugal (afferent) axons toward the posterior commissure and 2) neurons which emit pinealopetal (efferent) axons into or toward the pineal organ.The nervous pathways connecting the pineal organ with the diencephalomesencephalic border area are represented by a distinct pineal pedicle and several accessory pineal tracts.Granular nerve fibers run within the posterior commissure and establish synaptic contacts in the commissural region adjacent to the pineal organ. Some of these granular elements enter the pineal organ.The morphology of the nervous apparatus of the pineal organ of Ambystoma tigrinum is discussed in context with evidence from physiological experiments.In partial fulfillment of the requirements for the degree of Dr. med., Faculty of Medicine, Justus Liebig University, GiessenThe author is indebted to Professors A. Oksche and M. Ueck for their interest in this study. Thanks are due to Professor Ch. Baumann, Giessen, and Professor H. Langer, Bochum, for stimulating discussions. The technical assistance of Miss R. Liesner is gratefully acknowledgedDedicated to Professor Berta Scharrer on the occasion of her 70th birthday. Supported by grants from the Deutsche Forschungsgemeinschaft to A.O. and M.U.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号