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   17498篇
  免费   1573篇
  国内免费   766篇
  19837篇
  2025年   64篇
  2024年   237篇
  2023年   334篇
  2022年   357篇
  2021年   565篇
  2020年   499篇
  2019年   580篇
  2018年   448篇
  2017年   499篇
  2016年   588篇
  2015年   619篇
  2014年   804篇
  2013年   982篇
  2012年   612篇
  2011年   699篇
  2010年   601篇
  2009年   750篇
  2008年   813篇
  2007年   893篇
  2006年   776篇
  2005年   723篇
  2004年   700篇
  2003年   613篇
  2002年   553篇
  2001年   463篇
  2000年   462篇
  1999年   398篇
  1998年   442篇
  1997年   343篇
  1996年   376篇
  1995年   326篇
  1994年   274篇
  1993年   251篇
  1992年   268篇
  1991年   233篇
  1990年   191篇
  1989年   184篇
  1988年   148篇
  1987年   131篇
  1986年   116篇
  1985年   160篇
  1984年   123篇
  1983年   76篇
  1982年   111篇
  1981年   97篇
  1980年   71篇
  1979年   76篇
  1978年   60篇
  1977年   43篇
  1976年   30篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
We studied the nectar characteristics in relation to flower age of the summer flowering Mediterranean shrubCapparis spinosa in three localities in Southern Greece. Anthesis was nocturnal. Nectar volume, concentration, and sucrose/hexose ratio varied with site, year, and between individual plants; amino acid concentration varied only with site. The sucrose/hexose ratio decreased considerably with flower age, while the glucose/fructose ratio remained constant (ca. 1), implying that nectar sucrose broke down in the course of anthesis. Sugar breakdown increased with water content of nectar. Amino acid concentration was strongly age-dependent: It was low in fresh flowers, relatively high in middle-aged ones (except aspartic acid that was extremely increased), and very high in senescent ones. We attribute the amino acid changes to phenomena related to flower senescence in the dark.  相似文献   
3.
4.
In this study we examined the biosynthesis of abscisic acid (ABA) by developing corn (Zea mays L.) embryos. Three comparisons were made: ABA biosynthesis in embryos isolated from kernels grown in vitro with those grown in the field; the developmental profile of ABA content with that of biosynthesis; and ABA biosynthesis in corn embryos lacking carotenoid precursors with ABA biosynthesis in normal embryos. Embryos were harvested at various times during seed development and divided into two groups. Endogenous levels of ABA were measured in one group of embryos and ABA biosynthetic capacity was measured in the other group. The ABA biosynthetic capacity was measured with and without tetcyclacis (an inhibitor of ABA degradation) in embryos from both field-grown and in-vitro-grown corn kernels. Reduced-carotenoid (either fluridone-treated or genetically viviparous) embryos were also included in the study. Corn kernels developing under field and in-vitro conditions differed from each other in their responses to tetcyclacis and in their profiles of ABA biosynthesis during development. Therefore, in-vitro kernel culture may not be an appropriate substitute for field conditions for studies of embryo development. The developmental profiles of endogenous ABA content differed from those of ABA biosynthesis in isolated embryos of both in-vitro-and field-grown kernels. This indicated that ABA levels in the developing embryos were determined by import from the maternal tissues available to the embryos rather than by in-situ biosynthesis. In embryos with reduced levels of carotenoids, either fluridone-treated or genetically viviparous embryos, ABA biosynthesis was low or nonexistent. This result is expected for the presence of an indirect pathway of ABA biosynthesis and in the absence of ABA precursors.Abbreviations ABA abscisic acid - DAP days after pollination  相似文献   
5.
The Ih and lh i alleles have been shown previously to reduce the level of endogenous gibberellin A1 (GA1) in shoots of pea (Pisum sativum L.), resulting in a dwarf phenotype compared with the wild type, cv. Torsdag (Lh). In addition, plants homozygous for the lh i allele have reduced seed yield compared with Lh (tall, wild type) and lh (dwarf) plants. In this paper we show that the lh i mutation is expressed in developing seeds and pods. Comparison of GA levels in young shoots and developing seeds of genotypes lh and lh i demonstrates that the relative severity of the two mutations varies in different tissues. Homozygous h i seeds have reduced GA levels, weigh less, and are less likely to develop to maturity when compared with Lh seeds. However, fertilization of lh i plants with Lh pollen increases seed GA levels, seed weight and seed survival, indicating that an increase in seed GA levels due to the presence of the Lh allele can restore normal seed growth. Pods developing on self-pollinated lh i plants are shorter than pods on Lh (wild type) plants, although this may be an indirect effect of the increased seed abortion of lh i plants. Based on these results we suggest that endogenous GAs play an important role in the development of seeds of P. sativum L.Abbreviations GA(n) gibberellin An We wish to thank Katherine McPherson, Peter Newman, Leigh Johnson and Peter Bobbi for technical assistance, Professor L. Mander (ANU, Canberra) and Professor B.O. Phinney (UCLA, USA) for labelled GA standards, and the Australian Research Council for financial support.  相似文献   
6.
7.
A histological and gravimetric analysis of oocyte development in Encrasicholina heteroloba (Ruppell, 1858) indicated that this species spawns serially and has a group-synchronous mode of ovarian development. A six stage maturity scale, based on both external morphology and oocyte composition, was proposed to classify ovarian development in E. heteroloba . The incidence of females with hydrated oocytes and post-ovulatory follicles in samples from two regions; the south Java Sea and Roviana lagoon, in the Solomon Islands were used to estimate spawning frequency. Estimates of mean inter-spawning intervals ranged from around 2 days in fish from Roviana lagoon to up to 16.7 days in fish from the south Java Sea. Batch fecundity was determined from the number of oocytes in the largest oocyte size class in ripe stage ovaries. Batch fecundity was related to size and was significantly greater for a given size in the Roviana lagoon population ( F = 0.081 × L 4.89, Roviana population; F = 1.682 × L 2.83, Jepara population).  相似文献   
8.
9.
10.
In eukaryotic cells, the universal second messenger cAMP regulates various aspects of development and differentiation. The primary target for cAMP is the regulatory subunit of cAMP-dependent protein kinase A (PKA), which, upon cAMP binding, dissociates from the catalytic subunit and thus activates it. In the soil amoeba Dictyostelium discoideum, the function of PKA in growth, development and cell differentiation has been thoroughly investigated and substantial information is available. To obtain a more general view, we investigated the influence of PKA on development of the related species Polysphondylium pallidum. Cells were transformed to overexpress either a dominant negative mutant of the regulatory subunit (Rm) from Dictyostelium that cannot bind cAMP, or the catalytic subunit (PKA-C) from Dictyostelium. Cells overexpressing Rm rarely aggregated and the few multicellular structures developed slowly into very small fruiting bodies without branching of secondary sorogens, the prominent feature of Polysphondylium. Few round spores with reduced viability were formed. When mixed with wild-type cells and allowed to develop, the Rm cells were randomly distributed in aggregation streams, but were later found in the posterior region of the culminating slug or were left behind on the surface of the substratum. The PKA-C overexpressing cells exhibited precocious development and formed more aggregates of smaller size. Moreover, expression of PKA-C under the control of the prestalk-specific ecmB promoter of Dictyostelium leads to protrusions from aggregation streams. We conclude that Dictyostelium PKA subunits introduced into Polysphondylium cells are functional as signal components, indicating that a biochemically similar PKA mechanism works in Polysphondylium.  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