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   111335篇
  免费   7597篇
  国内免费   2300篇
  121232篇
  2023年   158篇
  2022年   189篇
  2021年   221篇
  2020年   194篇
  2019年   221篇
  2018年   201篇
  2017年   144篇
  2016年   182篇
  2015年   226篇
  2014年   331篇
  2013年   254篇
  2012年   14240篇
  2011年   15250篇
  2010年   2411篇
  2009年   1325篇
  2008年   10709篇
  2007年   10827篇
  2006年   9921篇
  2005年   9105篇
  2004年   8498篇
  2003年   7652篇
  2002年   6562篇
  2001年   4987篇
  2000年   6136篇
  1999年   2466篇
  1998年   429篇
  1997年   326篇
  1996年   273篇
  1995年   261篇
  1994年   269篇
  1993年   233篇
  1992年   231篇
  1991年   262篇
  1990年   169篇
  1989年   199篇
  1988年   132篇
  1987年   128篇
  1985年   114篇
  1984年   101篇
  1983年   122篇
  1959年   281篇
  1958年   540篇
  1957年   580篇
  1956年   495篇
  1955年   455篇
  1954年   462篇
  1953年   416篇
  1952年   392篇
  1951年   347篇
  1950年   293篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
101.
The structural polarity of statocytes of Lepidium sativum L. is converted to a physical stratification by a root-tip-directed centrifugal acceleration. Sedimentation of amyloplasts and nucleus to the centrifugal (distal) cell pole and the lateral displacement of the distal endoplasmic reticulum (ER) complex occur after centrifugation for 20 min at an acceleration of 50 g. With higher doses (20 min, 100-2,000 g), smaller organelles become increasingly displaced. From the centrifugal to the centripetal cell pole, the following stratification is observed: 1) amyloplasts with mitochondria; 2) nucleus with mitochondria and a few dictyosomes, as well as laterally located ER; 3) dictyosomes with a few mitochondria; 4) vacuoles; and 5) lipid droplets. Within the first 7.5 min, after the roots have been returned to 1 g, the original arrangement of the amyloplasts sedimented on the underlying ER complex is reestablished in 66% of the statocytes. When roots previously centrifuged in an apical direction are exposed in a horizontal position to 1 g, the latent period of the graviresponse is increased by 7.5 min relative to the non-centrifuged controls. The kinetics of the response are identical to the controls. Roots centrifuged first in an apical direction and then for 2 h in a lateral direction (1,000 g) have statocytes with a physical stratification perpendicular to the root axis. A gravitropic curvature does not take place during the lateral centrifugation. These results support the hypothesis that the distal ER complex is necessary and sufficient for graviperception.  相似文献   
102.
Rayder L  Ting IP 《Plant physiology》1983,72(3):611-615
Xerosicyos danguyi Humbert (Cucurbitaceae) is a leaf succulent endemic to Madagascar. Under well-watered conditions, the plant exhibited Crassulacean acid metabolism (CAM) but shifted to a dampened form of CAM, CAM-idling, when subjected to water stress. The purpose of this investigation was to examine the effects of a shift in carbon metabolism on phosphoenolpyruvate carboxylase and on NADP-malic enzyme in X. danguyi. Experiments were conducted to determine the diurnal patterns of enzyme activity and pH optima of both enzymes, as well as the approximate molecular mass, kinetic patterns, malate inhibition, and glucose-6-phosphate stimulation of phosphoenolpyruvate carboxylase. The two enzymes extracted from well-watered and water-stressed plants were similar in most parameters investigated; thus, CAM-idling appeared to be only a dampened form of CAM photosynthesis.  相似文献   
103.
104.
海南岛两种鸦鹃的生态和食性   总被引:2,自引:1,他引:1  
鸦鹃属在我国仅有两种,即褐翅鸦鹃Centropus sinensis(Stephens),又名大毛鸡,小鸦鹃Centropus toulou(P.L.S.Mǖller),又名小毛鸡。著名的毛鸡酒就是用这两种鸟体为主要原料浸制而成的。褐翅鸦鹃和小鸦鹃广布我国东南各省,数量较多。在60年代,仅两广的毛鸡年产量就达十万至数十万只,具有较大的经济意义。  相似文献   
105.
张克旭同志编著的《微生物发酵的代谢与控制》一书已由轻工业出版社于1982年出版。该书介绍了糖代谢与调节、脂类代谢与控制的基本原理,阐述了氨基酸、核苷酸和抗菌素发酵的代谢控制和实践,并从分子水平上突出了代谢控制的本质和育种要点。对反馈调节和用  相似文献   
106.
We have calculated translational and rotational diffusion coefficients and intrinsic viscosities of oligomeric structures composed of n identical subunits having a prolate ellipsoidal shape with axial ratio p. Results are presented for p = 1-6 for a variety of structures with n = 1-6. We compare our results with those obtained by a different modeling procedure, proposed by other workers, in which the monomeric subunit is represented as a string of touching, colinear spheres. If n and an estimate of p are known, the structure of the oligomer can be. in most cases, unambiguously determined by comparison of the experimental oligomer-to-monomer ratios of a given property with the numerical results of this work. As examples of the applicability of our results, we examine the relationship between structure and properties for neurophysin. bovine serum albumin, hemoglobin and phycocyanin.  相似文献   
107.
When an excised corn (Zea mays) root pretreated with chloride was exposed for 10 minutes to pulse labeling with 30Cl and then transferred to unlabeled chloride, the activity in the xylem exudate reached a maximum about 4 minutes after pulse labeling was discontinued and then declined sharply. The rate at which labeled chloride was transported across the root into the xylem and basipetally therein was on the order of 75 to 250 centimeters per hour. Consequently, symplasmic movement of chloride in corn roots is fast and may not be rate-limiting in transfer from the root surface to the xylem. Experiments on pulse labeling with 22Na gave similar results. A large fraction of the absorbed 22Na was not translocated into the exudate but was tightly sequestered in a cell compartment, probably the vacuole.  相似文献   
108.
109.
110.
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