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   7854篇
  免费   920篇
  国内免费   684篇
  2024年   20篇
  2023年   214篇
  2022年   214篇
  2021年   330篇
  2020年   394篇
  2019年   444篇
  2018年   353篇
  2017年   348篇
  2016年   331篇
  2015年   327篇
  2014年   454篇
  2013年   476篇
  2012年   335篇
  2011年   342篇
  2010年   234篇
  2009年   349篇
  2008年   346篇
  2007年   368篇
  2006年   328篇
  2005年   268篇
  2004年   293篇
  2003年   219篇
  2002年   232篇
  2001年   197篇
  2000年   167篇
  1999年   154篇
  1998年   167篇
  1997年   135篇
  1996年   142篇
  1995年   139篇
  1994年   134篇
  1993年   132篇
  1992年   130篇
  1991年   104篇
  1990年   84篇
  1989年   110篇
  1988年   70篇
  1987年   75篇
  1986年   54篇
  1985年   66篇
  1984年   45篇
  1983年   23篇
  1982年   21篇
  1981年   31篇
  1980年   18篇
  1979年   11篇
  1978年   10篇
  1977年   6篇
  1976年   4篇
  1972年   4篇
排序方式: 共有9458条查询结果,搜索用时 15 毫秒
81.
1. Initiation of subsynaptic sarcolemmal specialization and expression of different molecular forms of AChE were studied in fast extensor digitorum longus (EDL) and slow soleus (SOL) muscle of the rat under different experimental conditions in order to understand better the interplay of neural influences with intrinsic regulatory mechanisms of muscle cells. 2. Former junctional sarcolemma still accumulated AChE and continued to differentiate morphologically for at least 3 weeks after early postnatal denervation of EDL and SOL muscles. In noninnervated regenerating muscles, postsynaptic-like sarcolemmal specializations with AChE appeared (a) in the former junctional region, possibly induced by a substance in the former junctional basal lamina, and (b) in circumscribed areas along the whole length of myotubes. Therefore, the muscle cells seem to be able to produce a postsynaptic organization guiding substance, located in the basal lamina. The nerve may enhance the production or accumulation of this substance at the site of the future motor end plate. 3. Significant differences in the patterns of AChE molecular forms in EDL and SOL muscles arise between day 4 and day 10 after birth. The developmental process of downregulation of the asymmetric AChE forms, eliminating them extrajunctionally in the EDL, is less efficient in the SOL. The presence of these AChE forms in the extrajunctional regions of the SOL correlates with the ability to accumulate AChE in myotendinous junctions. The typical distribution of the asymmetric AChE forms in the EDL and SOL is maintained for at least 3 weeks after muscle denervation. 4. Different patterns of AChE molecular forms were observed in noninnervated EDL and SOL muscles regenerating in situ. In innervated regenerates, patterns of AChE molecular forms typical for mature muscles were instituted during the first week after reinnervation. 5. These results are consistent with the hypothesis that intrinsic differences between slow and fast muscle fibers, concerning the response of their AChE regulating mechanism to neural influences, may contribute to different AChE expression in fast and slow muscles, in addition to the influence of different stimulation patterns.  相似文献   
82.
Phosphatases; origin,characteristics and function in lakes   总被引:18,自引:4,他引:14  
Phosphatases catalyze the liberation of orthophosphate from organic phosphorus compounds. The total phosphatase activity in lake water results from a mixture of phosphatases localized on the cell surfaces of algae and bacteria and from dissolved enzymes supplied by autolysis or excretion from algae, bacteria and zooplankton. External lake water phosphatases usually have pH optima in the alkaline region. Acid phosphatases generally seem to be active in the internal cell metabolism. The synthesis of external alkaline phosphatases is often repressed at high phosphate concentrations and derepressed at low phosphate concentrations. Phosphatase activity has therefore been used as a phosphorus deficiency indicator in algae and in natural plankton populations. The possibilities for this interpretation of phosphatase activity in lake water are limited, however, and this is discussed. The in situ hydrolysis capacity, i.e. the rate by which orthophosphate is released from natural substrates, is unknown. However, we advocate that this process is important and that the rate of substrate supply, rather than phosphatase activity, limits the enzymatic phosphate regeneration.  相似文献   
83.
Urs Uehlinger 《Hydrobiologia》1986,135(3):197-206
The aerobic decomposition of the green alga Chlamydomonas reinhardii by a mixed population of lake bacteria was studied in batch and chemostat cultures. Bacterial chemostats were supplied with continuously heatkilled algae. The dead algae rapidly released most of their phosphorus as SRP. In the batch experiments bacteria acted as consumers of the released algal phosphorus. This phosphorus uptake was dependent on the C:P ratio of the algae. During the death phase of the bacteria most of the bacterial phosphorus itself was released. The continuous supply of energy in form of dead algae in the chemostat experiments prevented the death phase of the bacteria and thus any net regeneration of phosphorus. The influence of the C:P stoichiometry of algae and bacteria on the regeneration of algal phosphorus is discussed.  相似文献   
84.
In reviewing recent research published in Russian on regeneration and asexual reproduction, the following morphogenetic processes in the planarian Dugesia tigrina are considered: 1) regeneration of lost parts of the body; 2) regeneration of the whole worm from fragments of the body, either by normal regeneration when the inital polarity of the fragment is retained or by somatic embryogenesis when one or more new axes of polarity arise; 3) somatic embryogenesis, or development of individuals from somatic cells; 4) hypermorphosis, or the presence of more than the usual number of organs or body parts, a process that can be interpreted in terms of somatic embryogenesis; and 5) asexual reproduction. Some morphological, biochemical, and physiological studies of the division zone in D. tigrina demonstrate peculiarities of a local breakdown of integrative functions, a breakdown which in turn causes division of the individual to take place at this zone; timing of division is controlled by the organism as an integrated whole.  相似文献   
85.
On the origin of neoblasts in freshwater planarians (Turbellaria)   总被引:2,自引:2,他引:0  
Experiments on 1) regeneration of the cave-adapted planarian, Sphalloplana zeschi, 2) induction of sexuality in an asexual strain of Dugesia japonica japonica by feeding, and 3) culture of dissociated planarian cells, show that neoblasts originate from intestinal cells, i.e. phagocytic cells and granular clubs.  相似文献   
86.
Abstract: After the goldfish optic nerve was crushed, the total amount of protein in the nerve decreased by about 45% within 1 week as the axons degenerated, began to recover between 2 and 5 weeks as axonal regeneration occurred, and had returned to nearly normal by 12 weeks. Corresponding changes in the relative amounts of some individual proteins were investigated by separating the proteins by two-dimensional gel electrophoresis and performing a quantitative analysis of the Coomassie Brilliant Blue staining patterns of the gels. In addition, labelling patterns showing incorporation of [3H]proline into individual proteins were examined to differentiate between locally synthesized proteins (presumably produced mainly by the glial cells) and axonal proteins carried by fast or slow axonal transport. Some prominent nerve proteins, ON1 and ON2 (50–55 kD, pI ~6), decreased to almost undetectable levels and then reappeared with a time course corresponding to the changes in total protein content of the nerve. Similar changes were seen in a protein we have designated NF (~130 kD, pI ~5.2). These three proteins, which were labelled in association with slow axonal transport, may be neurofilament constituents. Large decreases following optic nerve crush were also seen in the relative amounts of α- and β-tubulin, which suggests that they are localized mainly in the optic axons rather than the glial cells. Another group of proteins, W2, W3, and W4 (35–45 kD, pI 6.5–7.0), which showed a somewhat slower time course of disappearance and were intensely labelled in the local synthesis pattern, may be associated with myelin. A small number of proteins increased in relative amount following nerve crush. These included some, P1 and P2 (35–40 kD, pIs 6.1–6.2) and NT (~50 kD, pI ~5.5), that appeared to be synthesized by the glial cells. Increases were also seen in one axonal protein, B (~45 kD, pI ~4.5), that is carried by fast axonal transport, as well as in two axonal proteins, HA1 and HA2 (~60 and 65 kD respectively, pIs 4.5–5.0), that are carried mainly by slow axonal transport. Other proteins, including actin, that showed no net changes in relative amount (but presumably changed in absolute amount in direct proportion to the changes in total protein content of the nerve), are apparently distributed in both the neuronal and nonneuronal compartments of the nerve.  相似文献   
87.
Regeneration of peach plants from callus derived from immature embryos   总被引:9,自引:0,他引:9  
Summary Peach plants were repeatedly regenerated from immature embryos but not from callus derived from mature embryos. A white, nodular, highly regenerative callus was obtained when friable, primary callus from immature embryos was transferred from medium containing 4.5 M 2,4-dichlorophenoxyacetic acid and 0.44 M benzyladenine (BA) to media containing 0.27 M -naphthaleneacetic acid (NAA) and 2.2 M BA. This callus retained its morphogenetic potential for a minimum of three subcultures. Green nodular callus, that lacked regenerative capacity, was produced from primary callus derived from mature embryos. Maximum regeneration of shoots occurred when highly regenerative callus was transferred to a medium in which the NAA concentration was reduced five times and the BA concentration was increased two times. Regenerated shoots were rooted in the dark on a medium containing 28.5 M indoleacetic acid. Cytogenetic analysis of regenerated plants indicated that all plants were diploid, 2n = 2x = 16. Phenotypic evaluation of regenerated plants, grown under field conditions, is now in progress.  相似文献   
88.
Callus was successfully initiated on root, mesocotyl and leaf base segments of 3- to 4-day-old seedlings of ragi (Eleusine coracana Gaertn.). 2,4-D along with casein hydrolysate for Murashige and Skoog's basal medium was found to be most effective for callus initiation and maintenance. Mesocotyl and leaf base tissue derived calli gave shoot buds in medium in which the 2,4-D concentration was lowered.  相似文献   
89.
Callus cultures were initiated from apical meristem explants of one to four-week-old aseptically-grown barley (Hordeum vulgare L. cv. Atlas 57) plants. Embryogenic callus and plants were produced in three separate experiments; the cultures have retained regenerative capacity for three years after initiation. Our results demonstrate that explants other than immature embryos are embryogenically competent in barley and that regeneration occurs by both somatic embryogenesis and organogenesis.  相似文献   
90.
Although both ribavirin (1--ribofuranosyl-1,2,4-triazole-3carboxamide) and adenine arabinoside inhibited the multiplication of tobacco mosaic virus (TMV) in mechanically inoculated leaf tissues, neither chemical inhibited virus multiplication in unorganized tobacco callus after in vitro inoculation. The adenine deaminase inhibitor, pentostatin, did not increase the activity of adenine arabinoside in cultured cells. Several different developmental conditions and media did not increase the ability of either chemical to eradicate the virus from tobacco tissue cultures. However, the virus was eradicated from TMV-infected callus when grown in the presence of combinations of ribavirin and adenine arabinoside in shoot inducing medium.  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