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   10788篇
  免费   851篇
  国内免费   3篇
  2023年   60篇
  2022年   173篇
  2021年   263篇
  2020年   152篇
  2019年   206篇
  2018年   252篇
  2017年   236篇
  2016年   326篇
  2015年   560篇
  2014年   605篇
  2013年   780篇
  2012年   983篇
  2011年   896篇
  2010年   597篇
  2009年   515篇
  2008年   672篇
  2007年   728篇
  2006年   665篇
  2005年   549篇
  2004年   546篇
  2003年   450篇
  2002年   434篇
  2001年   77篇
  2000年   62篇
  1999年   82篇
  1998年   95篇
  1997年   75篇
  1996年   60篇
  1995年   60篇
  1994年   32篇
  1993年   53篇
  1992年   35篇
  1991年   34篇
  1990年   21篇
  1989年   19篇
  1988年   20篇
  1986年   14篇
  1985年   23篇
  1984年   20篇
  1983年   17篇
  1982年   21篇
  1981年   12篇
  1980年   11篇
  1979年   10篇
  1977年   11篇
  1976年   8篇
  1975年   7篇
  1974年   8篇
  1969年   8篇
  1960年   7篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
The suitability of different host plants for Pieris rapae L. in the South of the Iberian Peninsula, was studied in relation to host plant phenology, female behaviour, and larval development. Capparis spinosa is the only host plant available during the dry season of the year in the area studied. D. virgata, R. raphanistrum, H. incana and S. alba being suitable spring hosts. Comparative studies on mortality, larval development and larval growth between C. spinosa and the spring hosts revealed no significant differences.
Résumé L'adéquation de différentes plantes comme hôtes de P. rapae a été examinée en fonction de la phénologie des végétaux, du comportement de la femelle et du développement larvaire dans le sud de la péninsule ibérique. Pendant la saison sèche, C. spinosa est la seule plant convenable dans la zone étudiée. Diplotaxis virgata, Raphanus raphanistrum, Hirschfeldia incana et Sinapis alba sont des hôtes printaniers convenables. La mortalité, le développement et la croissance larvaires ne sont pas différentes sur C. spinosa et sur les hôtes de printemps.
  相似文献   
22.
Summary UsingSchönwetter's data base regression equations are derived expressing egg length and egg breadth as a function of egg mass for Passerines (n=3929) and non-Passerines (n=3217). For both groups these show a variation around the mean which is twice as large for length as for breadth. The average elongation (length/breadth) ist presented for 27 orders ranging from 1.61 in Apterygiformes and Gaviiformes to 1.21 in Strigiformes as well as examples of a few families where elongation increases or decreases as egg mass becomes larger. Egg mass can be estimated from the relationship where egg mass=k(LB2). Mean values, SD, and range of k for both groups are given, but for any particular species are best derived from the dimensions of L, B, and egg mass inSchönwetter's tables.
Länge, Breite und Form der Vogeleier auf der Grundlage der Tabellen vonSchönwetter
Zusammenfassung Regressionsgleichungen für Eilänge und Eibreite als Funktion der Eimasse ergeben für Passeres (3929 Arten) und Non-Passeres (3217 Arten) eine Streuung um den Mittelwert, die für Länge doppelt so hoch wie für die Breite ist. Das Verhältnis Länge: Breite reicht bei 27 Ordnungen von 1.61 bei Apterygiformes und Gaviiformes bis 1.21 bei den Strigiformes. In Beispielen für einzelne Familien steigt oder fällt der Wert mit zunehmender Eimasse. Letztere kann bestimmt werden gemäß k · (L · B2), wobei k eine Konstante darstellt. Mittelwerte, Standardabweichung und Konstante werden für Passeriformes und Non-Passeriformes angegeben, doch für einzelne Arten hält man sich am besten an die Werte beiSchönwetter.
  相似文献   
23.
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   
24.
Curves relating net photosynthetic rate to irradiance [P(I) curve relation] were estimated and analysed inMercurialis perennis L. plants stemming from three forest (spruce, beech and ash) stands with different tree leaf canopy development and different light regime. The saturating irradiance (Is) reached the highest values in plants of all three stands in spring (spruce forest: 438 W m−2, beech forest: 440 W m−2 and ash forest: 367 W m−2), it declined sharply in the middle of the growing season (283, 285 and 297 W m-2, respectively) and this Is level persisted until autumn. A pronounced dynamics in plants from spruce and beech forests made itself manifest also in the adaptation (Ia) and compensating (Ic) irradiances, respectively. After a sudden decline in summer, values in autumn were close to those of the vernal season. The most pronounced parameter, which optimally expressed the adaptation ofMercurialis perennis to various light conditions, was the photosynthetic efficiency (α) calculated as the slope of the linear part of the curve relating net photosynthetic rate to irradiance. At the time of the highest PN sat. value in course of the growing season (August) (spruce forest: 100, beech forest: 98.7 and ash forest: 85.8 μg CO2 m−2 s−1), RD was in its minimum; in autumn PN sat. reached the lowest values which corresponded to the most intensive RD. It was found thatMercurialis perennis plants stemming from forest stands with different light conditions did not make use equally of the altering light conditions in the course of the growing season. By the underlying analysis of P(I) curves this rhizomatous perennial herb (geophyte) may be characterized as a shade tolerant species.  相似文献   
25.
Summary A high-yield microbiological transformation of polyfunctional 5-3-acetoxy steroids, containing an additional ring E, by Corynebacterium mediolanum strain B-964 was carried out, resulting in the corresponding 4-3-ketones. It was shown that the type of transformation and the yield of the reaction depend on the degree of saturation of the ring E and on the position of the oxygen-containing substituents in it.  相似文献   
26.
To determine the value of serum arabinitol concentrations in clinical practice, we identified all patients at the University of Utah Medical Center for whom a serum arabinitol determination had been requested by the attending physician or housestaff to assist in the management of candidiasis. The patient populaton was divided into three categories on the basis of clinical and pathological findings: (1) superficial candidiasis, (2) possible deep, invasive candidiasis, and (3) definite, deep invasive candidiasis. Abnormal renal function was associated with elevated concentrations of serum arabinitol in proportion to the degree of renal dysfunction. Both the serum arabinitol concentration and the arabinitol/creatinine ratio were increased in the combined patient population with candidiasis relative to normal uninfected controls (p=0.06 and 0.001, respectively). However, neither of these tests reliably distinguished patients with invasive candidiasis from those with only superficial candidal disease.  相似文献   
27.
Chromosome pairing in females and males of diploid (2n = 22) and tetraploid (2n = 44) Odontophrynus americanus and diploid Ceratophrys cranwelli (2n = 26) and tetraploid C. ornata (2n = 104) showed that diploid females formed more chiasmata per paired arm than diploid males and polyploids of both sexes. There was a reduction in the level of recombination in female polyploids by forming multivalents with terminal chiasmata. The reduction reflected a change in the genetic control of pairing in females after polyploidization.  相似文献   
28.
The pool size and composition of free fatty acids (FFA) and diglycerides (DG) from the cerebrum and cerebellum of rats undergoing bicuculline-induced seizures were studied. A fourfold increase in cerebral FFA occurred 3-4 min after bicuculline injection; arachidonic and stearic acids were the principal fatty acids accumulated. Cerebellar FFA also increased, but to a lesser extent. An increased production of arachidonic acid took place in the cerebrum as a function of time after bicuculline injection. Other fatty acids produced were oleic, palmitic, and docosahexaenoic acids. A twofold increase in cerebral arachidonic acid was seen at the time of the first generalized tonic-clonic convulsion. However, a 13- to 17-fold increase in arachidonic acid was seen approximately 5-6 min after bicuculline injection. The rise in other FFA was much smaller. Stearoyl- and arachidonoyl-DG were also accumulated. The drug alpha-methyl-p-tyrosine was found to (a) potentiate the bicuculline-stimulated release of cerebellar FFA, and (b) inhibit by 70% the production of stearoyl- and arachidonoyl-DG in the cerebrum and cerebellum. Basal production of FFA was stimulated by p-chlorophenylalanine, but the drug had no effect on the bicuculline-induced changes. Hydrolysis of phospholipids enriched in stearoyl-arachidonoyl groups, such as phosphatidylinositol of excitable membranes, may be stimulated during seizures.  相似文献   
29.
The interaction between deoxyguanosine (dG) and cis-dichlorodiammineplatinum(II) (cis-Pt) leads to the 2:1 and the 1:1 dG-Pt adducts. These adducts were separated on an Aminex A6 cationexchange column by use ot 0.01 M K2CO3 (pH 11) as an eluent. The stoichiometry of the adducts was determined from the 195mPt radioactivity and from the absorbance of the guanine chromophore at 280 nm. Time-course studies show that dG reacts initially with cis-Pt to form the 1:1 adduct, which then interacts with a second molecule of dG to form the 2:1 adduct. Acid hydrolysis (100°C in 88% formic acid for 5–15 min) of the 1:1 and 2:1 adducts results in their conversion to two new products, which elute differently from the column but which still contain Pt bound in the same stoichiometric ratio to dG as in the nonhydrolyzed adducts. The hydrolyzed adducts show a negative diphenylamine reaction indicative ot cleavage of the glycosidic bond. It is concluded that mild acid hydrolysis converts the 1:1 and 2:1 dG-Pt adducts into the corresponding guanine-Pt adducts, which are chromatographically distinguishable. This acid hydrolysis-high pressure liquid chromatography (HPLC) procedure has application to the identification of the Pt adducts formed in DNA.  相似文献   
30.
The stimulation of human lymphocytes with phytohaemoagglutinin induces the appearance or increase of several enzymes of DNA metabolism [Pedrini etal., Biochem. Biophys. Res. Comm., 47:1221(1972)]. With long times of stimulation, two phenomena are observed; an increase in the levels of DNA polymerase, of a DNase acting on single-stranded DNA, and of an endonuclease, occurring between the third and fourth day, in parallel with a wave of DNA synthesis;a second wave of increase of the same enzymes and of DNA ligase,occurring between the fifth and eight day when the DNA replication rate, as measured by thymidine-pulses, has decreased to values close to the background.  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