全文获取类型
收费全文 | 16802篇 |
免费 | 1692篇 |
国内免费 | 1779篇 |
专业分类
20273篇 |
出版年
2024年 | 61篇 |
2023年 | 383篇 |
2022年 | 356篇 |
2021年 | 539篇 |
2020年 | 622篇 |
2019年 | 814篇 |
2018年 | 703篇 |
2017年 | 733篇 |
2016年 | 652篇 |
2015年 | 695篇 |
2014年 | 778篇 |
2013年 | 1035篇 |
2012年 | 688篇 |
2011年 | 725篇 |
2010年 | 621篇 |
2009年 | 859篇 |
2008年 | 896篇 |
2007年 | 1003篇 |
2006年 | 877篇 |
2005年 | 788篇 |
2004年 | 694篇 |
2003年 | 631篇 |
2002年 | 560篇 |
2001年 | 455篇 |
2000年 | 386篇 |
1999年 | 427篇 |
1998年 | 387篇 |
1997年 | 284篇 |
1996年 | 298篇 |
1995年 | 259篇 |
1994年 | 193篇 |
1993年 | 224篇 |
1992年 | 196篇 |
1991年 | 186篇 |
1990年 | 162篇 |
1989年 | 155篇 |
1988年 | 135篇 |
1987年 | 122篇 |
1986年 | 95篇 |
1985年 | 89篇 |
1984年 | 91篇 |
1983年 | 61篇 |
1982年 | 80篇 |
1981年 | 59篇 |
1980年 | 47篇 |
1979年 | 34篇 |
1978年 | 39篇 |
1977年 | 21篇 |
1976年 | 21篇 |
1975年 | 15篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
91.
Chloroplast number per cell and mesophyll cell plan area were determined in populations of separated cells from the primary leaves of different wheat species representing three levels of ploidy. Mean chloroplast number per cell increases with ploidy level as mean cell size increases. But in addition the analysis of individual cells clearly shows that cells of a similar size but from species of different ploidies have similar numbers of chloroplasts. We conclude that the number of chloroplasts within a cell is closely correlated (P<0.001) with the size of the cell and this relationship is consistent for species of different ploidies over a wide range of cell sizes. These results are discussed in relation to the hypothesis that chloroplast number in leaf mesophyll cells is determined by the size of the cell. 相似文献
92.
Emergence and growth of barley was severely decreased by short periods (less than 24 hours) of pre-emergence waterlogging
at 20°C. The extent of damage depended on a combination of duration of waterlogging, soil water potential and aggregate size.
Potentials of less than—4kPa prevented loss of plants developing in aggregates of less than 2 mm diameter after a transitory
period of waterlogging although some shoot and root damage occurred. By comparison seeds growing in soil consisting of aggregates
greater than 2 mm in diameter were not damaged by transitory waterlogging even when drainage only occurred at−0.8kPa. The
severity of damage increased with the period of waterlogging. A criterion obtained as the product of mean size grade and water
potential gave a single value (−4NM−1) below which emergence was satisfactory. Waterlogging halfway through germination gave more severe damage than near sowing
date or near emergence. 相似文献
93.
The fate of15N labeled nitrogen applied to mature citrus trees 总被引:1,自引:0,他引:1
Summary The efficiency and balance of nitrogen from one year's application was studied in a long-term fertigation experiment. Enriched
nitrogen fertilizer, K15NO3, was applied to a 22-year-old Shamouti orange tree with a history of high N applications (N3) and to an N-starved tree (N1). The distribution of N in the different parts of the trees and in the soil was determined after the experimental trees were
excavated.
Similar total recovery of the labeled fertilizer N was found in the trees and soil in both treatments (N1−61.7% N3−56%). However, the distribution between tree and soil was different. The amount of recovered residual fertilizer in the soil
was much larger in the N3 treatment than in N1. The highest percentage of fertilizer N was found in the new organs,i.e. fruits, twigs and leaves. The roots and branches took up only 6–14% from the labeled fertilizer.
Only 20.9% of the leaf N and 23.4% of the fruit N in the N3 tree originated in the labeled fertilizer, indicating translocation of N from older parts of the tree to new growth.
Evidence was found of storage of N in the wooded branches, while the roots contained a surprisingly small part of labeled
fertilizer.
Contribution 1599E. 相似文献
94.
G. R. Findenegg 《Plant and Soil》1987,103(2):239-243
Seedlings of 14 species were grown for 14–28 days on nutrient solution with 6 mmol.l−1 NH4 as the sole nitrogen source. Solutions acidity was were kept constant at pH 4.0, 5.0, 6.0 and 7.0 by continuous titration
with diluted KOH. The following species were used: barley, maize, oats sorghum, yellow and white lupin, pea, soybean, carrot,
flax, castor-oil, spinach, sugarbeet and sunflower. Most plant species grew optimally at pH 6.0 with slight reductions at
pH 5.0. Growth of many species was severely inhibited at pH 4.0, but this inhibition was not observed with the legume and
cereal species. Yield depressions at pH 4.0 relative to pH 6.0 were well correlated with the respective relative decreases
of the K concentration in their roots (P<0.002). In the roots of two species (sunflower and flax) total N concentrations were also strongly reduced at pH 4.0. apparently,
the interactions between uptake of K, NH4 and H ions become the prevalent problem at suboptimal pH. At pH 7.0, yields were also considerably decreased, with the exception
of the lupines. At this pH, the roots of the growth inhibited plants were characterized by increased levels of total N and
free NH4. It is thought that the binding capacity of the roots for NH4 is an important factor in preventing NH4/NH3 toxicity at supraoptimal pH. 相似文献
95.
Inbreeding under a cyclical mating system 总被引:1,自引:0,他引:1
A. Farid M. Makarechian C. Strobeck 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1987,73(4):506-515
Summary General recursion formulae for the coefficient of inbreeding under a cyclical mating system were derived in which one male and one female are selected from each of the n families per generation (population size N = 2 n). Each male is given the family number of his sire in each generation, while his mate comes from another family, varying systematically in different generations. Males of the r-th family in generations 1, 2, 3,..., t = n–1 within each cycle mate with females from families r+1, r+2, r+3,..., r+t to produce generations 2, 3, 4,..., t+1=1, respectively. The change in heterozygosity shows a cyclical pattern of rises and falls, repeating in cycles of n–1 generations. The rate of inbreeding oscillates between <-3% to >6% in different generations within each cycle, irrespective of the population size. The average rate of inbreeding per generation is approximately 1/[4 N-(Log2N+1)], which is the rate for the maximum avoidance of inbreeding. The average inbreeding effective population size is approximately 2 N–2. 相似文献
96.
A. Pete Klimley 《Environmental Biology of Fishes》1987,18(1):27-40
Synopsis Female scalloped hammerhead sharks move offshore at a smaller size than do males to form schools composed primarily of intermediate size female sharks. This movement results in smaller females feeding more on pelagic prey than do males and with greater predatory success. It is contended that this change in habitat causes females to grow more rapidly to reproductive size. Intermediate size females grow at a more rapid rate than males. Female scalloped hammerhead sharks mature at a size larger than males. For many elasmobranch species, females: (1) occupy a different habitat, (2) grow more rapidly prior to maturity and continue growth following maturation, (3) feed on different prey with increased feeding success, and (4) reproduce at a size larger than males. It is suggested that female segregation increases fitness, resulting in more rapid growth for the former sex. The females reach maturity at the larger size necessary to support embryonic young, yet similar age to males, matching the female reproductive lifetime to that of males. 相似文献
97.
G. Modiano G. Cermele C. Santolamazza S. Biagioni G. Scarsella L. E. Pacifici G. Toschi 《International Journal of Anthropology》1987,2(1):61-73
392 random patients treated with SCC prior to surgery were assayed for pseudocholinesterase activity and electrophoretic pattern.
The estimate of the percent frequencies of theE
1
a
allele were 1.16±0.38 and of the C5 (+) phenotype 9.7±1.5, both typical of Caucasian populations. By combining the presentE
1
a
gene frequency estimate with that from another sample of the same population (Cermele
et al., 1987) a better estimate with smaller confidence limits was obtained, that is: 1,14±0,27. One subject interpreted asE
1
a
E
1
a
on phenotypic grounds was also found (expected 0,05) in this random sample.
A correlation coefficient of −0.521 was found between E activity and myorelaxation time in the whole sample. High correlation
values (r=−0.55 and r=−0.46) were found forE
1
u
E
1
u
; C5 (−) andE
1
u
E
1
u
; C5 (+) individuals, respectively, showing a strong dependence of the latter variable on the former one even within apparently
homogeneous classes. The use of the product of these two variables as a classification criterion allowed the identification
of a subject with very long myorelaxation time but normal activity. 相似文献
98.
Two secretary birds and three Kori bustards were studied to determine differences between their body size and gastrointestinal morphology. Body measurements were made on captive, live birds and gastrointestinal measurements on fresh postmortem specimens. For predator species, such as the Kori bustard and secretary bird, body size is a function of their ability to capture and destroy prey. While the secretary bird was clearly the taller of the two species, superior body weight, wing length, and therefore body size was noted for the Kori bustard. The size and length of the gastrointestinal tract varied between species. The secretary bird had the shorter, less complex digestive tract, with a foregut well adapted for consumption of large quantities of flesh. The large intestine was devoid of ceca. The gastrointestinal tract of the Kori bustard was markedly different from that of the secretary bird. The foregut was less complex and the large intestine possessed large, voluminous ceca. 相似文献
99.
100.
Summary Corner's rules for plant form relate the degree of branching to branch diameter, and branch diameter to leaf or inflorescence size. We report the first interspecific test of these rules for inflorescence size and branch diameter. We derived a simple corollary of Corner's rules; since leaf size and inflorescence size are both correlated to branch thickness, they may be correlated to each other. This corollary holds for Leucadendron and Protea (Proteaceae), and in certain other taxa in the Asteraceae, Bruniaceae and Pinaceae which also have leaves and reproductive structures on the same shoot. For such taxa this implies that selection for aspects of floral display (inflorescence size, pollination type) may also be expressed at the level of leaf size and vice versa. This has implications for many aspects of botany and also points to the importance of the co-ordinating role of plant architecture for aspects of plant form. 相似文献