全文获取类型
收费全文 | 2260篇 |
免费 | 233篇 |
出版年
2021年 | 28篇 |
2019年 | 28篇 |
2018年 | 17篇 |
2017年 | 21篇 |
2016年 | 46篇 |
2015年 | 85篇 |
2014年 | 80篇 |
2013年 | 95篇 |
2012年 | 117篇 |
2011年 | 124篇 |
2010年 | 59篇 |
2009年 | 69篇 |
2008年 | 80篇 |
2007年 | 89篇 |
2006年 | 76篇 |
2005年 | 71篇 |
2004年 | 79篇 |
2003年 | 79篇 |
2002年 | 95篇 |
2001年 | 65篇 |
2000年 | 66篇 |
1999年 | 62篇 |
1998年 | 32篇 |
1997年 | 30篇 |
1996年 | 30篇 |
1995年 | 22篇 |
1994年 | 23篇 |
1993年 | 20篇 |
1992年 | 49篇 |
1991年 | 49篇 |
1990年 | 40篇 |
1989年 | 44篇 |
1988年 | 30篇 |
1987年 | 39篇 |
1986年 | 30篇 |
1985年 | 25篇 |
1984年 | 29篇 |
1983年 | 33篇 |
1982年 | 22篇 |
1981年 | 21篇 |
1980年 | 19篇 |
1979年 | 26篇 |
1978年 | 19篇 |
1977年 | 20篇 |
1976年 | 20篇 |
1975年 | 32篇 |
1974年 | 26篇 |
1973年 | 25篇 |
1972年 | 19篇 |
1969年 | 17篇 |
排序方式: 共有2493条查询结果,搜索用时 531 毫秒
91.
The dynamics of stomatal resistance and osmotic adjustment in response to plant water deficits and stage of physiological development was studied in the leaves of spring wheat ( Triticum aestivum L., GWO 1809). Plants were germinated and grown in pots in a growth chamber at the Duke University Phytotron to four physiological stages of development (4th leaf, 7th leaf, anthesis, and soft dough), during which time stomatal resistance, total water potential and osmotic potential were measured on the last fully developed leaf of water stressed and non-stressed plants. Pressure potential was obtained by difference. Stomatal closure of the abaxial and adaxial surfaces were independent of each other, each having a different critical total water potential. The total water potential required to close the stomata on the last fully developed leaf were different at different stages of physiological development, decreasing as the plants grew older. The development of osmoregulation in wheat allows the closure of stomata during the vegetative stage at a high total water potential, but insures that stomata remain open from anthesis through the ear filling period to a lower total water potential. 相似文献
92.
The formation of the triplet state of carotenoids (detected by an absorption peak at 515 nm) and the photo-oxidation of the primary donor of Photosystem II, P-680 (detected by an absorption increase at 820 nm) have been measured by flash absorption spectroscopy in chloroplasts in which the oxygen evolution was inhibited by treatment with Tris. The amount of each transient form has been followed versus excitation flash intensity (at 590 or 694 nm). At low excitation energy the quantum yield of triplet formation (with the Photosystem II reaction center in the state Q−) is about 30% that of P-680 photo-oxidation. The yield of carotenoid triplet formation is higher in the state Q− than in the state Q, in nearly the same proportion as chlorophyll a fluorescence. It is concluded that, for excited chlorophyll a, the relative rates of intersystem crossing to the triplet state and of fluorescence emission are the same in vivo as in organic solvent. At high flash intensity the signal of P-680+ completely saturates, whereas that of carotenoid triplet continues to increase.
The rate of triplet-triplet energy transfer from chlorophyll a to carotenoids has been derived from the rise time of the absorption change at 515 nm, in chloroplasts and in several light-harvesting pigment-protein complexes. In all cases the rate is very high, around 8 · 107 s−1 at 294 K. It is about 2–3 times slower at 5 K. The transitory formation of chlorophyll triplet has been verified in two pigment-protein complexes, at 5 K. 相似文献
93.
Orians & Pearson (1979) considered the optimal foraging strategies of ‘central place foragers’, animals that repeatedly return with their food to a fixed location. We tested some of their predictions on eastern chipmunks, Tamias striatus. The rate of cheek pouch loading declines as the pouches fill; thus the optimal load size may vary, depending on the time required to travel between the feeding site and burrow. This ‘travel time’ may also affect the choice of feeding site. A method was developed to test this, and preliminary results confirm the hypothesis. Methodological and theoretical implications of these empirical results are discussed. 相似文献
94.
95.
96.
Respiration of the air breathing fishPiabucina festae 总被引:1,自引:1,他引:0
Jeffrey B. Graham Donald L. Kramer Elpidio Pineda 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1977,122(3):295-310
Summary
Piabucina festae, a Central American stream fish, breathes air frequently, even in air saturated water, however, is not an obligate air breather. Without access to air, it can maintain routine
by aquatic respiration down to aP
wO2 of about 70 Torr which is its critical O2 tension (P
cO2, Fig. 5). Aerial respiration averages 10% of total
in air saturated water and 70% in hypoxic water (Fig. 4). At lowP
wO2 air breathing is more frequent (Fig. 1), and more O2 is utilized from each air breath (Table 3), and tidal volume may increase (Fig. 7). Vascularized respiratory compartments or cells (Fig. 6), located in the second chamber of the physostomus gas bladder, function for aerial respiration. In ventilation air is gulped and forced through a large pneumatic duct into the gas bladder, excess gas is then released through opercula. Inspiration always precedes expiration and tidal volume is small, keeping gas bladderP
O
2 low (Table 4). Major differences in the air breathing physiology ofP. festae and other species are its higherP
cO2, a low aerial
in normoxic water, even though air gulps are frequent, and its pattern of inhalation prior to expiration. The interrelationship and optimization of the three gas bladder functions (buoyancy, sound reception, and air breathing) inP. festae is discussed. Aerial respiration may have evolved secondarily to the gas bladder's function in buoyancy control. 相似文献
97.
98.
Niels H. Andersen Yoshimoto Ohta C.-B. Liu C. Michael Kramer Ken Allison S. Huneck 《Phytochemistry》1977,16(11):1727-1729
Thalloid liverworts of orders Metzgeriales and Marchantiales elaborate essential oils distinguishable from those of the Jungermanniales by the absence of β-barbatene and anastreptene. Riccardia sinuata elaborates a novel tricyclic exomethylene sesquiterpene of as yet undetermined structure. Conocephalum conicum elaborates cadinene-type sesquiterpenes. β-Cadinene from the latter species is clearly enantiomeric to the same product from vascular plants. 相似文献
99.
100.