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Stewart A. Hill Stephen E. Scheckler James F. Basinger 《American journal of botany》1997,84(1):85-103
A new fern-like fossil plant is described from the lower Upper Devonian of southern Ellesmere Island, Canadian Arctic Archipelago. The plant occurs in an Archaeopteris-dominated flora preserved in the Nordstrand Point Formation (Mid-Late Frasnian) near Bird Fiord. The plant has a pinnate vegetative system with three branch orders and laminate sphenopteroid pinnules. Primary pinnae usually diverge from the main axis in distichous pairs (quadriseriate), but can depart singly (biseriate). Each primary pinna bears a basal catadromic aphlebia. Anatomically, the plant exhibits a mesarch, bipolar protostele that is ribbon- to clepsydropsoid-shaped in the main axis. Primary pinna traces are also initially bipolar and crescent-shaped, but may become four-ribbed before dividing into a pair of bipolar traces. The morphology and anatomy of this plant are nongymnospermous and are most similar to Zygopteridales (particularly Rhacophytaceae and Zygopteridaceae). The Frasnian age of Ellesmeris shows that laminated foliage had evolved in some zygopterid ferns much earlier than previously recognized. The Sphenopteris-like pinnules of Ellesmeris indicate the need for caution when attributing such a convergent foliar design to other plant groups, such as the Devonian gymnosperms. 相似文献
24.
Food availability is important to the dynamics of animal social organizations or populations. However, the role of winter
food availability in animal population dynamics is still controversial. We carried out an experimental study to test Lack’s
hypothesis that reduced food in winter limits survival and spring numbers of breeding individuals of social groups, using
the Mongolian gerbil (Meriones unguiculatus) as model species. We established 24 gerbil social groups in 24, 10 × 10 m, pens in September 2008. We provided wheat seeds
as supplemental food in 12 enclosures from September 2008 to March 2009; the other 12 enclosures, not provided with supplemental
food, served as controls. We live-trapped gerbils at a 2-week interval from September to April. Supplemental food during winter
increased biweekly survival by 10% relative to that in control groups. Only four control social groups survived to the end
of our study whereas all 12 food-supplemented social groups survived through our study period. Supplemental food also increased
cumulative numbers of recruits and group sizes of gerbils. We conclude that winter food availability limits winter survival
and spring social groups or population sizes of Mongolian gerbils. 相似文献
25.
M Minnich F Kueppers H James 《Comparative biochemistry and physiology. B, Comparative biochemistry》1984,78(2):413-419
Alpha-1-antitrypsin (alpha-1-protease inhibitor) was isolated from mouse serum by a series of electrophoretic and chromatographic steps. We found it to be a glycoprotein of a mass ratio of 57.7 Kd. The extinction coefficient was E1%1cm,280=4.74. It inhibits bovine trypsin, human granulocytic and porcine pancreatic elastase. Its concentration in serum differs between inbred strains. Of those tested the concentration in C57BL/6J males was lowest with 5.58 +/- 0.71 mg/ml (females: 3.02 +/- 0.39) and that in DBA/2J was highest: 8.5 +/- 0.87 mg/ml (females: 4.09 +/- 0.51). The concentration of alpha-1-antitrypsin in male serum was almost twice as high as that in females of all strains tested. 相似文献
26.
David M. Anderson Richard H. Scheller James W. Posakony Linda B. McAllister Steven G. Trabert Clifford Beall Roy J. Britten Eric H. Davidson 《Journal of molecular biology》1981,145(1):5-28
Three repetitive sequence families from the sea urchin genome were studied, each defined by homology with a specific cloned probe one to a few hundred nucleotides long. Recombinant λ-sea urchin DNA libraries were screened with these probes, and individual recombinants were selected that include genomic members of these families. Restriction mapping, gel blot, and kinetic analyses were carried out to determine the organization of each repeat family. Sequence elements belonging to the first of the three repeat families were found to be embedded in longer repeat sequences. These repeat sequences frequently occur in small clusters. Members of the second repeat family are also found in a long repetitive sequence environment, but these repeats usually occur singly in any given region of the DNA. The sequences of the third repeat are only 200 to 300 nucleotides long, and are generally terminated by single copy DNA, though a few examples were found associated with other repeats. These three repeat sequence families constitute sets of homologous sequence elements that relate distant regions of the DNA. 相似文献
27.
James F. Richards Kulgite Lit Roland Fuca Catherine Bourgeault 《Biochemical and biophysical research communications》1981,99(4):1461-1467
Several species of ornithine decarboxylase were separated by chromatography of rat thymus and kidney extracts on DEAE-Sepharose CL-6B. One major and one minor species were absent from thymus of rats two hours after hormone treatment but otherwise, the elution profile was identical to thymus from control animals. The elution patterns of ODC activity in kidneys of rats treated 2.5 or 5 hours before sacrifice with dexamethasone differ from that of control kidney and from each other. Enzyme from kidneys early after hormone treatment is eluted earlier than enzyme from control tissue, while at 5 hours, the enzyme is eluted much later than in the control. This suggests that the hormone-induced activity is subsequently modified. 相似文献
28.
Carbon and nitrogen stable isotope analyses have improved our understanding of food webs and movement patterns of aquatic
organisms. These techniques have recently been applied to diet studies of elasmobranch fishes, but isotope turnover rates
and isotope diet–tissue discrimination are still poorly understood for this group. We performed a diet switch experiment on
captive sandbar sharks (Carcharhinus plumbeus) as a model shark species to determine tissue turnover rates for liver, whole blood, and white muscle. In a second experiment,
we subjected captive coastal skates (Leucoraja spp.) to serial salinity reductions to measure possible impacts of tissue urea content on nitrogen stable isotope values.
We extracted urea from spiny dogfish (Squalus acanthias) white muscle to test for effects on nitrogen stable isotopes. Isotope turnover was slow for shark tissues and similar to
previously published estimates for stingrays and teleost fishes with low growth rates. Muscle isotope data would likely fail
to capture seasonal migrations or diet switches in sharks, while liver and whole blood would more closely reflect shorter
term movement or shifts in diet. Nitrogen stable isotope values of skate blood and skate and dogfish white muscle were not
affected by tissue urea content, suggesting that available diet–tissue discrimination estimates for teleost fishes with similar
physiologies would provide accurate estimates for elasmobranchs. 相似文献
29.
IN addition to its well known antiviral activity, interferon has recently been shown to inhibit the multiplication of tumour and mammalian cells in cell culture1–6. We report here the inhibition by interferon of DNA synthesis induced in mouse spleen lymphocytes by the non-viral stimuli phytohaemagglutinin (PHA) and allogeneic lymphocytes. These findings are in accord with our contention that interferon affects cell function and, furthermore, they suggest that by acting on lymphocytes, interferon plays a role in the immunological response of the host. 相似文献
30.